YQ Connector: support managed ClickHouse

Со стороны dqrun можно обратиться к инстансу коннектора, который работает на streaming стенде, и извлечь данные из облачного CH.

1 files changed, 11102 insertions, 0 deletions

diff --git a/contrib/libs/clang14/lib/Sema/SemaTemplate.cpp b/contrib/libs/clang14/lib/Sema/SemaTemplate.cpp
new file mode 100644
index 0000000000..64a0b45feb
--- /dev/null
+++ b/contrib/libs/clang14/lib/Sema/SemaTemplate.cpp
@@ -0,0 +1,11102 @@
+//===------- SemaTemplate.cpp - Semantic Analysis for C++ Templates -------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//===----------------------------------------------------------------------===//
+//
+//  This file implements semantic analysis for C++ templates.
+//===----------------------------------------------------------------------===//
+
+#include "TreeTransform.h"
+#include "clang/AST/ASTConsumer.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/DeclFriend.h"
+#include "clang/AST/DeclTemplate.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/ExprCXX.h"
+#include "clang/AST/RecursiveASTVisitor.h"
+#include "clang/AST/TypeVisitor.h"
+#include "clang/Basic/Builtins.h"
+#include "clang/Basic/LangOptions.h"
+#include "clang/Basic/PartialDiagnostic.h"
+#include "clang/Basic/Stack.h"
+#include "clang/Basic/TargetInfo.h"
+#include "clang/Sema/DeclSpec.h"
+#include "clang/Sema/Initialization.h"
+#include "clang/Sema/Lookup.h"
+#include "clang/Sema/Overload.h"
+#include "clang/Sema/ParsedTemplate.h"
+#include "clang/Sema/Scope.h"
+#include "clang/Sema/SemaInternal.h"
+#include "clang/Sema/Template.h"
+#include "clang/Sema/TemplateDeduction.h"
+#include "llvm/ADT/SmallBitVector.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringExtras.h"
+
+#include <iterator>
+using namespace clang;
+using namespace sema;
+
+// Exported for use by Parser.
+SourceRange
+clang::getTemplateParamsRange(TemplateParameterList const * const *Ps,
+                              unsigned N) {
+  if (!N) return SourceRange();
+  return SourceRange(Ps[0]->getTemplateLoc(), Ps[N-1]->getRAngleLoc());
+}
+
+unsigned Sema::getTemplateDepth(Scope *S) const {
+  unsigned Depth = 0;
+
+  // Each template parameter scope represents one level of template parameter
+  // depth.
+  for (Scope *TempParamScope = S->getTemplateParamParent(); TempParamScope;
+       TempParamScope = TempParamScope->getParent()->getTemplateParamParent()) {
+    ++Depth;
+  }
+
+  // Note that there are template parameters with the given depth.
+  auto ParamsAtDepth = [&](unsigned D) { Depth = std::max(Depth, D + 1); };
+
+  // Look for parameters of an enclosing generic lambda. We don't create a
+  // template parameter scope for these.
+  for (FunctionScopeInfo *FSI : getFunctionScopes()) {
+    if (auto *LSI = dyn_cast<LambdaScopeInfo>(FSI)) {
+      if (!LSI->TemplateParams.empty()) {
+        ParamsAtDepth(LSI->AutoTemplateParameterDepth);
+        break;
+      }
+      if (LSI->GLTemplateParameterList) {
+        ParamsAtDepth(LSI->GLTemplateParameterList->getDepth());
+        break;
+      }
+    }
+  }
+
+  // Look for parameters of an enclosing terse function template. We don't
+  // create a template parameter scope for these either.
+  for (const InventedTemplateParameterInfo &Info :
+       getInventedParameterInfos()) {
+    if (!Info.TemplateParams.empty()) {
+      ParamsAtDepth(Info.AutoTemplateParameterDepth);
+      break;
+    }
+  }
+
+  return Depth;
+}
+
+/// \brief Determine whether the declaration found is acceptable as the name
+/// of a template and, if so, return that template declaration. Otherwise,
+/// returns null.
+///
+/// Note that this may return an UnresolvedUsingValueDecl if AllowDependent
+/// is true. In all other cases it will return a TemplateDecl (or null).
+NamedDecl *Sema::getAsTemplateNameDecl(NamedDecl *D,
+                                       bool AllowFunctionTemplates,
+                                       bool AllowDependent) {
+  D = D->getUnderlyingDecl();
+
+  if (isa<TemplateDecl>(D)) {
+    if (!AllowFunctionTemplates && isa<FunctionTemplateDecl>(D))
+      return nullptr;
+
+    return D;
+  }
+
+  if (CXXRecordDecl *Record = dyn_cast<CXXRecordDecl>(D)) {
+    // C++ [temp.local]p1:
+    //   Like normal (non-template) classes, class templates have an
+    //   injected-class-name (Clause 9). The injected-class-name
+    //   can be used with or without a template-argument-list. When
+    //   it is used without a template-argument-list, it is
+    //   equivalent to the injected-class-name followed by the
+    //   template-parameters of the class template enclosed in
+    //   <>. When it is used with a template-argument-list, it
+    //   refers to the specified class template specialization,
+    //   which could be the current specialization or another
+    //   specialization.
+    if (Record->isInjectedClassName()) {
+      Record = cast<CXXRecordDecl>(Record->getDeclContext());
+      if (Record->getDescribedClassTemplate())
+        return Record->getDescribedClassTemplate();
+
+      if (ClassTemplateSpecializationDecl *Spec
+            = dyn_cast<ClassTemplateSpecializationDecl>(Record))
+        return Spec->getSpecializedTemplate();
+    }
+
+    return nullptr;
+  }
+
+  // 'using Dependent::foo;' can resolve to a template name.
+  // 'using typename Dependent::foo;' cannot (not even if 'foo' is an
+  // injected-class-name).
+  if (AllowDependent && isa<UnresolvedUsingValueDecl>(D))
+    return D;
+
+  return nullptr;
+}
+
+void Sema::FilterAcceptableTemplateNames(LookupResult &R,
+                                         bool AllowFunctionTemplates,
+                                         bool AllowDependent) {
+  LookupResult::Filter filter = R.makeFilter();
+  while (filter.hasNext()) {
+    NamedDecl *Orig = filter.next();
+    if (!getAsTemplateNameDecl(Orig, AllowFunctionTemplates, AllowDependent))
+      filter.erase();
+  }
+  filter.done();
+}
+
+bool Sema::hasAnyAcceptableTemplateNames(LookupResult &R,
+                                         bool AllowFunctionTemplates,
+                                         bool AllowDependent,
+                                         bool AllowNonTemplateFunctions) {
+  for (LookupResult::iterator I = R.begin(), IEnd = R.end(); I != IEnd; ++I) {
+    if (getAsTemplateNameDecl(*I, AllowFunctionTemplates, AllowDependent))
+      return true;
+    if (AllowNonTemplateFunctions &&
+        isa<FunctionDecl>((*I)->getUnderlyingDecl()))
+      return true;
+  }
+
+  return false;
+}
+
+TemplateNameKind Sema::isTemplateName(Scope *S,
+                                      CXXScopeSpec &SS,
+                                      bool hasTemplateKeyword,
+                                      const UnqualifiedId &Name,
+                                      ParsedType ObjectTypePtr,
+                                      bool EnteringContext,
+                                      TemplateTy &TemplateResult,
+                                      bool &MemberOfUnknownSpecialization,
+                                      bool Disambiguation) {
+  assert(getLangOpts().CPlusPlus && "No template names in C!");
+
+  DeclarationName TName;
+  MemberOfUnknownSpecialization = false;
+
+  switch (Name.getKind()) {
+  case UnqualifiedIdKind::IK_Identifier:
+    TName = DeclarationName(Name.Identifier);
+    break;
+
+  case UnqualifiedIdKind::IK_OperatorFunctionId:
+    TName = Context.DeclarationNames.getCXXOperatorName(
+                                              Name.OperatorFunctionId.Operator);
+    break;
+
+  case UnqualifiedIdKind::IK_LiteralOperatorId:
+    TName = Context.DeclarationNames.getCXXLiteralOperatorName(Name.Identifier);
+    break;
+
+  default:
+    return TNK_Non_template;
+  }
+
+  QualType ObjectType = ObjectTypePtr.get();
+
+  AssumedTemplateKind AssumedTemplate;
+  LookupResult R(*this, TName, Name.getBeginLoc(), LookupOrdinaryName);
+  if (LookupTemplateName(R, S, SS, ObjectType, EnteringContext,
+                         MemberOfUnknownSpecialization, SourceLocation(),
+                         &AssumedTemplate,
+                         /*AllowTypoCorrection=*/!Disambiguation))
+    return TNK_Non_template;
+
+  if (AssumedTemplate != AssumedTemplateKind::None) {
+    TemplateResult = TemplateTy::make(Context.getAssumedTemplateName(TName));
+    // Let the parser know whether we found nothing or found functions; if we
+    // found nothing, we want to more carefully check whether this is actually
+    // a function template name versus some other kind of undeclared identifier.
+    return AssumedTemplate == AssumedTemplateKind::FoundNothing
+               ? TNK_Undeclared_template
+               : TNK_Function_template;
+  }
+
+  if (R.empty())
+    return TNK_Non_template;
+
+  NamedDecl *D = nullptr;
+  if (R.isAmbiguous()) {
+    // If we got an ambiguity involving a non-function template, treat this
+    // as a template name, and pick an arbitrary template for error recovery.
+    bool AnyFunctionTemplates = false;
+    for (NamedDecl *FoundD : R) {
+      if (NamedDecl *FoundTemplate = getAsTemplateNameDecl(FoundD)) {
+        if (isa<FunctionTemplateDecl>(FoundTemplate))
+          AnyFunctionTemplates = true;
+        else {
+          D = FoundTemplate;
+          break;
+        }
+      }
+    }
+
+    // If we didn't find any templates at all, this isn't a template name.
+    // Leave the ambiguity for a later lookup to diagnose.
+    if (!D && !AnyFunctionTemplates) {
+      R.suppressDiagnostics();
+      return TNK_Non_template;
+    }
+
+    // If the only templates were function templates, filter out the rest.
+    // We'll diagnose the ambiguity later.
+    if (!D)
+      FilterAcceptableTemplateNames(R);
+  }
+
+  // At this point, we have either picked a single template name declaration D
+  // or we have a non-empty set of results R containing either one template name
+  // declaration or a set of function templates.
+
+  TemplateName Template;
+  TemplateNameKind TemplateKind;
+
+  unsigned ResultCount = R.end() - R.begin();
+  if (!D && ResultCount > 1) {
+    // We assume that we'll preserve the qualifier from a function
+    // template name in other ways.
+    Template = Context.getOverloadedTemplateName(R.begin(), R.end());
+    TemplateKind = TNK_Function_template;
+
+    // We'll do this lookup again later.
+    R.suppressDiagnostics();
+  } else {
+    if (!D) {
+      D = getAsTemplateNameDecl(*R.begin());
+      assert(D && "unambiguous result is not a template name");
+    }
+
+    if (isa<UnresolvedUsingValueDecl>(D)) {
+      // We don't yet know whether this is a template-name or not.
+      MemberOfUnknownSpecialization = true;
+      return TNK_Non_template;
+    }
+
+    TemplateDecl *TD = cast<TemplateDecl>(D);
+
+    if (SS.isSet() && !SS.isInvalid()) {
+      NestedNameSpecifier *Qualifier = SS.getScopeRep();
+      Template = Context.getQualifiedTemplateName(Qualifier,
+                                                  hasTemplateKeyword, TD);
+    } else {
+      Template = TemplateName(TD);
+    }
+
+    if (isa<FunctionTemplateDecl>(TD)) {
+      TemplateKind = TNK_Function_template;
+
+      // We'll do this lookup again later.
+      R.suppressDiagnostics();
+    } else {
+      assert(isa<ClassTemplateDecl>(TD) || isa<TemplateTemplateParmDecl>(TD) ||
+             isa<TypeAliasTemplateDecl>(TD) || isa<VarTemplateDecl>(TD) ||
+             isa<BuiltinTemplateDecl>(TD) || isa<ConceptDecl>(TD));
+      TemplateKind =
+          isa<VarTemplateDecl>(TD) ? TNK_Var_template :
+          isa<ConceptDecl>(TD) ? TNK_Concept_template :
+          TNK_Type_template;
+    }
+  }
+
+  TemplateResult = TemplateTy::make(Template);
+  return TemplateKind;
+}
+
+bool Sema::isDeductionGuideName(Scope *S, const IdentifierInfo &Name,
+                                SourceLocation NameLoc,
+                                ParsedTemplateTy *Template) {
+  CXXScopeSpec SS;
+  bool MemberOfUnknownSpecialization = false;
+
+  // We could use redeclaration lookup here, but we don't need to: the
+  // syntactic form of a deduction guide is enough to identify it even
+  // if we can't look up the template name at all.
+  LookupResult R(*this, DeclarationName(&Name), NameLoc, LookupOrdinaryName);
+  if (LookupTemplateName(R, S, SS, /*ObjectType*/ QualType(),
+                         /*EnteringContext*/ false,
+                         MemberOfUnknownSpecialization))
+    return false;
+
+  if (R.empty()) return false;
+  if (R.isAmbiguous()) {
+    // FIXME: Diagnose an ambiguity if we find at least one template.
+    R.suppressDiagnostics();
+    return false;
+  }
+
+  // We only treat template-names that name type templates as valid deduction
+  // guide names.
+  TemplateDecl *TD = R.getAsSingle<TemplateDecl>();
+  if (!TD || !getAsTypeTemplateDecl(TD))
+    return false;
+
+  if (Template)
+    *Template = TemplateTy::make(TemplateName(TD));
+  return true;
+}
+
+bool Sema::DiagnoseUnknownTemplateName(const IdentifierInfo &II,
+                                       SourceLocation IILoc,
+                                       Scope *S,
+                                       const CXXScopeSpec *SS,
+                                       TemplateTy &SuggestedTemplate,
+                                       TemplateNameKind &SuggestedKind) {
+  // We can't recover unless there's a dependent scope specifier preceding the
+  // template name.
+  // FIXME: Typo correction?
+  if (!SS || !SS->isSet() || !isDependentScopeSpecifier(*SS) ||
+      computeDeclContext(*SS))
+    return false;
+
+  // The code is missing a 'template' keyword prior to the dependent template
+  // name.
+  NestedNameSpecifier *Qualifier = (NestedNameSpecifier*)SS->getScopeRep();
+  Diag(IILoc, diag::err_template_kw_missing)
+    << Qualifier << II.getName()
+    << FixItHint::CreateInsertion(IILoc, "template ");
+  SuggestedTemplate
+    = TemplateTy::make(Context.getDependentTemplateName(Qualifier, &II));
+  SuggestedKind = TNK_Dependent_template_name;
+  return true;
+}
+
+bool Sema::LookupTemplateName(LookupResult &Found,
+                              Scope *S, CXXScopeSpec &SS,
+                              QualType ObjectType,
+                              bool EnteringContext,
+                              bool &MemberOfUnknownSpecialization,
+                              RequiredTemplateKind RequiredTemplate,
+                              AssumedTemplateKind *ATK,
+                              bool AllowTypoCorrection) {
+  if (ATK)
+    *ATK = AssumedTemplateKind::None;
+
+  if (SS.isInvalid())
+    return true;
+
+  Found.setTemplateNameLookup(true);
+
+  // Determine where to perform name lookup
+  MemberOfUnknownSpecialization = false;
+  DeclContext *LookupCtx = nullptr;
+  bool IsDependent = false;
+  if (!ObjectType.isNull()) {
+    // This nested-name-specifier occurs in a member access expression, e.g.,
+    // x->B::f, and we are looking into the type of the object.
+    assert(SS.isEmpty() && "ObjectType and scope specifier cannot coexist");
+    LookupCtx = computeDeclContext(ObjectType);
+    IsDependent = !LookupCtx && ObjectType->isDependentType();
+    assert((IsDependent || !ObjectType->isIncompleteType() ||
+            ObjectType->castAs<TagType>()->isBeingDefined()) &&
+           "Caller should have completed object type");
+
+    // Template names cannot appear inside an Objective-C class or object type
+    // or a vector type.
+    //
+    // FIXME: This is wrong. For example:
+    //
+    //   template<typename T> using Vec = T __attribute__((ext_vector_type(4)));
+    //   Vec<int> vi;
+    //   vi.Vec<int>::~Vec<int>();
+    //
+    // ... should be accepted but we will not treat 'Vec' as a template name
+    // here. The right thing to do would be to check if the name is a valid
+    // vector component name, and look up a template name if not. And similarly
+    // for lookups into Objective-C class and object types, where the same
+    // problem can arise.
+    if (ObjectType->isObjCObjectOrInterfaceType() ||
+        ObjectType->isVectorType()) {
+      Found.clear();
+      return false;
+    }
+  } else if (SS.isNotEmpty()) {
+    // This nested-name-specifier occurs after another nested-name-specifier,
+    // so long into the context associated with the prior nested-name-specifier.
+    LookupCtx = computeDeclContext(SS, EnteringContext);
+    IsDependent = !LookupCtx && isDependentScopeSpecifier(SS);
+
+    // The declaration context must be complete.
+    if (LookupCtx && RequireCompleteDeclContext(SS, LookupCtx))
+      return true;
+  }
+
+  bool ObjectTypeSearchedInScope = false;
+  bool AllowFunctionTemplatesInLookup = true;
+  if (LookupCtx) {
+    // Perform "qualified" name lookup into the declaration context we
+    // computed, which is either the type of the base of a member access
+    // expression or the declaration context associated with a prior
+    // nested-name-specifier.
+    LookupQualifiedName(Found, LookupCtx);
+
+    // FIXME: The C++ standard does not clearly specify what happens in the
+    // case where the object type is dependent, and implementations vary. In
+    // Clang, we treat a name after a . or -> as a template-name if lookup
+    // finds a non-dependent member or member of the current instantiation that
+    // is a type template, or finds no such members and lookup in the context
+    // of the postfix-expression finds a type template. In the latter case, the
+    // name is nonetheless dependent, and we may resolve it to a member of an
+    // unknown specialization when we come to instantiate the template.
+    IsDependent |= Found.wasNotFoundInCurrentInstantiation();
+  }
+
+  if (SS.isEmpty() && (ObjectType.isNull() || Found.empty())) {
+    // C++ [basic.lookup.classref]p1:
+    //   In a class member access expression (5.2.5), if the . or -> token is
+    //   immediately followed by an identifier followed by a <, the
+    //   identifier must be looked up to determine whether the < is the
+    //   beginning of a template argument list (14.2) or a less-than operator.
+    //   The identifier is first looked up in the class of the object
+    //   expression. If the identifier is not found, it is then looked up in
+    //   the context of the entire postfix-expression and shall name a class
+    //   template.
+    if (S)
+      LookupName(Found, S);
+
+    if (!ObjectType.isNull()) {
+      //  FIXME: We should filter out all non-type templates here, particularly
+      //  variable templates and concepts. But the exclusion of alias templates
+      //  and template template parameters is a wording defect.
+      AllowFunctionTemplatesInLookup = false;
+      ObjectTypeSearchedInScope = true;
+    }
+
+    IsDependent |= Found.wasNotFoundInCurrentInstantiation();
+  }
+
+  if (Found.isAmbiguous())
+    return false;
+
+  if (ATK && SS.isEmpty() && ObjectType.isNull() &&
+      !RequiredTemplate.hasTemplateKeyword()) {
+    // C++2a [temp.names]p2:
+    //   A name is also considered to refer to a template if it is an
+    //   unqualified-id followed by a < and name lookup finds either one or more
+    //   functions or finds nothing.
+    //
+    // To keep our behavior consistent, we apply the "finds nothing" part in
+    // all language modes, and diagnose the empty lookup in ActOnCallExpr if we
+    // successfully form a call to an undeclared template-id.
+    bool AllFunctions =
+        getLangOpts().CPlusPlus20 && llvm::all_of(Found, [](NamedDecl *ND) {
+          return isa<FunctionDecl>(ND->getUnderlyingDecl());
+        });
+    if (AllFunctions || (Found.empty() && !IsDependent)) {
+      // If lookup found any functions, or if this is a name that can only be
+      // used for a function, then strongly assume this is a function
+      // template-id.
+      *ATK = (Found.empty() && Found.getLookupName().isIdentifier())
+                 ? AssumedTemplateKind::FoundNothing
+                 : AssumedTemplateKind::FoundFunctions;
+      Found.clear();
+      return false;
+    }
+  }
+
+  if (Found.empty() && !IsDependent && AllowTypoCorrection) {
+    // If we did not find any names, and this is not a disambiguation, attempt
+    // to correct any typos.
+    DeclarationName Name = Found.getLookupName();
+    Found.clear();
+    // Simple filter callback that, for keywords, only accepts the C++ *_cast
+    DefaultFilterCCC FilterCCC{};
+    FilterCCC.WantTypeSpecifiers = false;
+    FilterCCC.WantExpressionKeywords = false;
+    FilterCCC.WantRemainingKeywords = false;
+    FilterCCC.WantCXXNamedCasts = true;
+    if (TypoCorrection Corrected =
+            CorrectTypo(Found.getLookupNameInfo(), Found.getLookupKind(), S,
+                        &SS, FilterCCC, CTK_ErrorRecovery, LookupCtx)) {
+      if (auto *ND = Corrected.getFoundDecl())
+        Found.addDecl(ND);
+      FilterAcceptableTemplateNames(Found);
+      if (Found.isAmbiguous()) {
+        Found.clear();
+      } else if (!Found.empty()) {
+        Found.setLookupName(Corrected.getCorrection());
+        if (LookupCtx) {
+          std::string CorrectedStr(Corrected.getAsString(getLangOpts()));
+          bool DroppedSpecifier = Corrected.WillReplaceSpecifier() &&
+                                  Name.getAsString() == CorrectedStr;
+          diagnoseTypo(Corrected, PDiag(diag::err_no_member_template_suggest)
+                                    << Name << LookupCtx << DroppedSpecifier
+                                    << SS.getRange());
+        } else {
+          diagnoseTypo(Corrected, PDiag(diag::err_no_template_suggest) << Name);
+        }
+      }
+    }
+  }
+
+  NamedDecl *ExampleLookupResult =
+      Found.empty() ? nullptr : Found.getRepresentativeDecl();
+  FilterAcceptableTemplateNames(Found, AllowFunctionTemplatesInLookup);
+  if (Found.empty()) {
+    if (IsDependent) {
+      MemberOfUnknownSpecialization = true;
+      return false;
+    }
+
+    // If a 'template' keyword was used, a lookup that finds only non-template
+    // names is an error.
+    if (ExampleLookupResult && RequiredTemplate) {
+      Diag(Found.getNameLoc(), diag::err_template_kw_refers_to_non_template)
+          << Found.getLookupName() << SS.getRange()
+          << RequiredTemplate.hasTemplateKeyword()
+          << RequiredTemplate.getTemplateKeywordLoc();
+      Diag(ExampleLookupResult->getUnderlyingDecl()->getLocation(),
+           diag::note_template_kw_refers_to_non_template)
+          << Found.getLookupName();
+      return true;
+    }
+
+    return false;
+  }
+
+  if (S && !ObjectType.isNull() && !ObjectTypeSearchedInScope &&
+      !getLangOpts().CPlusPlus11) {
+    // C++03 [basic.lookup.classref]p1:
+    //   [...] If the lookup in the class of the object expression finds a
+    //   template, the name is also looked up in the context of the entire
+    //   postfix-expression and [...]
+    //
+    // Note: C++11 does not perform this second lookup.
+    LookupResult FoundOuter(*this, Found.getLookupName(), Found.getNameLoc(),
+                            LookupOrdinaryName);
+    FoundOuter.setTemplateNameLookup(true);
+    LookupName(FoundOuter, S);
+    // FIXME: We silently accept an ambiguous lookup here, in violation of
+    // [basic.lookup]/1.
+    FilterAcceptableTemplateNames(FoundOuter, /*AllowFunctionTemplates=*/false);
+
+    NamedDecl *OuterTemplate;
+    if (FoundOuter.empty()) {
+      //   - if the name is not found, the name found in the class of the
+      //     object expression is used, otherwise
+    } else if (FoundOuter.isAmbiguous() || !FoundOuter.isSingleResult() ||
+               !(OuterTemplate =
+                     getAsTemplateNameDecl(FoundOuter.getFoundDecl()))) {
+      //   - if the name is found in the context of the entire
+      //     postfix-expression and does not name a class template, the name
+      //     found in the class of the object expression is used, otherwise
+      FoundOuter.clear();
+    } else if (!Found.isSuppressingDiagnostics()) {
+      //   - if the name found is a class template, it must refer to the same
+      //     entity as the one found in the class of the object expression,
+      //     otherwise the program is ill-formed.
+      if (!Found.isSingleResult() ||
+          getAsTemplateNameDecl(Found.getFoundDecl())->getCanonicalDecl() !=
+              OuterTemplate->getCanonicalDecl()) {
+        Diag(Found.getNameLoc(),
+             diag::ext_nested_name_member_ref_lookup_ambiguous)
+          << Found.getLookupName()
+          << ObjectType;
+        Diag(Found.getRepresentativeDecl()->getLocation(),
+             diag::note_ambig_member_ref_object_type)
+          << ObjectType;
+        Diag(FoundOuter.getFoundDecl()->getLocation(),
+             diag::note_ambig_member_ref_scope);
+
+        // Recover by taking the template that we found in the object
+        // expression's type.
+      }
+    }
+  }
+
+  return false;
+}
+
+void Sema::diagnoseExprIntendedAsTemplateName(Scope *S, ExprResult TemplateName,
+                                              SourceLocation Less,
+                                              SourceLocation Greater) {
+  if (TemplateName.isInvalid())
+    return;
+
+  DeclarationNameInfo NameInfo;
+  CXXScopeSpec SS;
+  LookupNameKind LookupKind;
+
+  DeclContext *LookupCtx = nullptr;
+  NamedDecl *Found = nullptr;
+  bool MissingTemplateKeyword = false;
+
+  // Figure out what name we looked up.
+  if (auto *DRE = dyn_cast<DeclRefExpr>(TemplateName.get())) {
+    NameInfo = DRE->getNameInfo();
+    SS.Adopt(DRE->getQualifierLoc());
+    LookupKind = LookupOrdinaryName;
+    Found = DRE->getFoundDecl();
+  } else if (auto *ME = dyn_cast<MemberExpr>(TemplateName.get())) {
+    NameInfo = ME->getMemberNameInfo();
+    SS.Adopt(ME->getQualifierLoc());
+    LookupKind = LookupMemberName;
+    LookupCtx = ME->getBase()->getType()->getAsCXXRecordDecl();
+    Found = ME->getMemberDecl();
+  } else if (auto *DSDRE =
+                 dyn_cast<DependentScopeDeclRefExpr>(TemplateName.get())) {
+    NameInfo = DSDRE->getNameInfo();
+    SS.Adopt(DSDRE->getQualifierLoc());
+    MissingTemplateKeyword = true;
+  } else if (auto *DSME =
+                 dyn_cast<CXXDependentScopeMemberExpr>(TemplateName.get())) {
+    NameInfo = DSME->getMemberNameInfo();
+    SS.Adopt(DSME->getQualifierLoc());
+    MissingTemplateKeyword = true;
+  } else {
+    llvm_unreachable("unexpected kind of potential template name");
+  }
+
+  // If this is a dependent-scope lookup, diagnose that the 'template' keyword
+  // was missing.
+  if (MissingTemplateKeyword) {
+    Diag(NameInfo.getBeginLoc(), diag::err_template_kw_missing)
+        << "" << NameInfo.getName().getAsString() << SourceRange(Less, Greater);
+    return;
+  }
+
+  // Try to correct the name by looking for templates and C++ named casts.
+  struct TemplateCandidateFilter : CorrectionCandidateCallback {
+    Sema &S;
+    TemplateCandidateFilter(Sema &S) : S(S) {
+      WantTypeSpecifiers = false;
+      WantExpressionKeywords = false;
+      WantRemainingKeywords = false;
+      WantCXXNamedCasts = true;
+    };
+    bool ValidateCandidate(const TypoCorrection &Candidate) override {
+      if (auto *ND = Candidate.getCorrectionDecl())
+        return S.getAsTemplateNameDecl(ND);
+      return Candidate.isKeyword();
+    }
+
+    std::unique_ptr<CorrectionCandidateCallback> clone() override {
+      return std::make_unique<TemplateCandidateFilter>(*this);
+    }
+  };
+
+  DeclarationName Name = NameInfo.getName();
+  TemplateCandidateFilter CCC(*this);
+  if (TypoCorrection Corrected = CorrectTypo(NameInfo, LookupKind, S, &SS, CCC,
+                                             CTK_ErrorRecovery, LookupCtx)) {
+    auto *ND = Corrected.getFoundDecl();
+    if (ND)
+      ND = getAsTemplateNameDecl(ND);
+    if (ND || Corrected.isKeyword()) {
+      if (LookupCtx) {
+        std::string CorrectedStr(Corrected.getAsString(getLangOpts()));
+        bool DroppedSpecifier = Corrected.WillReplaceSpecifier() &&
+                                Name.getAsString() == CorrectedStr;
+        diagnoseTypo(Corrected,
+                     PDiag(diag::err_non_template_in_member_template_id_suggest)
+                         << Name << LookupCtx << DroppedSpecifier
+                         << SS.getRange(), false);
+      } else {
+        diagnoseTypo(Corrected,
+                     PDiag(diag::err_non_template_in_template_id_suggest)
+                         << Name, false);
+      }
+      if (Found)
+        Diag(Found->getLocation(),
+             diag::note_non_template_in_template_id_found);
+      return;
+    }
+  }
+
+  Diag(NameInfo.getLoc(), diag::err_non_template_in_template_id)
+    << Name << SourceRange(Less, Greater);
+  if (Found)
+    Diag(Found->getLocation(), diag::note_non_template_in_template_id_found);
+}
+
+/// ActOnDependentIdExpression - Handle a dependent id-expression that
+/// was just parsed.  This is only possible with an explicit scope
+/// specifier naming a dependent type.
+ExprResult
+Sema::ActOnDependentIdExpression(const CXXScopeSpec &SS,
+                                 SourceLocation TemplateKWLoc,
+                                 const DeclarationNameInfo &NameInfo,
+                                 bool isAddressOfOperand,
+                           const TemplateArgumentListInfo *TemplateArgs) {
+  DeclContext *DC = getFunctionLevelDeclContext();
+
+  // C++11 [expr.prim.general]p12:
+  //   An id-expression that denotes a non-static data member or non-static
+  //   member function of a class can only be used:
+  //   (...)
+  //   - if that id-expression denotes a non-static data member and it
+  //     appears in an unevaluated operand.
+  //
+  // If this might be the case, form a DependentScopeDeclRefExpr instead of a
+  // CXXDependentScopeMemberExpr. The former can instantiate to either
+  // DeclRefExpr or MemberExpr depending on lookup results, while the latter is
+  // always a MemberExpr.
+  bool MightBeCxx11UnevalField =
+      getLangOpts().CPlusPlus11 && isUnevaluatedContext();
+
+  // Check if the nested name specifier is an enum type.
+  bool IsEnum = false;
+  if (NestedNameSpecifier *NNS = SS.getScopeRep())
+    IsEnum = isa_and_nonnull<EnumType>(NNS->getAsType());
+
+  if (!MightBeCxx11UnevalField && !isAddressOfOperand && !IsEnum &&
+      isa<CXXMethodDecl>(DC) && cast<CXXMethodDecl>(DC)->isInstance()) {
+    QualType ThisType = cast<CXXMethodDecl>(DC)->getThisType();
+
+    // Since the 'this' expression is synthesized, we don't need to
+    // perform the double-lookup check.
+    NamedDecl *FirstQualifierInScope = nullptr;
+
+    return CXXDependentScopeMemberExpr::Create(
+        Context, /*This*/ nullptr, ThisType, /*IsArrow*/ true,
+        /*Op*/ SourceLocation(), SS.getWithLocInContext(Context), TemplateKWLoc,
+        FirstQualifierInScope, NameInfo, TemplateArgs);
+  }
+
+  return BuildDependentDeclRefExpr(SS, TemplateKWLoc, NameInfo, TemplateArgs);
+}
+
+ExprResult
+Sema::BuildDependentDeclRefExpr(const CXXScopeSpec &SS,
+                                SourceLocation TemplateKWLoc,
+                                const DeclarationNameInfo &NameInfo,
+                                const TemplateArgumentListInfo *TemplateArgs) {
+  // DependentScopeDeclRefExpr::Create requires a valid QualifierLoc
+  NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);
+  if (!QualifierLoc)
+    return ExprError();
+
+  return DependentScopeDeclRefExpr::Create(
+      Context, QualifierLoc, TemplateKWLoc, NameInfo, TemplateArgs);
+}
+
+
+/// Determine whether we would be unable to instantiate this template (because
+/// it either has no definition, or is in the process of being instantiated).
+bool Sema::DiagnoseUninstantiableTemplate(SourceLocation PointOfInstantiation,
+                                          NamedDecl *Instantiation,
+                                          bool InstantiatedFromMember,
+                                          const NamedDecl *Pattern,
+                                          const NamedDecl *PatternDef,
+                                          TemplateSpecializationKind TSK,
+                                          bool Complain /*= true*/) {
+  assert(isa<TagDecl>(Instantiation) || isa<FunctionDecl>(Instantiation) ||
+         isa<VarDecl>(Instantiation));
+
+  bool IsEntityBeingDefined = false;
+  if (const TagDecl *TD = dyn_cast_or_null<TagDecl>(PatternDef))
+    IsEntityBeingDefined = TD->isBeingDefined();
+
+  if (PatternDef && !IsEntityBeingDefined) {
+    NamedDecl *SuggestedDef = nullptr;
+    if (!hasVisibleDefinition(const_cast<NamedDecl*>(PatternDef), &SuggestedDef,
+                              /*OnlyNeedComplete*/false)) {
+      // If we're allowed to diagnose this and recover, do so.
+      bool Recover = Complain && !isSFINAEContext();
+      if (Complain)
+        diagnoseMissingImport(PointOfInstantiation, SuggestedDef,
+                              Sema::MissingImportKind::Definition, Recover);
+      return !Recover;
+    }
+    return false;
+  }
+
+  if (!Complain || (PatternDef && PatternDef->isInvalidDecl()))
+    return true;
+
+  llvm::Optional<unsigned> Note;
+  QualType InstantiationTy;
+  if (TagDecl *TD = dyn_cast<TagDecl>(Instantiation))
+    InstantiationTy = Context.getTypeDeclType(TD);
+  if (PatternDef) {
+    Diag(PointOfInstantiation,
+         diag::err_template_instantiate_within_definition)
+      << /*implicit|explicit*/(TSK != TSK_ImplicitInstantiation)
+      << InstantiationTy;
+    // Not much point in noting the template declaration here, since
+    // we're lexically inside it.
+    Instantiation->setInvalidDecl();
+  } else if (InstantiatedFromMember) {
+    if (isa<FunctionDecl>(Instantiation)) {
+      Diag(PointOfInstantiation,
+           diag::err_explicit_instantiation_undefined_member)
+        << /*member function*/ 1 << Instantiation->getDeclName()
+        << Instantiation->getDeclContext();
+      Note = diag::note_explicit_instantiation_here;
+    } else {
+      assert(isa<TagDecl>(Instantiation) && "Must be a TagDecl!");
+      Diag(PointOfInstantiation,
+           diag::err_implicit_instantiate_member_undefined)
+        << InstantiationTy;
+      Note = diag::note_member_declared_at;
+    }
+  } else {
+    if (isa<FunctionDecl>(Instantiation)) {
+      Diag(PointOfInstantiation,
+           diag::err_explicit_instantiation_undefined_func_template)
+        << Pattern;
+      Note = diag::note_explicit_instantiation_here;
+    } else if (isa<TagDecl>(Instantiation)) {
+      Diag(PointOfInstantiation, diag::err_template_instantiate_undefined)
+        << (TSK != TSK_ImplicitInstantiation)
+        << InstantiationTy;
+      Note = diag::note_template_decl_here;
+    } else {
+      assert(isa<VarDecl>(Instantiation) && "Must be a VarDecl!");
+      if (isa<VarTemplateSpecializationDecl>(Instantiation)) {
+        Diag(PointOfInstantiation,
+             diag::err_explicit_instantiation_undefined_var_template)
+          << Instantiation;
+        Instantiation->setInvalidDecl();
+      } else
+        Diag(PointOfInstantiation,
+             diag::err_explicit_instantiation_undefined_member)
+          << /*static data member*/ 2 << Instantiation->getDeclName()
+          << Instantiation->getDeclContext();
+      Note = diag::note_explicit_instantiation_here;
+    }
+  }
+  if (Note) // Diagnostics were emitted.
+    Diag(Pattern->getLocation(), Note.getValue());
+
+  // In general, Instantiation isn't marked invalid to get more than one
+  // error for multiple undefined instantiations. But the code that does
+  // explicit declaration -> explicit definition conversion can't handle
+  // invalid declarations, so mark as invalid in that case.
+  if (TSK == TSK_ExplicitInstantiationDeclaration)
+    Instantiation->setInvalidDecl();
+  return true;
+}
+
+/// DiagnoseTemplateParameterShadow - Produce a diagnostic complaining
+/// that the template parameter 'PrevDecl' is being shadowed by a new
+/// declaration at location Loc. Returns true to indicate that this is
+/// an error, and false otherwise.
+void Sema::DiagnoseTemplateParameterShadow(SourceLocation Loc, Decl *PrevDecl) {
+  assert(PrevDecl->isTemplateParameter() && "Not a template parameter");
+
+  // C++ [temp.local]p4:
+  //   A template-parameter shall not be redeclared within its
+  //   scope (including nested scopes).
+  //
+  // Make this a warning when MSVC compatibility is requested.
+  unsigned DiagId = getLangOpts().MSVCCompat ? diag::ext_template_param_shadow
+                                             : diag::err_template_param_shadow;
+  Diag(Loc, DiagId) << cast<NamedDecl>(PrevDecl)->getDeclName();
+  Diag(PrevDecl->getLocation(), diag::note_template_param_here);
+}
+
+/// AdjustDeclIfTemplate - If the given decl happens to be a template, reset
+/// the parameter D to reference the templated declaration and return a pointer
+/// to the template declaration. Otherwise, do nothing to D and return null.
+TemplateDecl *Sema::AdjustDeclIfTemplate(Decl *&D) {
+  if (TemplateDecl *Temp = dyn_cast_or_null<TemplateDecl>(D)) {
+    D = Temp->getTemplatedDecl();
+    return Temp;
+  }
+  return nullptr;
+}
+
+ParsedTemplateArgument ParsedTemplateArgument::getTemplatePackExpansion(
+                                             SourceLocation EllipsisLoc) const {
+  assert(Kind == Template &&
+         "Only template template arguments can be pack expansions here");
+  assert(getAsTemplate().get().containsUnexpandedParameterPack() &&
+         "Template template argument pack expansion without packs");
+  ParsedTemplateArgument Result(*this);
+  Result.EllipsisLoc = EllipsisLoc;
+  return Result;
+}
+
+static TemplateArgumentLoc translateTemplateArgument(Sema &SemaRef,
+                                            const ParsedTemplateArgument &Arg) {
+
+  switch (Arg.getKind()) {
+  case ParsedTemplateArgument::Type: {
+    TypeSourceInfo *DI;
+    QualType T = SemaRef.GetTypeFromParser(Arg.getAsType(), &DI);
+    if (!DI)
+      DI = SemaRef.Context.getTrivialTypeSourceInfo(T, Arg.getLocation());
+    return TemplateArgumentLoc(TemplateArgument(T), DI);
+  }
+
+  case ParsedTemplateArgument::NonType: {
+    Expr *E = static_cast<Expr *>(Arg.getAsExpr());
+    return TemplateArgumentLoc(TemplateArgument(E), E);
+  }
+
+  case ParsedTemplateArgument::Template: {
+    TemplateName Template = Arg.getAsTemplate().get();
+    TemplateArgument TArg;
+    if (Arg.getEllipsisLoc().isValid())
+      TArg = TemplateArgument(Template, Optional<unsigned int>());
+    else
+      TArg = Template;
+    return TemplateArgumentLoc(
+        SemaRef.Context, TArg,
+        Arg.getScopeSpec().getWithLocInContext(SemaRef.Context),
+        Arg.getLocation(), Arg.getEllipsisLoc());
+  }
+  }
+
+  llvm_unreachable("Unhandled parsed template argument");
+}
+
+/// Translates template arguments as provided by the parser
+/// into template arguments used by semantic analysis.
+void Sema::translateTemplateArguments(const ASTTemplateArgsPtr &TemplateArgsIn,
+                                      TemplateArgumentListInfo &TemplateArgs) {
+ for (unsigned I = 0, Last = TemplateArgsIn.size(); I != Last; ++I)
+   TemplateArgs.addArgument(translateTemplateArgument(*this,
+                                                      TemplateArgsIn[I]));
+}
+
+static void maybeDiagnoseTemplateParameterShadow(Sema &SemaRef, Scope *S,
+                                                 SourceLocation Loc,
+                                                 IdentifierInfo *Name) {
+  NamedDecl *PrevDecl = SemaRef.LookupSingleName(
+      S, Name, Loc, Sema::LookupOrdinaryName, Sema::ForVisibleRedeclaration);
+  if (PrevDecl && PrevDecl->isTemplateParameter())
+    SemaRef.DiagnoseTemplateParameterShadow(Loc, PrevDecl);
+}
+
+/// Convert a parsed type into a parsed template argument. This is mostly
+/// trivial, except that we may have parsed a C++17 deduced class template
+/// specialization type, in which case we should form a template template
+/// argument instead of a type template argument.
+ParsedTemplateArgument Sema::ActOnTemplateTypeArgument(TypeResult ParsedType) {
+  TypeSourceInfo *TInfo;
+  QualType T = GetTypeFromParser(ParsedType.get(), &TInfo);
+  if (T.isNull())
+    return ParsedTemplateArgument();
+  assert(TInfo && "template argument with no location");
+
+  // If we might have formed a deduced template specialization type, convert
+  // it to a template template argument.
+  if (getLangOpts().CPlusPlus17) {
+    TypeLoc TL = TInfo->getTypeLoc();
+    SourceLocation EllipsisLoc;
+    if (auto PET = TL.getAs<PackExpansionTypeLoc>()) {
+      EllipsisLoc = PET.getEllipsisLoc();
+      TL = PET.getPatternLoc();
+    }
+
+    CXXScopeSpec SS;
+    if (auto ET = TL.getAs<ElaboratedTypeLoc>()) {
+      SS.Adopt(ET.getQualifierLoc());
+      TL = ET.getNamedTypeLoc();
+    }
+
+    if (auto DTST = TL.getAs<DeducedTemplateSpecializationTypeLoc>()) {
+      TemplateName Name = DTST.getTypePtr()->getTemplateName();
+      if (SS.isSet())
+        Name = Context.getQualifiedTemplateName(SS.getScopeRep(),
+                                                /*HasTemplateKeyword*/ false,
+                                                Name.getAsTemplateDecl());
+      ParsedTemplateArgument Result(SS, TemplateTy::make(Name),
+                                    DTST.getTemplateNameLoc());
+      if (EllipsisLoc.isValid())
+        Result = Result.getTemplatePackExpansion(EllipsisLoc);
+      return Result;
+    }
+  }
+
+  // This is a normal type template argument. Note, if the type template
+  // argument is an injected-class-name for a template, it has a dual nature
+  // and can be used as either a type or a template. We handle that in
+  // convertTypeTemplateArgumentToTemplate.
+  return ParsedTemplateArgument(ParsedTemplateArgument::Type,
+                                ParsedType.get().getAsOpaquePtr(),
+                                TInfo->getTypeLoc().getBeginLoc());
+}
+
+/// ActOnTypeParameter - Called when a C++ template type parameter
+/// (e.g., "typename T") has been parsed. Typename specifies whether
+/// the keyword "typename" was used to declare the type parameter
+/// (otherwise, "class" was used), and KeyLoc is the location of the
+/// "class" or "typename" keyword. ParamName is the name of the
+/// parameter (NULL indicates an unnamed template parameter) and
+/// ParamNameLoc is the location of the parameter name (if any).
+/// If the type parameter has a default argument, it will be added
+/// later via ActOnTypeParameterDefault.
+NamedDecl *Sema::ActOnTypeParameter(Scope *S, bool Typename,
+                                    SourceLocation EllipsisLoc,
+                                    SourceLocation KeyLoc,
+                                    IdentifierInfo *ParamName,
+                                    SourceLocation ParamNameLoc,
+                                    unsigned Depth, unsigned Position,
+                                    SourceLocation EqualLoc,
+                                    ParsedType DefaultArg,
+                                    bool HasTypeConstraint) {
+  assert(S->isTemplateParamScope() &&
+         "Template type parameter not in template parameter scope!");
+
+  bool IsParameterPack = EllipsisLoc.isValid();
+  TemplateTypeParmDecl *Param
+    = TemplateTypeParmDecl::Create(Context, Context.getTranslationUnitDecl(),
+                                   KeyLoc, ParamNameLoc, Depth, Position,
+                                   ParamName, Typename, IsParameterPack,
+                                   HasTypeConstraint);
+  Param->setAccess(AS_public);
+
+  if (Param->isParameterPack())
+    if (auto *LSI = getEnclosingLambda())
+      LSI->LocalPacks.push_back(Param);
+
+  if (ParamName) {
+    maybeDiagnoseTemplateParameterShadow(*this, S, ParamNameLoc, ParamName);
+
+    // Add the template parameter into the current scope.
+    S->AddDecl(Param);
+    IdResolver.AddDecl(Param);
+  }
+
+  // C++0x [temp.param]p9:
+  //   A default template-argument may be specified for any kind of
+  //   template-parameter that is not a template parameter pack.
+  if (DefaultArg && IsParameterPack) {
+    Diag(EqualLoc, diag::err_template_param_pack_default_arg);
+    DefaultArg = nullptr;
+  }
+
+  // Handle the default argument, if provided.
+  if (DefaultArg) {
+    TypeSourceInfo *DefaultTInfo;
+    GetTypeFromParser(DefaultArg, &DefaultTInfo);
+
+    assert(DefaultTInfo && "expected source information for type");
+
+    // Check for unexpanded parameter packs.
+    if (DiagnoseUnexpandedParameterPack(ParamNameLoc, DefaultTInfo,
+                                        UPPC_DefaultArgument))
+      return Param;
+
+    // Check the template argument itself.
+    if (CheckTemplateArgument(DefaultTInfo)) {
+      Param->setInvalidDecl();
+      return Param;
+    }
+
+    Param->setDefaultArgument(DefaultTInfo);
+  }
+
+  return Param;
+}
+
+/// Convert the parser's template argument list representation into our form.
+static TemplateArgumentListInfo
+makeTemplateArgumentListInfo(Sema &S, TemplateIdAnnotation &TemplateId) {
+  TemplateArgumentListInfo TemplateArgs(TemplateId.LAngleLoc,
+                                        TemplateId.RAngleLoc);
+  ASTTemplateArgsPtr TemplateArgsPtr(TemplateId.getTemplateArgs(),
+                                     TemplateId.NumArgs);
+  S.translateTemplateArguments(TemplateArgsPtr, TemplateArgs);
+  return TemplateArgs;
+}
+
+bool Sema::ActOnTypeConstraint(const CXXScopeSpec &SS,
+                               TemplateIdAnnotation *TypeConstr,
+                               TemplateTypeParmDecl *ConstrainedParameter,
+                               SourceLocation EllipsisLoc) {
+  return BuildTypeConstraint(SS, TypeConstr, ConstrainedParameter, EllipsisLoc,
+                             false);
+}
+
+bool Sema::BuildTypeConstraint(const CXXScopeSpec &SS,
+                               TemplateIdAnnotation *TypeConstr,
+                               TemplateTypeParmDecl *ConstrainedParameter,
+                               SourceLocation EllipsisLoc,
+                               bool AllowUnexpandedPack) {
+  TemplateName TN = TypeConstr->Template.get();
+  ConceptDecl *CD = cast<ConceptDecl>(TN.getAsTemplateDecl());
+
+  // C++2a [temp.param]p4:
+  //     [...] The concept designated by a type-constraint shall be a type
+  //     concept ([temp.concept]).
+  if (!CD->isTypeConcept()) {
+    Diag(TypeConstr->TemplateNameLoc,
+         diag::err_type_constraint_non_type_concept);
+    return true;
+  }
+
+  bool WereArgsSpecified = TypeConstr->LAngleLoc.isValid();
+
+  if (!WereArgsSpecified &&
+      CD->getTemplateParameters()->getMinRequiredArguments() > 1) {
+    Diag(TypeConstr->TemplateNameLoc,
+         diag::err_type_constraint_missing_arguments) << CD;
+    return true;
+  }
+
+  DeclarationNameInfo ConceptName(DeclarationName(TypeConstr->Name),
+                                  TypeConstr->TemplateNameLoc);
+
+  TemplateArgumentListInfo TemplateArgs;
+  if (TypeConstr->LAngleLoc.isValid()) {
+    TemplateArgs =
+        makeTemplateArgumentListInfo(*this, *TypeConstr);
+
+    if (EllipsisLoc.isInvalid() && !AllowUnexpandedPack) {
+      for (TemplateArgumentLoc Arg : TemplateArgs.arguments()) {
+        if (DiagnoseUnexpandedParameterPack(Arg, UPPC_TypeConstraint))
+          return true;
+      }
+    }
+  }
+  return AttachTypeConstraint(
+      SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc(),
+      ConceptName, CD,
+      TypeConstr->LAngleLoc.isValid() ? &TemplateArgs : nullptr,
+      ConstrainedParameter, EllipsisLoc);
+}
+
+template<typename ArgumentLocAppender>
+static ExprResult formImmediatelyDeclaredConstraint(
+    Sema &S, NestedNameSpecifierLoc NS, DeclarationNameInfo NameInfo,
+    ConceptDecl *NamedConcept, SourceLocation LAngleLoc,
+    SourceLocation RAngleLoc, QualType ConstrainedType,
+    SourceLocation ParamNameLoc, ArgumentLocAppender Appender,
+    SourceLocation EllipsisLoc) {
+
+  TemplateArgumentListInfo ConstraintArgs;
+  ConstraintArgs.addArgument(
+    S.getTrivialTemplateArgumentLoc(TemplateArgument(ConstrainedType),
+                                    /*NTTPType=*/QualType(), ParamNameLoc));
+
+  ConstraintArgs.setRAngleLoc(RAngleLoc);
+  ConstraintArgs.setLAngleLoc(LAngleLoc);
+  Appender(ConstraintArgs);
+
+  // C++2a [temp.param]p4:
+  //     [...] This constraint-expression E is called the immediately-declared
+  //     constraint of T. [...]
+  CXXScopeSpec SS;
+  SS.Adopt(NS);
+  ExprResult ImmediatelyDeclaredConstraint = S.CheckConceptTemplateId(
+      SS, /*TemplateKWLoc=*/SourceLocation(), NameInfo,
+      /*FoundDecl=*/NamedConcept, NamedConcept, &ConstraintArgs);
+  if (ImmediatelyDeclaredConstraint.isInvalid() || !EllipsisLoc.isValid())
+    return ImmediatelyDeclaredConstraint;
+
+  // C++2a [temp.param]p4:
+  //     [...] If T is not a pack, then E is E', otherwise E is (E' && ...).
+  //
+  // We have the following case:
+  //
+  // template<typename T> concept C1 = true;
+  // template<C1... T> struct s1;
+  //
+  // The constraint: (C1<T> && ...)
+  //
+  // Note that the type of C1<T> is known to be 'bool', so we don't need to do
+  // any unqualified lookups for 'operator&&' here.
+  return S.BuildCXXFoldExpr(/*UnqualifiedLookup=*/nullptr,
+                            /*LParenLoc=*/SourceLocation(),
+                            ImmediatelyDeclaredConstraint.get(), BO_LAnd,
+                            EllipsisLoc, /*RHS=*/nullptr,
+                            /*RParenLoc=*/SourceLocation(),
+                            /*NumExpansions=*/None);
+}
+
+/// Attach a type-constraint to a template parameter.
+/// \returns true if an error occurred. This can happen if the
+/// immediately-declared constraint could not be formed (e.g. incorrect number
+/// of arguments for the named concept).
+bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
+                                DeclarationNameInfo NameInfo,
+                                ConceptDecl *NamedConcept,
+                                const TemplateArgumentListInfo *TemplateArgs,
+                                TemplateTypeParmDecl *ConstrainedParameter,
+                                SourceLocation EllipsisLoc) {
+  // C++2a [temp.param]p4:
+  //     [...] If Q is of the form C<A1, ..., An>, then let E' be
+  //     C<T, A1, ..., An>. Otherwise, let E' be C<T>. [...]
+  const ASTTemplateArgumentListInfo *ArgsAsWritten =
+    TemplateArgs ? ASTTemplateArgumentListInfo::Create(Context,
+                                                       *TemplateArgs) : nullptr;
+
+  QualType ParamAsArgument(ConstrainedParameter->getTypeForDecl(), 0);
+
+  ExprResult ImmediatelyDeclaredConstraint =
+      formImmediatelyDeclaredConstraint(
+          *this, NS, NameInfo, NamedConcept,
+          TemplateArgs ? TemplateArgs->getLAngleLoc() : SourceLocation(),
+          TemplateArgs ? TemplateArgs->getRAngleLoc() : SourceLocation(),
+          ParamAsArgument, ConstrainedParameter->getLocation(),
+          [&] (TemplateArgumentListInfo &ConstraintArgs) {
+            if (TemplateArgs)
+              for (const auto &ArgLoc : TemplateArgs->arguments())
+                ConstraintArgs.addArgument(ArgLoc);
+          }, EllipsisLoc);
+  if (ImmediatelyDeclaredConstraint.isInvalid())
+    return true;
+
+  ConstrainedParameter->setTypeConstraint(NS, NameInfo,
+                                          /*FoundDecl=*/NamedConcept,
+                                          NamedConcept, ArgsAsWritten,
+                                          ImmediatelyDeclaredConstraint.get());
+  return false;
+}
+
+bool Sema::AttachTypeConstraint(AutoTypeLoc TL, NonTypeTemplateParmDecl *NTTP,
+                                SourceLocation EllipsisLoc) {
+  if (NTTP->getType() != TL.getType() ||
+      TL.getAutoKeyword() != AutoTypeKeyword::Auto) {
+    Diag(NTTP->getTypeSourceInfo()->getTypeLoc().getBeginLoc(),
+         diag::err_unsupported_placeholder_constraint)
+       << NTTP->getTypeSourceInfo()->getTypeLoc().getSourceRange();
+    return true;
+  }
+  // FIXME: Concepts: This should be the type of the placeholder, but this is
+  // unclear in the wording right now.
+  DeclRefExpr *Ref =
+      BuildDeclRefExpr(NTTP, NTTP->getType(), VK_PRValue, NTTP->getLocation());
+  if (!Ref)
+    return true;
+  ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
+      *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
+      TL.getNamedConcept(), TL.getLAngleLoc(), TL.getRAngleLoc(),
+      BuildDecltypeType(Ref), NTTP->getLocation(),
+      [&](TemplateArgumentListInfo &ConstraintArgs) {
+        for (unsigned I = 0, C = TL.getNumArgs(); I != C; ++I)
+          ConstraintArgs.addArgument(TL.getArgLoc(I));
+      },
+      EllipsisLoc);
+  if (ImmediatelyDeclaredConstraint.isInvalid() ||
+     !ImmediatelyDeclaredConstraint.isUsable())
+    return true;
+
+  NTTP->setPlaceholderTypeConstraint(ImmediatelyDeclaredConstraint.get());
+  return false;
+}
+
+/// Check that the type of a non-type template parameter is
+/// well-formed.
+///
+/// \returns the (possibly-promoted) parameter type if valid;
+/// otherwise, produces a diagnostic and returns a NULL type.
+QualType Sema::CheckNonTypeTemplateParameterType(TypeSourceInfo *&TSI,
+                                                 SourceLocation Loc) {
+  if (TSI->getType()->isUndeducedType()) {
+    // C++17 [temp.dep.expr]p3:
+    //   An id-expression is type-dependent if it contains
+    //    - an identifier associated by name lookup with a non-type
+    //      template-parameter declared with a type that contains a
+    //      placeholder type (7.1.7.4),
+    TSI = SubstAutoTypeSourceInfoDependent(TSI);
+  }
+
+  return CheckNonTypeTemplateParameterType(TSI->getType(), Loc);
+}
+
+/// Require the given type to be a structural type, and diagnose if it is not.
+///
+/// \return \c true if an error was produced.
+bool Sema::RequireStructuralType(QualType T, SourceLocation Loc) {
+  if (T->isDependentType())
+    return false;
+
+  if (RequireCompleteType(Loc, T, diag::err_template_nontype_parm_incomplete))
+    return true;
+
+  if (T->isStructuralType())
+    return false;
+
+  // Structural types are required to be object types or lvalue references.
+  if (T->isRValueReferenceType()) {
+    Diag(Loc, diag::err_template_nontype_parm_rvalue_ref) << T;
+    return true;
+  }
+
+  // Don't mention structural types in our diagnostic prior to C++20. Also,
+  // there's not much more we can say about non-scalar non-class types --
+  // because we can't see functions or arrays here, those can only be language
+  // extensions.
+  if (!getLangOpts().CPlusPlus20 ||
+      (!T->isScalarType() && !T->isRecordType())) {
+    Diag(Loc, diag::err_template_nontype_parm_bad_type) << T;
+    return true;
+  }
+
+  // Structural types are required to be literal types.
+  if (RequireLiteralType(Loc, T, diag::err_template_nontype_parm_not_literal))
+    return true;
+
+  Diag(Loc, diag::err_template_nontype_parm_not_structural) << T;
+
+  // Drill down into the reason why the class is non-structural.
+  while (const CXXRecordDecl *RD = T->getAsCXXRecordDecl()) {
+    // All members are required to be public and non-mutable, and can't be of
+    // rvalue reference type. Check these conditions first to prefer a "local"
+    // reason over a more distant one.
+    for (const FieldDecl *FD : RD->fields()) {
+      if (FD->getAccess() != AS_public) {
+        Diag(FD->getLocation(), diag::note_not_structural_non_public) << T << 0;
+        return true;
+      }
+      if (FD->isMutable()) {
+        Diag(FD->getLocation(), diag::note_not_structural_mutable_field) << T;
+        return true;
+      }
+      if (FD->getType()->isRValueReferenceType()) {
+        Diag(FD->getLocation(), diag::note_not_structural_rvalue_ref_field)
+            << T;
+        return true;
+      }
+    }
+
+    // All bases are required to be public.
+    for (const auto &BaseSpec : RD->bases()) {
+      if (BaseSpec.getAccessSpecifier() != AS_public) {
+        Diag(BaseSpec.getBaseTypeLoc(), diag::note_not_structural_non_public)
+            << T << 1;
+        return true;
+      }
+    }
+
+    // All subobjects are required to be of structural types.
+    SourceLocation SubLoc;
+    QualType SubType;
+    int Kind = -1;
+
+    for (const FieldDecl *FD : RD->fields()) {
+      QualType T = Context.getBaseElementType(FD->getType());
+      if (!T->isStructuralType()) {
+        SubLoc = FD->getLocation();
+        SubType = T;
+        Kind = 0;
+        break;
+      }
+    }
+
+    if (Kind == -1) {
+      for (const auto &BaseSpec : RD->bases()) {
+        QualType T = BaseSpec.getType();
+        if (!T->isStructuralType()) {
+          SubLoc = BaseSpec.getBaseTypeLoc();
+          SubType = T;
+          Kind = 1;
+          break;
+        }
+      }
+    }
+
+    assert(Kind != -1 && "couldn't find reason why type is not structural");
+    Diag(SubLoc, diag::note_not_structural_subobject)
+        << T << Kind << SubType;
+    T = SubType;
+    RD = T->getAsCXXRecordDecl();
+  }
+
+  return true;
+}
+
+QualType Sema::CheckNonTypeTemplateParameterType(QualType T,
+                                                 SourceLocation Loc) {
+  // We don't allow variably-modified types as the type of non-type template
+  // parameters.
+  if (T->isVariablyModifiedType()) {
+    Diag(Loc, diag::err_variably_modified_nontype_template_param)
+      << T;
+    return QualType();
+  }
+
+  // C++ [temp.param]p4:
+  //
+  // A non-type template-parameter shall have one of the following
+  // (optionally cv-qualified) types:
+  //
+  //       -- integral or enumeration type,
+  if (T->isIntegralOrEnumerationType() ||
+      //   -- pointer to object or pointer to function,
+      T->isPointerType() ||
+      //   -- lvalue reference to object or lvalue reference to function,
+      T->isLValueReferenceType() ||
+      //   -- pointer to member,
+      T->isMemberPointerType() ||
+      //   -- std::nullptr_t, or
+      T->isNullPtrType() ||
+      //   -- a type that contains a placeholder type.
+      T->isUndeducedType()) {
+    // C++ [temp.param]p5: The top-level cv-qualifiers on the template-parameter
+    // are ignored when determining its type.
+    return T.getUnqualifiedType();
+  }
+
+  // C++ [temp.param]p8:
+  //
+  //   A non-type template-parameter of type "array of T" or
+  //   "function returning T" is adjusted to be of type "pointer to
+  //   T" or "pointer to function returning T", respectively.
+  if (T->isArrayType() || T->isFunctionType())
+    return Context.getDecayedType(T);
+
+  // If T is a dependent type, we can't do the check now, so we
+  // assume that it is well-formed. Note that stripping off the
+  // qualifiers here is not really correct if T turns out to be
+  // an array type, but we'll recompute the type everywhere it's
+  // used during instantiation, so that should be OK. (Using the
+  // qualified type is equally wrong.)
+  if (T->isDependentType())
+    return T.getUnqualifiedType();
+
+  // C++20 [temp.param]p6:
+  //   -- a structural type
+  if (RequireStructuralType(T, Loc))
+    return QualType();
+
+  if (!getLangOpts().CPlusPlus20) {
+    // FIXME: Consider allowing structural types as an extension in C++17. (In
+    // earlier language modes, the template argument evaluation rules are too
+    // inflexible.)
+    Diag(Loc, diag::err_template_nontype_parm_bad_structural_type) << T;
+    return QualType();
+  }
+
+  Diag(Loc, diag::warn_cxx17_compat_template_nontype_parm_type) << T;
+  return T.getUnqualifiedType();
+}
+
+NamedDecl *Sema::ActOnNonTypeTemplateParameter(Scope *S, Declarator &D,
+                                          unsigned Depth,
+                                          unsigned Position,
+                                          SourceLocation EqualLoc,
+                                          Expr *Default) {
+  TypeSourceInfo *TInfo = GetTypeForDeclarator(D, S);
+
+  // Check that we have valid decl-specifiers specified.
+  auto CheckValidDeclSpecifiers = [this, &D] {
+    // C++ [temp.param]
+    // p1
+    //   template-parameter:
+    //     ...
+    //     parameter-declaration
+    // p2
+    //   ... A storage class shall not be specified in a template-parameter
+    //   declaration.
+    // [dcl.typedef]p1:
+    //   The typedef specifier [...] shall not be used in the decl-specifier-seq
+    //   of a parameter-declaration
+    const DeclSpec &DS = D.getDeclSpec();
+    auto EmitDiag = [this](SourceLocation Loc) {
+      Diag(Loc, diag::err_invalid_decl_specifier_in_nontype_parm)
+          << FixItHint::CreateRemoval(Loc);
+    };
+    if (DS.getStorageClassSpec() != DeclSpec::SCS_unspecified)
+      EmitDiag(DS.getStorageClassSpecLoc());
+
+    if (DS.getThreadStorageClassSpec() != TSCS_unspecified)
+      EmitDiag(DS.getThreadStorageClassSpecLoc());
+
+    // [dcl.inline]p1:
+    //   The inline specifier can be applied only to the declaration or
+    //   definition of a variable or function.
+
+    if (DS.isInlineSpecified())
+      EmitDiag(DS.getInlineSpecLoc());
+
+    // [dcl.constexpr]p1:
+    //   The constexpr specifier shall be applied only to the definition of a
+    //   variable or variable template or the declaration of a function or
+    //   function template.
+
+    if (DS.hasConstexprSpecifier())
+      EmitDiag(DS.getConstexprSpecLoc());
+
+    // [dcl.fct.spec]p1:
+    //   Function-specifiers can be used only in function declarations.
+
+    if (DS.isVirtualSpecified())
+      EmitDiag(DS.getVirtualSpecLoc());
+
+    if (DS.hasExplicitSpecifier())
+      EmitDiag(DS.getExplicitSpecLoc());
+
+    if (DS.isNoreturnSpecified())
+      EmitDiag(DS.getNoreturnSpecLoc());
+  };
+
+  CheckValidDeclSpecifiers();
+
+  if (TInfo->getType()->isUndeducedType()) {
+    Diag(D.getIdentifierLoc(),
+         diag::warn_cxx14_compat_template_nontype_parm_auto_type)
+      << QualType(TInfo->getType()->getContainedAutoType(), 0);
+  }
+
+  assert(S->isTemplateParamScope() &&
+         "Non-type template parameter not in template parameter scope!");
+  bool Invalid = false;
+
+  QualType T = CheckNonTypeTemplateParameterType(TInfo, D.getIdentifierLoc());
+  if (T.isNull()) {
+    T = Context.IntTy; // Recover with an 'int' type.
+    Invalid = true;
+  }
+
+  CheckFunctionOrTemplateParamDeclarator(S, D);
+
+  IdentifierInfo *ParamName = D.getIdentifier();
+  bool IsParameterPack = D.hasEllipsis();
+  NonTypeTemplateParmDecl *Param = NonTypeTemplateParmDecl::Create(
+      Context, Context.getTranslationUnitDecl(), D.getBeginLoc(),
+      D.getIdentifierLoc(), Depth, Position, ParamName, T, IsParameterPack,
+      TInfo);
+  Param->setAccess(AS_public);
+
+  if (AutoTypeLoc TL = TInfo->getTypeLoc().getContainedAutoTypeLoc())
+    if (TL.isConstrained())
+      if (AttachTypeConstraint(TL, Param, D.getEllipsisLoc()))
+        Invalid = true;
+
+  if (Invalid)
+    Param->setInvalidDecl();
+
+  if (Param->isParameterPack())
+    if (auto *LSI = getEnclosingLambda())
+      LSI->LocalPacks.push_back(Param);
+
+  if (ParamName) {
+    maybeDiagnoseTemplateParameterShadow(*this, S, D.getIdentifierLoc(),
+                                         ParamName);
+
+    // Add the template parameter into the current scope.
+    S->AddDecl(Param);
+    IdResolver.AddDecl(Param);
+  }
+
+  // C++0x [temp.param]p9:
+  //   A default template-argument may be specified for any kind of
+  //   template-parameter that is not a template parameter pack.
+  if (Default && IsParameterPack) {
+    Diag(EqualLoc, diag::err_template_param_pack_default_arg);
+    Default = nullptr;
+  }
+
+  // Check the well-formedness of the default template argument, if provided.
+  if (Default) {
+    // Check for unexpanded parameter packs.
+    if (DiagnoseUnexpandedParameterPack(Default, UPPC_DefaultArgument))
+      return Param;
+
+    TemplateArgument Converted;
+    ExprResult DefaultRes =
+        CheckTemplateArgument(Param, Param->getType(), Default, Converted);
+    if (DefaultRes.isInvalid()) {
+      Param->setInvalidDecl();
+      return Param;
+    }
+    Default = DefaultRes.get();
+
+    Param->setDefaultArgument(Default);
+  }
+
+  return Param;
+}
+
+/// ActOnTemplateTemplateParameter - Called when a C++ template template
+/// parameter (e.g. T in template <template \<typename> class T> class array)
+/// has been parsed. S is the current scope.
+NamedDecl *Sema::ActOnTemplateTemplateParameter(Scope* S,
+                                           SourceLocation TmpLoc,
+                                           TemplateParameterList *Params,
+                                           SourceLocation EllipsisLoc,
+                                           IdentifierInfo *Name,
+                                           SourceLocation NameLoc,
+                                           unsigned Depth,
+                                           unsigned Position,
+                                           SourceLocation EqualLoc,
+                                           ParsedTemplateArgument Default) {
+  assert(S->isTemplateParamScope() &&
+         "Template template parameter not in template parameter scope!");
+
+  // Construct the parameter object.
+  bool IsParameterPack = EllipsisLoc.isValid();
+  TemplateTemplateParmDecl *Param =
+    TemplateTemplateParmDecl::Create(Context, Context.getTranslationUnitDecl(),
+                                     NameLoc.isInvalid()? TmpLoc : NameLoc,
+                                     Depth, Position, IsParameterPack,
+                                     Name, Params);
+  Param->setAccess(AS_public);
+
+  if (Param->isParameterPack())
+    if (auto *LSI = getEnclosingLambda())
+      LSI->LocalPacks.push_back(Param);
+
+  // If the template template parameter has a name, then link the identifier
+  // into the scope and lookup mechanisms.
+  if (Name) {
+    maybeDiagnoseTemplateParameterShadow(*this, S, NameLoc, Name);
+
+    S->AddDecl(Param);
+    IdResolver.AddDecl(Param);
+  }
+
+  if (Params->size() == 0) {
+    Diag(Param->getLocation(), diag::err_template_template_parm_no_parms)
+    << SourceRange(Params->getLAngleLoc(), Params->getRAngleLoc());
+    Param->setInvalidDecl();
+  }
+
+  // C++0x [temp.param]p9:
+  //   A default template-argument may be specified for any kind of
+  //   template-parameter that is not a template parameter pack.
+  if (IsParameterPack && !Default.isInvalid()) {
+    Diag(EqualLoc, diag::err_template_param_pack_default_arg);
+    Default = ParsedTemplateArgument();
+  }
+
+  if (!Default.isInvalid()) {
+    // Check only that we have a template template argument. We don't want to
+    // try to check well-formedness now, because our template template parameter
+    // might have dependent types in its template parameters, which we wouldn't
+    // be able to match now.
+    //
+    // If none of the template template parameter's template arguments mention
+    // other template parameters, we could actually perform more checking here.
+    // However, it isn't worth doing.
+    TemplateArgumentLoc DefaultArg = translateTemplateArgument(*this, Default);
+    if (DefaultArg.getArgument().getAsTemplate().isNull()) {
+      Diag(DefaultArg.getLocation(), diag::err_template_arg_not_valid_template)
+        << DefaultArg.getSourceRange();
+      return Param;
+    }
+
+    // Check for unexpanded parameter packs.
+    if (DiagnoseUnexpandedParameterPack(DefaultArg.getLocation(),
+                                        DefaultArg.getArgument().getAsTemplate(),
+                                        UPPC_DefaultArgument))
+      return Param;
+
+    Param->setDefaultArgument(Context, DefaultArg);
+  }
+
+  return Param;
+}
+
+/// ActOnTemplateParameterList - Builds a TemplateParameterList, optionally
+/// constrained by RequiresClause, that contains the template parameters in
+/// Params.
+TemplateParameterList *
+Sema::ActOnTemplateParameterList(unsigned Depth,
+                                 SourceLocation ExportLoc,
+                                 SourceLocation TemplateLoc,
+                                 SourceLocation LAngleLoc,
+                                 ArrayRef<NamedDecl *> Params,
+                                 SourceLocation RAngleLoc,
+                                 Expr *RequiresClause) {
+  if (ExportLoc.isValid())
+    Diag(ExportLoc, diag::warn_template_export_unsupported);
+
+  for (NamedDecl *P : Params)
+    warnOnReservedIdentifier(P);
+
+  return TemplateParameterList::Create(
+      Context, TemplateLoc, LAngleLoc,
+      llvm::makeArrayRef(Params.data(), Params.size()),
+      RAngleLoc, RequiresClause);
+}
+
+static void SetNestedNameSpecifier(Sema &S, TagDecl *T,
+                                   const CXXScopeSpec &SS) {
+  if (SS.isSet())
+    T->setQualifierInfo(SS.getWithLocInContext(S.Context));
+}
+
+DeclResult Sema::CheckClassTemplate(
+    Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
+    CXXScopeSpec &SS, IdentifierInfo *Name, SourceLocation NameLoc,
+    const ParsedAttributesView &Attr, TemplateParameterList *TemplateParams,
+    AccessSpecifier AS, SourceLocation ModulePrivateLoc,
+    SourceLocation FriendLoc, unsigned NumOuterTemplateParamLists,
+    TemplateParameterList **OuterTemplateParamLists, SkipBodyInfo *SkipBody) {
+  assert(TemplateParams && TemplateParams->size() > 0 &&
+         "No template parameters");
+  assert(TUK != TUK_Reference && "Can only declare or define class templates");
+  bool Invalid = false;
+
+  // Check that we can declare a template here.
+  if (CheckTemplateDeclScope(S, TemplateParams))
+    return true;
+
+  TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForTypeSpec(TagSpec);
+  assert(Kind != TTK_Enum && "can't build template of enumerated type");
+
+  // There is no such thing as an unnamed class template.
+  if (!Name) {
+    Diag(KWLoc, diag::err_template_unnamed_class);
+    return true;
+  }
+
+  // Find any previous declaration with this name. For a friend with no
+  // scope explicitly specified, we only look for tag declarations (per
+  // C++11 [basic.lookup.elab]p2).
+  DeclContext *SemanticContext;
+  LookupResult Previous(*this, Name, NameLoc,
+                        (SS.isEmpty() && TUK == TUK_Friend)
+                          ? LookupTagName : LookupOrdinaryName,
+                        forRedeclarationInCurContext());
+  if (SS.isNotEmpty() && !SS.isInvalid()) {
+    SemanticContext = computeDeclContext(SS, true);
+    if (!SemanticContext) {
+      // FIXME: Horrible, horrible hack! We can't currently represent this
+      // in the AST, and historically we have just ignored such friend
+      // class templates, so don't complain here.
+      Diag(NameLoc, TUK == TUK_Friend
+                        ? diag::warn_template_qualified_friend_ignored
+                        : diag::err_template_qualified_declarator_no_match)
+          << SS.getScopeRep() << SS.getRange();
+      return TUK != TUK_Friend;
+    }
+
+    if (RequireCompleteDeclContext(SS, SemanticContext))
+      return true;
+
+    // If we're adding a template to a dependent context, we may need to
+    // rebuilding some of the types used within the template parameter list,
+    // now that we know what the current instantiation is.
+    if (SemanticContext->isDependentContext()) {
+      ContextRAII SavedContext(*this, SemanticContext);
+      if (RebuildTemplateParamsInCurrentInstantiation(TemplateParams))
+        Invalid = true;
+    } else if (TUK != TUK_Friend && TUK != TUK_Reference)
+      diagnoseQualifiedDeclaration(SS, SemanticContext, Name, NameLoc, false);
+
+    LookupQualifiedName(Previous, SemanticContext);
+  } else {
+    SemanticContext = CurContext;
+
+    // C++14 [class.mem]p14:
+    //   If T is the name of a class, then each of the following shall have a
+    //   name different from T:
+    //    -- every member template of class T
+    if (TUK != TUK_Friend &&
+        DiagnoseClassNameShadow(SemanticContext,
+                                DeclarationNameInfo(Name, NameLoc)))
+      return true;
+
+    LookupName(Previous, S);
+  }
+
+  if (Previous.isAmbiguous())
+    return true;
+
+  NamedDecl *PrevDecl = nullptr;
+  if (Previous.begin() != Previous.end())
+    PrevDecl = (*Previous.begin())->getUnderlyingDecl();
+
+  if (PrevDecl && PrevDecl->isTemplateParameter()) {
+    // Maybe we will complain about the shadowed template parameter.
+    DiagnoseTemplateParameterShadow(NameLoc, PrevDecl);
+    // Just pretend that we didn't see the previous declaration.
+    PrevDecl = nullptr;
+  }
+
+  // If there is a previous declaration with the same name, check
+  // whether this is a valid redeclaration.
+  ClassTemplateDecl *PrevClassTemplate =
+      dyn_cast_or_null<ClassTemplateDecl>(PrevDecl);
+
+  // We may have found the injected-class-name of a class template,
+  // class template partial specialization, or class template specialization.
+  // In these cases, grab the template that is being defined or specialized.
+  if (!PrevClassTemplate && PrevDecl && isa<CXXRecordDecl>(PrevDecl) &&
+      cast<CXXRecordDecl>(PrevDecl)->isInjectedClassName()) {
+    PrevDecl = cast<CXXRecordDecl>(PrevDecl->getDeclContext());
+    PrevClassTemplate
+      = cast<CXXRecordDecl>(PrevDecl)->getDescribedClassTemplate();
+    if (!PrevClassTemplate && isa<ClassTemplateSpecializationDecl>(PrevDecl)) {
+      PrevClassTemplate
+        = cast<ClassTemplateSpecializationDecl>(PrevDecl)
+            ->getSpecializedTemplate();
+    }
+  }
+
+  if (TUK == TUK_Friend) {
+    // C++ [namespace.memdef]p3:
+    //   [...] When looking for a prior declaration of a class or a function
+    //   declared as a friend, and when the name of the friend class or
+    //   function is neither a qualified name nor a template-id, scopes outside
+    //   the innermost enclosing namespace scope are not considered.
+    if (!SS.isSet()) {
+      DeclContext *OutermostContext = CurContext;
+      while (!OutermostContext->isFileContext())
+        OutermostContext = OutermostContext->getLookupParent();
+
+      if (PrevDecl &&
+          (OutermostContext->Equals(PrevDecl->getDeclContext()) ||
+           OutermostContext->Encloses(PrevDecl->getDeclContext()))) {
+        SemanticContext = PrevDecl->getDeclContext();
+      } else {
+        // Declarations in outer scopes don't matter. However, the outermost
+        // context we computed is the semantic context for our new
+        // declaration.
+        PrevDecl = PrevClassTemplate = nullptr;
+        SemanticContext = OutermostContext;
+
+        // Check that the chosen semantic context doesn't already contain a
+        // declaration of this name as a non-tag type.
+        Previous.clear(LookupOrdinaryName);
+        DeclContext *LookupContext = SemanticContext;
+        while (LookupContext->isTransparentContext())
+          LookupContext = LookupContext->getLookupParent();
+        LookupQualifiedName(Previous, LookupContext);
+
+        if (Previous.isAmbiguous())
+          return true;
+
+        if (Previous.begin() != Previous.end())
+          PrevDecl = (*Previous.begin())->getUnderlyingDecl();
+      }
+    }
+  } else if (PrevDecl &&
+             !isDeclInScope(Previous.getRepresentativeDecl(), SemanticContext,
+                            S, SS.isValid()))
+    PrevDecl = PrevClassTemplate = nullptr;
+
+  if (auto *Shadow = dyn_cast_or_null<UsingShadowDecl>(
+          PrevDecl ? Previous.getRepresentativeDecl() : nullptr)) {
+    if (SS.isEmpty() &&
+        !(PrevClassTemplate &&
+          PrevClassTemplate->getDeclContext()->getRedeclContext()->Equals(
+              SemanticContext->getRedeclContext()))) {
+      Diag(KWLoc, diag::err_using_decl_conflict_reverse);
+      Diag(Shadow->getTargetDecl()->getLocation(),
+           diag::note_using_decl_target);
+      Diag(Shadow->getIntroducer()->getLocation(), diag::note_using_decl) << 0;
+      // Recover by ignoring the old declaration.
+      PrevDecl = PrevClassTemplate = nullptr;
+    }
+  }
+
+  if (PrevClassTemplate) {
+    // Ensure that the template parameter lists are compatible. Skip this check
+    // for a friend in a dependent context: the template parameter list itself
+    // could be dependent.
+    if (!(TUK == TUK_Friend && CurContext->isDependentContext()) &&
+        !TemplateParameterListsAreEqual(TemplateParams,
+                                   PrevClassTemplate->getTemplateParameters(),
+                                        /*Complain=*/true,
+                                        TPL_TemplateMatch))
+      return true;
+
+    // C++ [temp.class]p4:
+    //   In a redeclaration, partial specialization, explicit
+    //   specialization or explicit instantiation of a class template,
+    //   the class-key shall agree in kind with the original class
+    //   template declaration (7.1.5.3).
+    RecordDecl *PrevRecordDecl = PrevClassTemplate->getTemplatedDecl();
+    if (!isAcceptableTagRedeclaration(PrevRecordDecl, Kind,
+                                      TUK == TUK_Definition,  KWLoc, Name)) {
+      Diag(KWLoc, diag::err_use_with_wrong_tag)
+        << Name
+        << FixItHint::CreateReplacement(KWLoc, PrevRecordDecl->getKindName());
+      Diag(PrevRecordDecl->getLocation(), diag::note_previous_use);
+      Kind = PrevRecordDecl->getTagKind();
+    }
+
+    // Check for redefinition of this class template.
+    if (TUK == TUK_Definition) {
+      if (TagDecl *Def = PrevRecordDecl->getDefinition()) {
+        // If we have a prior definition that is not visible, treat this as
+        // simply making that previous definition visible.
+        NamedDecl *Hidden = nullptr;
+        if (SkipBody && !hasVisibleDefinition(Def, &Hidden)) {
+          SkipBody->ShouldSkip = true;
+          SkipBody->Previous = Def;
+          auto *Tmpl = cast<CXXRecordDecl>(Hidden)->getDescribedClassTemplate();
+          assert(Tmpl && "original definition of a class template is not a "
+                         "class template?");
+          makeMergedDefinitionVisible(Hidden);
+          makeMergedDefinitionVisible(Tmpl);
+        } else {
+          Diag(NameLoc, diag::err_redefinition) << Name;
+          Diag(Def->getLocation(), diag::note_previous_definition);
+          // FIXME: Would it make sense to try to "forget" the previous
+          // definition, as part of error recovery?
+          return true;
+        }
+      }
+    }
+  } else if (PrevDecl) {
+    // C++ [temp]p5:
+    //   A class template shall not have the same name as any other
+    //   template, class, function, object, enumeration, enumerator,
+    //   namespace, or type in the same scope (3.3), except as specified
+    //   in (14.5.4).
+    Diag(NameLoc, diag::err_redefinition_different_kind) << Name;
+    Diag(PrevDecl->getLocation(), diag::note_previous_definition);
+    return true;
+  }
+
+  // Check the template parameter list of this declaration, possibly
+  // merging in the template parameter list from the previous class
+  // template declaration. Skip this check for a friend in a dependent
+  // context, because the template parameter list might be dependent.
+  if (!(TUK == TUK_Friend && CurContext->isDependentContext()) &&
+      CheckTemplateParameterList(
+          TemplateParams,
+          PrevClassTemplate
+              ? PrevClassTemplate->getMostRecentDecl()->getTemplateParameters()
+              : nullptr,
+          (SS.isSet() && SemanticContext && SemanticContext->isRecord() &&
+           SemanticContext->isDependentContext())
+              ? TPC_ClassTemplateMember
+              : TUK == TUK_Friend ? TPC_FriendClassTemplate : TPC_ClassTemplate,
+          SkipBody))
+    Invalid = true;
+
+  if (SS.isSet()) {
+    // If the name of the template was qualified, we must be defining the
+    // template out-of-line.
+    if (!SS.isInvalid() && !Invalid && !PrevClassTemplate) {
+      Diag(NameLoc, TUK == TUK_Friend ? diag::err_friend_decl_does_not_match
+                                      : diag::err_member_decl_does_not_match)
+        << Name << SemanticContext << /*IsDefinition*/true << SS.getRange();
+      Invalid = true;
+    }
+  }
+
+  // If this is a templated friend in a dependent context we should not put it
+  // on the redecl chain. In some cases, the templated friend can be the most
+  // recent declaration tricking the template instantiator to make substitutions
+  // there.
+  // FIXME: Figure out how to combine with shouldLinkDependentDeclWithPrevious
+  bool ShouldAddRedecl
+    = !(TUK == TUK_Friend && CurContext->isDependentContext());
+
+  CXXRecordDecl *NewClass =
+    CXXRecordDecl::Create(Context, Kind, SemanticContext, KWLoc, NameLoc, Name,
+                          PrevClassTemplate && ShouldAddRedecl ?
+                            PrevClassTemplate->getTemplatedDecl() : nullptr,
+                          /*DelayTypeCreation=*/true);
+  SetNestedNameSpecifier(*this, NewClass, SS);
+  if (NumOuterTemplateParamLists > 0)
+    NewClass->setTemplateParameterListsInfo(
+        Context, llvm::makeArrayRef(OuterTemplateParamLists,
+                                    NumOuterTemplateParamLists));
+
+  // Add alignment attributes if necessary; these attributes are checked when
+  // the ASTContext lays out the structure.
+  if (TUK == TUK_Definition && (!SkipBody || !SkipBody->ShouldSkip)) {
+    AddAlignmentAttributesForRecord(NewClass);
+    AddMsStructLayoutForRecord(NewClass);
+  }
+
+  ClassTemplateDecl *NewTemplate
+    = ClassTemplateDecl::Create(Context, SemanticContext, NameLoc,
+                                DeclarationName(Name), TemplateParams,
+                                NewClass);
+
+  if (ShouldAddRedecl)
+    NewTemplate->setPreviousDecl(PrevClassTemplate);
+
+  NewClass->setDescribedClassTemplate(NewTemplate);
+
+  if (ModulePrivateLoc.isValid())
+    NewTemplate->setModulePrivate();
+
+  // Build the type for the class template declaration now.
+  QualType T = NewTemplate->getInjectedClassNameSpecialization();
+  T = Context.getInjectedClassNameType(NewClass, T);
+  assert(T->isDependentType() && "Class template type is not dependent?");
+  (void)T;
+
+  // If we are providing an explicit specialization of a member that is a
+  // class template, make a note of that.
+  if (PrevClassTemplate &&
+      PrevClassTemplate->getInstantiatedFromMemberTemplate())
+    PrevClassTemplate->setMemberSpecialization();
+
+  // Set the access specifier.
+  if (!Invalid && TUK != TUK_Friend && NewTemplate->getDeclContext()->isRecord())
+    SetMemberAccessSpecifier(NewTemplate, PrevClassTemplate, AS);
+
+  // Set the lexical context of these templates
+  NewClass->setLexicalDeclContext(CurContext);
+  NewTemplate->setLexicalDeclContext(CurContext);
+
+  if (TUK == TUK_Definition && (!SkipBody || !SkipBody->ShouldSkip))
+    NewClass->startDefinition();
+
+  ProcessDeclAttributeList(S, NewClass, Attr);
+
+  if (PrevClassTemplate)
+    mergeDeclAttributes(NewClass, PrevClassTemplate->getTemplatedDecl());
+
+  AddPushedVisibilityAttribute(NewClass);
+  inferGslOwnerPointerAttribute(NewClass);
+
+  if (TUK != TUK_Friend) {
+    // Per C++ [basic.scope.temp]p2, skip the template parameter scopes.
+    Scope *Outer = S;
+    while ((Outer->getFlags() & Scope::TemplateParamScope) != 0)
+      Outer = Outer->getParent();
+    PushOnScopeChains(NewTemplate, Outer);
+  } else {
+    if (PrevClassTemplate && PrevClassTemplate->getAccess() != AS_none) {
+      NewTemplate->setAccess(PrevClassTemplate->getAccess());
+      NewClass->setAccess(PrevClassTemplate->getAccess());
+    }
+
+    NewTemplate->setObjectOfFriendDecl();
+
+    // Friend templates are visible in fairly strange ways.
+    if (!CurContext->isDependentContext()) {
+      DeclContext *DC = SemanticContext->getRedeclContext();
+      DC->makeDeclVisibleInContext(NewTemplate);
+      if (Scope *EnclosingScope = getScopeForDeclContext(S, DC))
+        PushOnScopeChains(NewTemplate, EnclosingScope,
+                          /* AddToContext = */ false);
+    }
+
+    FriendDecl *Friend = FriendDecl::Create(
+        Context, CurContext, NewClass->getLocation(), NewTemplate, FriendLoc);
+    Friend->setAccess(AS_public);
+    CurContext->addDecl(Friend);
+  }
+
+  if (PrevClassTemplate)
+    CheckRedeclarationInModule(NewTemplate, PrevClassTemplate);
+
+  if (Invalid) {
+    NewTemplate->setInvalidDecl();
+    NewClass->setInvalidDecl();
+  }
+
+  ActOnDocumentableDecl(NewTemplate);
+
+  if (SkipBody && SkipBody->ShouldSkip)
+    return SkipBody->Previous;
+
+  return NewTemplate;
+}
+
+namespace {
+/// Tree transform to "extract" a transformed type from a class template's
+/// constructor to a deduction guide.
+class ExtractTypeForDeductionGuide
+  : public TreeTransform<ExtractTypeForDeductionGuide> {
+  llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs;
+
+public:
+  typedef TreeTransform<ExtractTypeForDeductionGuide> Base;
+  ExtractTypeForDeductionGuide(
+      Sema &SemaRef,
+      llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs)
+      : Base(SemaRef), MaterializedTypedefs(MaterializedTypedefs) {}
+
+  TypeSourceInfo *transform(TypeSourceInfo *TSI) { return TransformType(TSI); }
+
+  QualType TransformTypedefType(TypeLocBuilder &TLB, TypedefTypeLoc TL) {
+    ASTContext &Context = SemaRef.getASTContext();
+    TypedefNameDecl *OrigDecl = TL.getTypedefNameDecl();
+    TypedefNameDecl *Decl = OrigDecl;
+    // Transform the underlying type of the typedef and clone the Decl only if
+    // the typedef has a dependent context.
+    if (OrigDecl->getDeclContext()->isDependentContext()) {
+      TypeLocBuilder InnerTLB;
+      QualType Transformed =
+          TransformType(InnerTLB, OrigDecl->getTypeSourceInfo()->getTypeLoc());
+      TypeSourceInfo *TSI = InnerTLB.getTypeSourceInfo(Context, Transformed);
+      if (isa<TypeAliasDecl>(OrigDecl))
+        Decl = TypeAliasDecl::Create(
+            Context, Context.getTranslationUnitDecl(), OrigDecl->getBeginLoc(),
+            OrigDecl->getLocation(), OrigDecl->getIdentifier(), TSI);
+      else {
+        assert(isa<TypedefDecl>(OrigDecl) && "Not a Type alias or typedef");
+        Decl = TypedefDecl::Create(
+            Context, Context.getTranslationUnitDecl(), OrigDecl->getBeginLoc(),
+            OrigDecl->getLocation(), OrigDecl->getIdentifier(), TSI);
+      }
+      MaterializedTypedefs.push_back(Decl);
+    }
+
+    QualType TDTy = Context.getTypedefType(Decl);
+    TypedefTypeLoc TypedefTL = TLB.push<TypedefTypeLoc>(TDTy);
+    TypedefTL.setNameLoc(TL.getNameLoc());
+
+    return TDTy;
+  }
+};
+
+/// Transform to convert portions of a constructor declaration into the
+/// corresponding deduction guide, per C++1z [over.match.class.deduct]p1.
+struct ConvertConstructorToDeductionGuideTransform {
+  ConvertConstructorToDeductionGuideTransform(Sema &S,
+                                              ClassTemplateDecl *Template)
+      : SemaRef(S), Template(Template) {}
+
+  Sema &SemaRef;
+  ClassTemplateDecl *Template;
+
+  DeclContext *DC = Template->getDeclContext();
+  CXXRecordDecl *Primary = Template->getTemplatedDecl();
+  DeclarationName DeductionGuideName =
+      SemaRef.Context.DeclarationNames.getCXXDeductionGuideName(Template);
+
+  QualType DeducedType = SemaRef.Context.getTypeDeclType(Primary);
+
+  // Index adjustment to apply to convert depth-1 template parameters into
+  // depth-0 template parameters.
+  unsigned Depth1IndexAdjustment = Template->getTemplateParameters()->size();
+
+  /// Transform a constructor declaration into a deduction guide.
+  NamedDecl *transformConstructor(FunctionTemplateDecl *FTD,
+                                  CXXConstructorDecl *CD) {
+    SmallVector<TemplateArgument, 16> SubstArgs;
+
+    LocalInstantiationScope Scope(SemaRef);
+
+    // C++ [over.match.class.deduct]p1:
+    // -- For each constructor of the class template designated by the
+    //    template-name, a function template with the following properties:
+
+    //    -- The template parameters are the template parameters of the class
+    //       template followed by the template parameters (including default
+    //       template arguments) of the constructor, if any.
+    TemplateParameterList *TemplateParams = Template->getTemplateParameters();
+    if (FTD) {
+      TemplateParameterList *InnerParams = FTD->getTemplateParameters();
+      SmallVector<NamedDecl *, 16> AllParams;
+      AllParams.reserve(TemplateParams->size() + InnerParams->size());
+      AllParams.insert(AllParams.begin(),
+                       TemplateParams->begin(), TemplateParams->end());
+      SubstArgs.reserve(InnerParams->size());
+
+      // Later template parameters could refer to earlier ones, so build up
+      // a list of substituted template arguments as we go.
+      for (NamedDecl *Param : *InnerParams) {
+        MultiLevelTemplateArgumentList Args;
+        Args.setKind(TemplateSubstitutionKind::Rewrite);
+        Args.addOuterTemplateArguments(SubstArgs);
+        Args.addOuterRetainedLevel();
+        NamedDecl *NewParam = transformTemplateParameter(Param, Args);
+        if (!NewParam)
+          return nullptr;
+        AllParams.push_back(NewParam);
+        SubstArgs.push_back(SemaRef.Context.getCanonicalTemplateArgument(
+            SemaRef.Context.getInjectedTemplateArg(NewParam)));
+      }
+      TemplateParams = TemplateParameterList::Create(
+          SemaRef.Context, InnerParams->getTemplateLoc(),
+          InnerParams->getLAngleLoc(), AllParams, InnerParams->getRAngleLoc(),
+          /*FIXME: RequiresClause*/ nullptr);
+    }
+
+    // If we built a new template-parameter-list, track that we need to
+    // substitute references to the old parameters into references to the
+    // new ones.
+    MultiLevelTemplateArgumentList Args;
+    Args.setKind(TemplateSubstitutionKind::Rewrite);
+    if (FTD) {
+      Args.addOuterTemplateArguments(SubstArgs);
+      Args.addOuterRetainedLevel();
+    }
+
+    FunctionProtoTypeLoc FPTL = CD->getTypeSourceInfo()->getTypeLoc()
+                                   .getAsAdjusted<FunctionProtoTypeLoc>();
+    assert(FPTL && "no prototype for constructor declaration");
+
+    // Transform the type of the function, adjusting the return type and
+    // replacing references to the old parameters with references to the
+    // new ones.
+    TypeLocBuilder TLB;
+    SmallVector<ParmVarDecl*, 8> Params;
+    SmallVector<TypedefNameDecl *, 4> MaterializedTypedefs;
+    QualType NewType = transformFunctionProtoType(TLB, FPTL, Params, Args,
+                                                  MaterializedTypedefs);
+    if (NewType.isNull())
+      return nullptr;
+    TypeSourceInfo *NewTInfo = TLB.getTypeSourceInfo(SemaRef.Context, NewType);
+
+    return buildDeductionGuide(TemplateParams, CD, CD->getExplicitSpecifier(),
+                               NewTInfo, CD->getBeginLoc(), CD->getLocation(),
+                               CD->getEndLoc(), MaterializedTypedefs);
+  }
+
+  /// Build a deduction guide with the specified parameter types.
+  NamedDecl *buildSimpleDeductionGuide(MutableArrayRef<QualType> ParamTypes) {
+    SourceLocation Loc = Template->getLocation();
+
+    // Build the requested type.
+    FunctionProtoType::ExtProtoInfo EPI;
+    EPI.HasTrailingReturn = true;
+    QualType Result = SemaRef.BuildFunctionType(DeducedType, ParamTypes, Loc,
+                                                DeductionGuideName, EPI);
+    TypeSourceInfo *TSI = SemaRef.Context.getTrivialTypeSourceInfo(Result, Loc);
+
+    FunctionProtoTypeLoc FPTL =
+        TSI->getTypeLoc().castAs<FunctionProtoTypeLoc>();
+
+    // Build the parameters, needed during deduction / substitution.
+    SmallVector<ParmVarDecl*, 4> Params;
+    for (auto T : ParamTypes) {
+      ParmVarDecl *NewParam = ParmVarDecl::Create(
+          SemaRef.Context, DC, Loc, Loc, nullptr, T,
+          SemaRef.Context.getTrivialTypeSourceInfo(T, Loc), SC_None, nullptr);
+      NewParam->setScopeInfo(0, Params.size());
+      FPTL.setParam(Params.size(), NewParam);
+      Params.push_back(NewParam);
+    }
+
+    return buildDeductionGuide(Template->getTemplateParameters(), nullptr,
+                               ExplicitSpecifier(), TSI, Loc, Loc, Loc);
+  }
+
+private:
+  /// Transform a constructor template parameter into a deduction guide template
+  /// parameter, rebuilding any internal references to earlier parameters and
+  /// renumbering as we go.
+  NamedDecl *transformTemplateParameter(NamedDecl *TemplateParam,
+                                        MultiLevelTemplateArgumentList &Args) {
+    if (auto *TTP = dyn_cast<TemplateTypeParmDecl>(TemplateParam)) {
+      // TemplateTypeParmDecl's index cannot be changed after creation, so
+      // substitute it directly.
+      auto *NewTTP = TemplateTypeParmDecl::Create(
+          SemaRef.Context, DC, TTP->getBeginLoc(), TTP->getLocation(),
+          /*Depth*/ 0, Depth1IndexAdjustment + TTP->getIndex(),
+          TTP->getIdentifier(), TTP->wasDeclaredWithTypename(),
+          TTP->isParameterPack(), TTP->hasTypeConstraint(),
+          TTP->isExpandedParameterPack() ?
+          llvm::Optional<unsigned>(TTP->getNumExpansionParameters()) : None);
+      if (const auto *TC = TTP->getTypeConstraint())
+        SemaRef.SubstTypeConstraint(NewTTP, TC, Args);
+      if (TTP->hasDefaultArgument()) {
+        TypeSourceInfo *InstantiatedDefaultArg =
+            SemaRef.SubstType(TTP->getDefaultArgumentInfo(), Args,
+                              TTP->getDefaultArgumentLoc(), TTP->getDeclName());
+        if (InstantiatedDefaultArg)
+          NewTTP->setDefaultArgument(InstantiatedDefaultArg);
+      }
+      SemaRef.CurrentInstantiationScope->InstantiatedLocal(TemplateParam,
+                                                           NewTTP);
+      return NewTTP;
+    }
+
+    if (auto *TTP = dyn_cast<TemplateTemplateParmDecl>(TemplateParam))
+      return transformTemplateParameterImpl(TTP, Args);
+
+    return transformTemplateParameterImpl(
+        cast<NonTypeTemplateParmDecl>(TemplateParam), Args);
+  }
+  template<typename TemplateParmDecl>
+  TemplateParmDecl *
+  transformTemplateParameterImpl(TemplateParmDecl *OldParam,
+                                 MultiLevelTemplateArgumentList &Args) {
+    // Ask the template instantiator to do the heavy lifting for us, then adjust
+    // the index of the parameter once it's done.
+    auto *NewParam =
+        cast<TemplateParmDecl>(SemaRef.SubstDecl(OldParam, DC, Args));
+    assert(NewParam->getDepth() == 0 && "unexpected template param depth");
+    NewParam->setPosition(NewParam->getPosition() + Depth1IndexAdjustment);
+    return NewParam;
+  }
+
+  QualType transformFunctionProtoType(
+      TypeLocBuilder &TLB, FunctionProtoTypeLoc TL,
+      SmallVectorImpl<ParmVarDecl *> &Params,
+      MultiLevelTemplateArgumentList &Args,
+      SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs) {
+    SmallVector<QualType, 4> ParamTypes;
+    const FunctionProtoType *T = TL.getTypePtr();
+
+    //    -- The types of the function parameters are those of the constructor.
+    for (auto *OldParam : TL.getParams()) {
+      ParmVarDecl *NewParam =
+          transformFunctionTypeParam(OldParam, Args, MaterializedTypedefs);
+      if (!NewParam)
+        return QualType();
+      ParamTypes.push_back(NewParam->getType());
+      Params.push_back(NewParam);
+    }
+
+    //    -- The return type is the class template specialization designated by
+    //       the template-name and template arguments corresponding to the
+    //       template parameters obtained from the class template.
+    //
+    // We use the injected-class-name type of the primary template instead.
+    // This has the convenient property that it is different from any type that
+    // the user can write in a deduction-guide (because they cannot enter the
+    // context of the template), so implicit deduction guides can never collide
+    // with explicit ones.
+    QualType ReturnType = DeducedType;
+    TLB.pushTypeSpec(ReturnType).setNameLoc(Primary->getLocation());
+
+    // Resolving a wording defect, we also inherit the variadicness of the
+    // constructor.
+    FunctionProtoType::ExtProtoInfo EPI;
+    EPI.Variadic = T->isVariadic();
+    EPI.HasTrailingReturn = true;
+
+    QualType Result = SemaRef.BuildFunctionType(
+        ReturnType, ParamTypes, TL.getBeginLoc(), DeductionGuideName, EPI);
+    if (Result.isNull())
+      return QualType();
+
+    FunctionProtoTypeLoc NewTL = TLB.push<FunctionProtoTypeLoc>(Result);
+    NewTL.setLocalRangeBegin(TL.getLocalRangeBegin());
+    NewTL.setLParenLoc(TL.getLParenLoc());
+    NewTL.setRParenLoc(TL.getRParenLoc());
+    NewTL.setExceptionSpecRange(SourceRange());
+    NewTL.setLocalRangeEnd(TL.getLocalRangeEnd());
+    for (unsigned I = 0, E = NewTL.getNumParams(); I != E; ++I)
+      NewTL.setParam(I, Params[I]);
+
+    return Result;
+  }
+
+  ParmVarDecl *transformFunctionTypeParam(
+      ParmVarDecl *OldParam, MultiLevelTemplateArgumentList &Args,
+      llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs) {
+    TypeSourceInfo *OldDI = OldParam->getTypeSourceInfo();
+    TypeSourceInfo *NewDI;
+    if (auto PackTL = OldDI->getTypeLoc().getAs<PackExpansionTypeLoc>()) {
+      // Expand out the one and only element in each inner pack.
+      Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, 0);
+      NewDI =
+          SemaRef.SubstType(PackTL.getPatternLoc(), Args,
+                            OldParam->getLocation(), OldParam->getDeclName());
+      if (!NewDI) return nullptr;
+      NewDI =
+          SemaRef.CheckPackExpansion(NewDI, PackTL.getEllipsisLoc(),
+                                     PackTL.getTypePtr()->getNumExpansions());
+    } else
+      NewDI = SemaRef.SubstType(OldDI, Args, OldParam->getLocation(),
+                                OldParam->getDeclName());
+    if (!NewDI)
+      return nullptr;
+
+    // Extract the type. This (for instance) replaces references to typedef
+    // members of the current instantiations with the definitions of those
+    // typedefs, avoiding triggering instantiation of the deduced type during
+    // deduction.
+    NewDI = ExtractTypeForDeductionGuide(SemaRef, MaterializedTypedefs)
+                .transform(NewDI);
+
+    // Resolving a wording defect, we also inherit default arguments from the
+    // constructor.
+    ExprResult NewDefArg;
+    if (OldParam->hasDefaultArg()) {
+      // We don't care what the value is (we won't use it); just create a
+      // placeholder to indicate there is a default argument.
+      QualType ParamTy = NewDI->getType();
+      NewDefArg = new (SemaRef.Context)
+          OpaqueValueExpr(OldParam->getDefaultArg()->getBeginLoc(),
+                          ParamTy.getNonLValueExprType(SemaRef.Context),
+                          ParamTy->isLValueReferenceType()   ? VK_LValue
+                          : ParamTy->isRValueReferenceType() ? VK_XValue
+                                                             : VK_PRValue);
+    }
+
+    ParmVarDecl *NewParam = ParmVarDecl::Create(SemaRef.Context, DC,
+                                                OldParam->getInnerLocStart(),
+                                                OldParam->getLocation(),
+                                                OldParam->getIdentifier(),
+                                                NewDI->getType(),
+                                                NewDI,
+                                                OldParam->getStorageClass(),
+                                                NewDefArg.get());
+    NewParam->setScopeInfo(OldParam->getFunctionScopeDepth(),
+                           OldParam->getFunctionScopeIndex());
+    SemaRef.CurrentInstantiationScope->InstantiatedLocal(OldParam, NewParam);
+    return NewParam;
+  }
+
+  FunctionTemplateDecl *buildDeductionGuide(
+      TemplateParameterList *TemplateParams, CXXConstructorDecl *Ctor,
+      ExplicitSpecifier ES, TypeSourceInfo *TInfo, SourceLocation LocStart,
+      SourceLocation Loc, SourceLocation LocEnd,
+      llvm::ArrayRef<TypedefNameDecl *> MaterializedTypedefs = {}) {
+    DeclarationNameInfo Name(DeductionGuideName, Loc);
+    ArrayRef<ParmVarDecl *> Params =
+        TInfo->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams();
+
+    // Build the implicit deduction guide template.
+    auto *Guide =
+        CXXDeductionGuideDecl::Create(SemaRef.Context, DC, LocStart, ES, Name,
+                                      TInfo->getType(), TInfo, LocEnd, Ctor);
+    Guide->setImplicit();
+    Guide->setParams(Params);
+
+    for (auto *Param : Params)
+      Param->setDeclContext(Guide);
+    for (auto *TD : MaterializedTypedefs)
+      TD->setDeclContext(Guide);
+
+    auto *GuideTemplate = FunctionTemplateDecl::Create(
+        SemaRef.Context, DC, Loc, DeductionGuideName, TemplateParams, Guide);
+    GuideTemplate->setImplicit();
+    Guide->setDescribedFunctionTemplate(GuideTemplate);
+
+    if (isa<CXXRecordDecl>(DC)) {
+      Guide->setAccess(AS_public);
+      GuideTemplate->setAccess(AS_public);
+    }
+
+    DC->addDecl(GuideTemplate);
+    return GuideTemplate;
+  }
+};
+}
+
+void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
+                                          SourceLocation Loc) {
+  if (CXXRecordDecl *DefRecord =
+          cast<CXXRecordDecl>(Template->getTemplatedDecl())->getDefinition()) {
+    TemplateDecl *DescribedTemplate = DefRecord->getDescribedClassTemplate();
+    Template = DescribedTemplate ? DescribedTemplate : Template;
+  }
+
+  DeclContext *DC = Template->getDeclContext();
+  if (DC->isDependentContext())
+    return;
+
+  ConvertConstructorToDeductionGuideTransform Transform(
+      *this, cast<ClassTemplateDecl>(Template));
+  if (!isCompleteType(Loc, Transform.DeducedType))
+    return;
+
+  // Check whether we've already declared deduction guides for this template.
+  // FIXME: Consider storing a flag on the template to indicate this.
+  auto Existing = DC->lookup(Transform.DeductionGuideName);
+  for (auto *D : Existing)
+    if (D->isImplicit())
+      return;
+
+  // In case we were expanding a pack when we attempted to declare deduction
+  // guides, turn off pack expansion for everything we're about to do.
+  ArgumentPackSubstitutionIndexRAII SubstIndex(*this, -1);
+  // Create a template instantiation record to track the "instantiation" of
+  // constructors into deduction guides.
+  // FIXME: Add a kind for this to give more meaningful diagnostics. But can
+  // this substitution process actually fail?
+  InstantiatingTemplate BuildingDeductionGuides(*this, Loc, Template);
+  if (BuildingDeductionGuides.isInvalid())
+    return;
+
+  // Convert declared constructors into deduction guide templates.
+  // FIXME: Skip constructors for which deduction must necessarily fail (those
+  // for which some class template parameter without a default argument never
+  // appears in a deduced context).
+  bool AddedAny = false;
+  for (NamedDecl *D : LookupConstructors(Transform.Primary)) {
+    D = D->getUnderlyingDecl();
+    if (D->isInvalidDecl() || D->isImplicit())
+      continue;
+    D = cast<NamedDecl>(D->getCanonicalDecl());
+
+    auto *FTD = dyn_cast<FunctionTemplateDecl>(D);
+    auto *CD =
+        dyn_cast_or_null<CXXConstructorDecl>(FTD ? FTD->getTemplatedDecl() : D);
+    // Class-scope explicit specializations (MS extension) do not result in
+    // deduction guides.
+    if (!CD || (!FTD && CD->isFunctionTemplateSpecialization()))
+      continue;
+
+    // Cannot make a deduction guide when unparsed arguments are present.
+    if (std::any_of(CD->param_begin(), CD->param_end(), [](ParmVarDecl *P) {
+          return !P || P->hasUnparsedDefaultArg();
+        }))
+      continue;
+
+    Transform.transformConstructor(FTD, CD);
+    AddedAny = true;
+  }
+
+  // C++17 [over.match.class.deduct]
+  //    --  If C is not defined or does not declare any constructors, an
+  //    additional function template derived as above from a hypothetical
+  //    constructor C().
+  if (!AddedAny)
+    Transform.buildSimpleDeductionGuide(None);
+
+  //    -- An additional function template derived as above from a hypothetical
+  //    constructor C(C), called the copy deduction candidate.
+  cast<CXXDeductionGuideDecl>(
+      cast<FunctionTemplateDecl>(
+          Transform.buildSimpleDeductionGuide(Transform.DeducedType))
+          ->getTemplatedDecl())
+      ->setIsCopyDeductionCandidate();
+}
+
+/// Diagnose the presence of a default template argument on a
+/// template parameter, which is ill-formed in certain contexts.
+///
+/// \returns true if the default template argument should be dropped.
+static bool DiagnoseDefaultTemplateArgument(Sema &S,
+                                            Sema::TemplateParamListContext TPC,
+                                            SourceLocation ParamLoc,
+                                            SourceRange DefArgRange) {
+  switch (TPC) {
+  case Sema::TPC_ClassTemplate:
+  case Sema::TPC_VarTemplate:
+  case Sema::TPC_TypeAliasTemplate:
+    return false;
+
+  case Sema::TPC_FunctionTemplate:
+  case Sema::TPC_FriendFunctionTemplateDefinition:
+    // C++ [temp.param]p9:
+    //   A default template-argument shall not be specified in a
+    //   function template declaration or a function template
+    //   definition [...]
+    //   If a friend function template declaration specifies a default
+    //   template-argument, that declaration shall be a definition and shall be
+    //   the only declaration of the function template in the translation unit.
+    // (C++98/03 doesn't have this wording; see DR226).
+    S.Diag(ParamLoc, S.getLangOpts().CPlusPlus11 ?
+         diag::warn_cxx98_compat_template_parameter_default_in_function_template
+           : diag::ext_template_parameter_default_in_function_template)
+      << DefArgRange;
+    return false;
+
+  case Sema::TPC_ClassTemplateMember:
+    // C++0x [temp.param]p9:
+    //   A default template-argument shall not be specified in the
+    //   template-parameter-lists of the definition of a member of a
+    //   class template that appears outside of the member's class.
+    S.Diag(ParamLoc, diag::err_template_parameter_default_template_member)
+      << DefArgRange;
+    return true;
+
+  case Sema::TPC_FriendClassTemplate:
+  case Sema::TPC_FriendFunctionTemplate:
+    // C++ [temp.param]p9:
+    //   A default template-argument shall not be specified in a
+    //   friend template declaration.
+    S.Diag(ParamLoc, diag::err_template_parameter_default_friend_template)
+      << DefArgRange;
+    return true;
+
+    // FIXME: C++0x [temp.param]p9 allows default template-arguments
+    // for friend function templates if there is only a single
+    // declaration (and it is a definition). Strange!
+  }
+
+  llvm_unreachable("Invalid TemplateParamListContext!");
+}
+
+/// Check for unexpanded parameter packs within the template parameters
+/// of a template template parameter, recursively.
+static bool DiagnoseUnexpandedParameterPacks(Sema &S,
+                                             TemplateTemplateParmDecl *TTP) {
+  // A template template parameter which is a parameter pack is also a pack
+  // expansion.
+  if (TTP->isParameterPack())
+    return false;
+
+  TemplateParameterList *Params = TTP->getTemplateParameters();
+  for (unsigned I = 0, N = Params->size(); I != N; ++I) {
+    NamedDecl *P = Params->getParam(I);
+    if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(P)) {
+      if (!TTP->isParameterPack())
+        if (const TypeConstraint *TC = TTP->getTypeConstraint())
+          if (TC->hasExplicitTemplateArgs())
+            for (auto &ArgLoc : TC->getTemplateArgsAsWritten()->arguments())
+              if (S.DiagnoseUnexpandedParameterPack(ArgLoc,
+                                                    Sema::UPPC_TypeConstraint))
+                return true;
+      continue;
+    }
+
+    if (NonTypeTemplateParmDecl *NTTP = dyn_cast<NonTypeTemplateParmDecl>(P)) {
+      if (!NTTP->isParameterPack() &&
+          S.DiagnoseUnexpandedParameterPack(NTTP->getLocation(),
+                                            NTTP->getTypeSourceInfo(),
+                                      Sema::UPPC_NonTypeTemplateParameterType))
+        return true;
+
+      continue;
+    }
+
+    if (TemplateTemplateParmDecl *InnerTTP
+                                        = dyn_cast<TemplateTemplateParmDecl>(P))
+      if (DiagnoseUnexpandedParameterPacks(S, InnerTTP))
+        return true;
+  }
+
+  return false;
+}
+
+/// Checks the validity of a template parameter list, possibly
+/// considering the template parameter list from a previous
+/// declaration.
+///
+/// If an "old" template parameter list is provided, it must be
+/// equivalent (per TemplateParameterListsAreEqual) to the "new"
+/// template parameter list.
+///
+/// \param NewParams Template parameter list for a new template
+/// declaration. This template parameter list will be updated with any
+/// default arguments that are carried through from the previous
+/// template parameter list.
+///
+/// \param OldParams If provided, template parameter list from a
+/// previous declaration of the same template. Default template
+/// arguments will be merged from the old template parameter list to
+/// the new template parameter list.
+///
+/// \param TPC Describes the context in which we are checking the given
+/// template parameter list.
+///
+/// \param SkipBody If we might have already made a prior merged definition
+/// of this template visible, the corresponding body-skipping information.
+/// Default argument redefinition is not an error when skipping such a body,
+/// because (under the ODR) we can assume the default arguments are the same
+/// as the prior merged definition.
+///
+/// \returns true if an error occurred, false otherwise.
+bool Sema::CheckTemplateParameterList(TemplateParameterList *NewParams,
+                                      TemplateParameterList *OldParams,
+                                      TemplateParamListContext TPC,
+                                      SkipBodyInfo *SkipBody) {
+  bool Invalid = false;
+
+  // C++ [temp.param]p10:
+  //   The set of default template-arguments available for use with a
+  //   template declaration or definition is obtained by merging the
+  //   default arguments from the definition (if in scope) and all
+  //   declarations in scope in the same way default function
+  //   arguments are (8.3.6).
+  bool SawDefaultArgument = false;
+  SourceLocation PreviousDefaultArgLoc;
+
+  // Dummy initialization to avoid warnings.
+  TemplateParameterList::iterator OldParam = NewParams->end();
+  if (OldParams)
+    OldParam = OldParams->begin();
+
+  bool RemoveDefaultArguments = false;
+  for (TemplateParameterList::iterator NewParam = NewParams->begin(),
+                                    NewParamEnd = NewParams->end();
+       NewParam != NewParamEnd; ++NewParam) {
+    // Variables used to diagnose redundant default arguments
+    bool RedundantDefaultArg = false;
+    SourceLocation OldDefaultLoc;
+    SourceLocation NewDefaultLoc;
+
+    // Variable used to diagnose missing default arguments
+    bool MissingDefaultArg = false;
+
+    // Variable used to diagnose non-final parameter packs
+    bool SawParameterPack = false;
+
+    if (TemplateTypeParmDecl *NewTypeParm
+          = dyn_cast<TemplateTypeParmDecl>(*NewParam)) {
+      // Check the presence of a default argument here.
+      if (NewTypeParm->hasDefaultArgument() &&
+          DiagnoseDefaultTemplateArgument(*this, TPC,
+                                          NewTypeParm->getLocation(),
+               NewTypeParm->getDefaultArgumentInfo()->getTypeLoc()
+                                                       .getSourceRange()))
+        NewTypeParm->removeDefaultArgument();
+
+      // Merge default arguments for template type parameters.
+      TemplateTypeParmDecl *OldTypeParm
+          = OldParams? cast<TemplateTypeParmDecl>(*OldParam) : nullptr;
+      if (NewTypeParm->isParameterPack()) {
+        assert(!NewTypeParm->hasDefaultArgument() &&
+               "Parameter packs can't have a default argument!");
+        SawParameterPack = true;
+      } else if (OldTypeParm && hasVisibleDefaultArgument(OldTypeParm) &&
+                 NewTypeParm->hasDefaultArgument() &&
+                 (!SkipBody || !SkipBody->ShouldSkip)) {
+        OldDefaultLoc = OldTypeParm->getDefaultArgumentLoc();
+        NewDefaultLoc = NewTypeParm->getDefaultArgumentLoc();
+        SawDefaultArgument = true;
+        RedundantDefaultArg = true;
+        PreviousDefaultArgLoc = NewDefaultLoc;
+      } else if (OldTypeParm && OldTypeParm->hasDefaultArgument()) {
+        // Merge the default argument from the old declaration to the
+        // new declaration.
+        NewTypeParm->setInheritedDefaultArgument(Context, OldTypeParm);
+        PreviousDefaultArgLoc = OldTypeParm->getDefaultArgumentLoc();
+      } else if (NewTypeParm->hasDefaultArgument()) {
+        SawDefaultArgument = true;
+        PreviousDefaultArgLoc = NewTypeParm->getDefaultArgumentLoc();
+      } else if (SawDefaultArgument)
+        MissingDefaultArg = true;
+    } else if (NonTypeTemplateParmDecl *NewNonTypeParm
+               = dyn_cast<NonTypeTemplateParmDecl>(*NewParam)) {
+      // Check for unexpanded parameter packs.
+      if (!NewNonTypeParm->isParameterPack() &&
+          DiagnoseUnexpandedParameterPack(NewNonTypeParm->getLocation(),
+                                          NewNonTypeParm->getTypeSourceInfo(),
+                                          UPPC_NonTypeTemplateParameterType)) {
+        Invalid = true;
+        continue;
+      }
+
+      // Check the presence of a default argument here.
+      if (NewNonTypeParm->hasDefaultArgument() &&
+          DiagnoseDefaultTemplateArgument(*this, TPC,
+                                          NewNonTypeParm->getLocation(),
+                    NewNonTypeParm->getDefaultArgument()->getSourceRange())) {
+        NewNonTypeParm->removeDefaultArgument();
+      }
+
+      // Merge default arguments for non-type template parameters
+      NonTypeTemplateParmDecl *OldNonTypeParm
+        = OldParams? cast<NonTypeTemplateParmDecl>(*OldParam) : nullptr;
+      if (NewNonTypeParm->isParameterPack()) {
+        assert(!NewNonTypeParm->hasDefaultArgument() &&
+               "Parameter packs can't have a default argument!");
+        if (!NewNonTypeParm->isPackExpansion())
+          SawParameterPack = true;
+      } else if (OldNonTypeParm && hasVisibleDefaultArgument(OldNonTypeParm) &&
+                 NewNonTypeParm->hasDefaultArgument() &&
+                 (!SkipBody || !SkipBody->ShouldSkip)) {
+        OldDefaultLoc = OldNonTypeParm->getDefaultArgumentLoc();
+        NewDefaultLoc = NewNonTypeParm->getDefaultArgumentLoc();
+        SawDefaultArgument = true;
+        RedundantDefaultArg = true;
+        PreviousDefaultArgLoc = NewDefaultLoc;
+      } else if (OldNonTypeParm && OldNonTypeParm->hasDefaultArgument()) {
+        // Merge the default argument from the old declaration to the
+        // new declaration.
+        NewNonTypeParm->setInheritedDefaultArgument(Context, OldNonTypeParm);
+        PreviousDefaultArgLoc = OldNonTypeParm->getDefaultArgumentLoc();
+      } else if (NewNonTypeParm->hasDefaultArgument()) {
+        SawDefaultArgument = true;
+        PreviousDefaultArgLoc = NewNonTypeParm->getDefaultArgumentLoc();
+      } else if (SawDefaultArgument)
+        MissingDefaultArg = true;
+    } else {
+      TemplateTemplateParmDecl *NewTemplateParm
+        = cast<TemplateTemplateParmDecl>(*NewParam);
+
+      // Check for unexpanded parameter packs, recursively.
+      if (::DiagnoseUnexpandedParameterPacks(*this, NewTemplateParm)) {
+        Invalid = true;
+        continue;
+      }
+
+      // Check the presence of a default argument here.
+      if (NewTemplateParm->hasDefaultArgument() &&
+          DiagnoseDefaultTemplateArgument(*this, TPC,
+                                          NewTemplateParm->getLocation(),
+                     NewTemplateParm->getDefaultArgument().getSourceRange()))
+        NewTemplateParm->removeDefaultArgument();
+
+      // Merge default arguments for template template parameters
+      TemplateTemplateParmDecl *OldTemplateParm
+        = OldParams? cast<TemplateTemplateParmDecl>(*OldParam) : nullptr;
+      if (NewTemplateParm->isParameterPack()) {
+        assert(!NewTemplateParm->hasDefaultArgument() &&
+               "Parameter packs can't have a default argument!");
+        if (!NewTemplateParm->isPackExpansion())
+          SawParameterPack = true;
+      } else if (OldTemplateParm &&
+                 hasVisibleDefaultArgument(OldTemplateParm) &&
+                 NewTemplateParm->hasDefaultArgument() &&
+                 (!SkipBody || !SkipBody->ShouldSkip)) {
+        OldDefaultLoc = OldTemplateParm->getDefaultArgument().getLocation();
+        NewDefaultLoc = NewTemplateParm->getDefaultArgument().getLocation();
+        SawDefaultArgument = true;
+        RedundantDefaultArg = true;
+        PreviousDefaultArgLoc = NewDefaultLoc;
+      } else if (OldTemplateParm && OldTemplateParm->hasDefaultArgument()) {
+        // Merge the default argument from the old declaration to the
+        // new declaration.
+        NewTemplateParm->setInheritedDefaultArgument(Context, OldTemplateParm);
+        PreviousDefaultArgLoc
+          = OldTemplateParm->getDefaultArgument().getLocation();
+      } else if (NewTemplateParm->hasDefaultArgument()) {
+        SawDefaultArgument = true;
+        PreviousDefaultArgLoc
+          = NewTemplateParm->getDefaultArgument().getLocation();
+      } else if (SawDefaultArgument)
+        MissingDefaultArg = true;
+    }
+
+    // C++11 [temp.param]p11:
+    //   If a template parameter of a primary class template or alias template
+    //   is a template parameter pack, it shall be the last template parameter.
+    if (SawParameterPack && (NewParam + 1) != NewParamEnd &&
+        (TPC == TPC_ClassTemplate || TPC == TPC_VarTemplate ||
+         TPC == TPC_TypeAliasTemplate)) {
+      Diag((*NewParam)->getLocation(),
+           diag::err_template_param_pack_must_be_last_template_parameter);
+      Invalid = true;
+    }
+
+    if (RedundantDefaultArg) {
+      // C++ [temp.param]p12:
+      //   A template-parameter shall not be given default arguments
+      //   by two different declarations in the same scope.
+      Diag(NewDefaultLoc, diag::err_template_param_default_arg_redefinition);
+      Diag(OldDefaultLoc, diag::note_template_param_prev_default_arg);
+      Invalid = true;
+    } else if (MissingDefaultArg && TPC != TPC_FunctionTemplate) {
+      // C++ [temp.param]p11:
+      //   If a template-parameter of a class template has a default
+      //   template-argument, each subsequent template-parameter shall either
+      //   have a default template-argument supplied or be a template parameter
+      //   pack.
+      Diag((*NewParam)->getLocation(),
+           diag::err_template_param_default_arg_missing);
+      Diag(PreviousDefaultArgLoc, diag::note_template_param_prev_default_arg);
+      Invalid = true;
+      RemoveDefaultArguments = true;
+    }
+
+    // If we have an old template parameter list that we're merging
+    // in, move on to the next parameter.
+    if (OldParams)
+      ++OldParam;
+  }
+
+  // We were missing some default arguments at the end of the list, so remove
+  // all of the default arguments.
+  if (RemoveDefaultArguments) {
+    for (TemplateParameterList::iterator NewParam = NewParams->begin(),
+                                      NewParamEnd = NewParams->end();
+         NewParam != NewParamEnd; ++NewParam) {
+      if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(*NewParam))
+        TTP->removeDefaultArgument();
+      else if (NonTypeTemplateParmDecl *NTTP
+                                = dyn_cast<NonTypeTemplateParmDecl>(*NewParam))
+        NTTP->removeDefaultArgument();
+      else
+        cast<TemplateTemplateParmDecl>(*NewParam)->removeDefaultArgument();
+    }
+  }
+
+  return Invalid;
+}
+
+namespace {
+
+/// A class which looks for a use of a certain level of template
+/// parameter.
+struct DependencyChecker : RecursiveASTVisitor<DependencyChecker> {
+  typedef RecursiveASTVisitor<DependencyChecker> super;
+
+  unsigned Depth;
+
+  // Whether we're looking for a use of a template parameter that makes the
+  // overall construct type-dependent / a dependent type. This is strictly
+  // best-effort for now; we may fail to match at all for a dependent type
+  // in some cases if this is set.
+  bool IgnoreNonTypeDependent;
+
+  bool Match;
+  SourceLocation MatchLoc;
+
+  DependencyChecker(unsigned Depth, bool IgnoreNonTypeDependent)
+      : Depth(Depth), IgnoreNonTypeDependent(IgnoreNonTypeDependent),
+        Match(false) {}
+
+  DependencyChecker(TemplateParameterList *Params, bool IgnoreNonTypeDependent)
+      : IgnoreNonTypeDependent(IgnoreNonTypeDependent), Match(false) {
+    NamedDecl *ND = Params->getParam(0);
+    if (TemplateTypeParmDecl *PD = dyn_cast<TemplateTypeParmDecl>(ND)) {
+      Depth = PD->getDepth();
+    } else if (NonTypeTemplateParmDecl *PD =
+                 dyn_cast<NonTypeTemplateParmDecl>(ND)) {
+      Depth = PD->getDepth();
+    } else {
+      Depth = cast<TemplateTemplateParmDecl>(ND)->getDepth();
+    }
+  }
+
+  bool Matches(unsigned ParmDepth, SourceLocation Loc = SourceLocation()) {
+    if (ParmDepth >= Depth) {
+      Match = true;
+      MatchLoc = Loc;
+      return true;
+    }
+    return false;
+  }
+
+  bool TraverseStmt(Stmt *S, DataRecursionQueue *Q = nullptr) {
+    // Prune out non-type-dependent expressions if requested. This can
+    // sometimes result in us failing to find a template parameter reference
+    // (if a value-dependent expression creates a dependent type), but this
+    // mode is best-effort only.
+    if (auto *E = dyn_cast_or_null<Expr>(S))
+      if (IgnoreNonTypeDependent && !E->isTypeDependent())
+        return true;
+    return super::TraverseStmt(S, Q);
+  }
+
+  bool TraverseTypeLoc(TypeLoc TL) {
+    if (IgnoreNonTypeDependent && !TL.isNull() &&
+        !TL.getType()->isDependentType())
+      return true;
+    return super::TraverseTypeLoc(TL);
+  }
+
+  bool VisitTemplateTypeParmTypeLoc(TemplateTypeParmTypeLoc TL) {
+    return !Matches(TL.getTypePtr()->getDepth(), TL.getNameLoc());
+  }
+
+  bool VisitTemplateTypeParmType(const TemplateTypeParmType *T) {
+    // For a best-effort search, keep looking until we find a location.
+    return IgnoreNonTypeDependent || !Matches(T->getDepth());
+  }
+
+  bool TraverseTemplateName(TemplateName N) {
+    if (TemplateTemplateParmDecl *PD =
+          dyn_cast_or_null<TemplateTemplateParmDecl>(N.getAsTemplateDecl()))
+      if (Matches(PD->getDepth()))
+        return false;
+    return super::TraverseTemplateName(N);
+  }
+
+  bool VisitDeclRefExpr(DeclRefExpr *E) {
+    if (NonTypeTemplateParmDecl *PD =
+          dyn_cast<NonTypeTemplateParmDecl>(E->getDecl()))
+      if (Matches(PD->getDepth(), E->getExprLoc()))
+        return false;
+    return super::VisitDeclRefExpr(E);
+  }
+
+  bool VisitSubstTemplateTypeParmType(const SubstTemplateTypeParmType *T) {
+    return TraverseType(T->getReplacementType());
+  }
+
+  bool
+  VisitSubstTemplateTypeParmPackType(const SubstTemplateTypeParmPackType *T) {
+    return TraverseTemplateArgument(T->getArgumentPack());
+  }
+
+  bool TraverseInjectedClassNameType(const InjectedClassNameType *T) {
+    return TraverseType(T->getInjectedSpecializationType());
+  }
+};
+} // end anonymous namespace
+
+/// Determines whether a given type depends on the given parameter
+/// list.
+static bool
+DependsOnTemplateParameters(QualType T, TemplateParameterList *Params) {
+  if (!Params->size())
+    return false;
+
+  DependencyChecker Checker(Params, /*IgnoreNonTypeDependent*/false);
+  Checker.TraverseType(T);
+  return Checker.Match;
+}
+
+// Find the source range corresponding to the named type in the given
+// nested-name-specifier, if any.
+static SourceRange getRangeOfTypeInNestedNameSpecifier(ASTContext &Context,
+                                                       QualType T,
+                                                       const CXXScopeSpec &SS) {
+  NestedNameSpecifierLoc NNSLoc(SS.getScopeRep(), SS.location_data());
+  while (NestedNameSpecifier *NNS = NNSLoc.getNestedNameSpecifier()) {
+    if (const Type *CurType = NNS->getAsType()) {
+      if (Context.hasSameUnqualifiedType(T, QualType(CurType, 0)))
+        return NNSLoc.getTypeLoc().getSourceRange();
+    } else
+      break;
+
+    NNSLoc = NNSLoc.getPrefix();
+  }
+
+  return SourceRange();
+}
+
+/// Match the given template parameter lists to the given scope
+/// specifier, returning the template parameter list that applies to the
+/// name.
+///
+/// \param DeclStartLoc the start of the declaration that has a scope
+/// specifier or a template parameter list.
+///
+/// \param DeclLoc The location of the declaration itself.
+///
+/// \param SS the scope specifier that will be matched to the given template
+/// parameter lists. This scope specifier precedes a qualified name that is
+/// being declared.
+///
+/// \param TemplateId The template-id following the scope specifier, if there
+/// is one. Used to check for a missing 'template<>'.
+///
+/// \param ParamLists the template parameter lists, from the outermost to the
+/// innermost template parameter lists.
+///
+/// \param IsFriend Whether to apply the slightly different rules for
+/// matching template parameters to scope specifiers in friend
+/// declarations.
+///
+/// \param IsMemberSpecialization will be set true if the scope specifier
+/// denotes a fully-specialized type, and therefore this is a declaration of
+/// a member specialization.
+///
+/// \returns the template parameter list, if any, that corresponds to the
+/// name that is preceded by the scope specifier @p SS. This template
+/// parameter list may have template parameters (if we're declaring a
+/// template) or may have no template parameters (if we're declaring a
+/// template specialization), or may be NULL (if what we're declaring isn't
+/// itself a template).
+TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
+    SourceLocation DeclStartLoc, SourceLocation DeclLoc, const CXXScopeSpec &SS,
+    TemplateIdAnnotation *TemplateId,
+    ArrayRef<TemplateParameterList *> ParamLists, bool IsFriend,
+    bool &IsMemberSpecialization, bool &Invalid, bool SuppressDiagnostic) {
+  IsMemberSpecialization = false;
+  Invalid = false;
+
+  // The sequence of nested types to which we will match up the template
+  // parameter lists. We first build this list by starting with the type named
+  // by the nested-name-specifier and walking out until we run out of types.
+  SmallVector<QualType, 4> NestedTypes;
+  QualType T;
+  if (SS.getScopeRep()) {
+    if (CXXRecordDecl *Record
+              = dyn_cast_or_null<CXXRecordDecl>(computeDeclContext(SS, true)))
+      T = Context.getTypeDeclType(Record);
+    else
+      T = QualType(SS.getScopeRep()->getAsType(), 0);
+  }
+
+  // If we found an explicit specialization that prevents us from needing
+  // 'template<>' headers, this will be set to the location of that
+  // explicit specialization.
+  SourceLocation ExplicitSpecLoc;
+
+  while (!T.isNull()) {
+    NestedTypes.push_back(T);
+
+    // Retrieve the parent of a record type.
+    if (CXXRecordDecl *Record = T->getAsCXXRecordDecl()) {
+      // If this type is an explicit specialization, we're done.
+      if (ClassTemplateSpecializationDecl *Spec
+          = dyn_cast<ClassTemplateSpecializationDecl>(Record)) {
+        if (!isa<ClassTemplatePartialSpecializationDecl>(Spec) &&
+            Spec->getSpecializationKind() == TSK_ExplicitSpecialization) {
+          ExplicitSpecLoc = Spec->getLocation();
+          break;
+        }
+      } else if (Record->getTemplateSpecializationKind()
+                                                == TSK_ExplicitSpecialization) {
+        ExplicitSpecLoc = Record->getLocation();
+        break;
+      }
+
+      if (TypeDecl *Parent = dyn_cast<TypeDecl>(Record->getParent()))
+        T = Context.getTypeDeclType(Parent);
+      else
+        T = QualType();
+      continue;
+    }
+
+    if (const TemplateSpecializationType *TST
+                                     = T->getAs<TemplateSpecializationType>()) {
+      if (TemplateDecl *Template = TST->getTemplateName().getAsTemplateDecl()) {
+        if (TypeDecl *Parent = dyn_cast<TypeDecl>(Template->getDeclContext()))
+          T = Context.getTypeDeclType(Parent);
+        else
+          T = QualType();
+        continue;
+      }
+    }
+
+    // Look one step prior in a dependent template specialization type.
+    if (const DependentTemplateSpecializationType *DependentTST
+                          = T->getAs<DependentTemplateSpecializationType>()) {
+      if (NestedNameSpecifier *NNS = DependentTST->getQualifier())
+        T = QualType(NNS->getAsType(), 0);
+      else
+        T = QualType();
+      continue;
+    }
+
+    // Look one step prior in a dependent name type.
+    if (const DependentNameType *DependentName = T->getAs<DependentNameType>()){
+      if (NestedNameSpecifier *NNS = DependentName->getQualifier())
+        T = QualType(NNS->getAsType(), 0);
+      else
+        T = QualType();
+      continue;
+    }
+
+    // Retrieve the parent of an enumeration type.
+    if (const EnumType *EnumT = T->getAs<EnumType>()) {
+      // FIXME: Forward-declared enums require a TSK_ExplicitSpecialization
+      // check here.
+      EnumDecl *Enum = EnumT->getDecl();
+
+      // Get to the parent type.
+      if (TypeDecl *Parent = dyn_cast<TypeDecl>(Enum->getParent()))
+        T = Context.getTypeDeclType(Parent);
+      else
+        T = QualType();
+      continue;
+    }
+
+    T = QualType();
+  }
+  // Reverse the nested types list, since we want to traverse from the outermost
+  // to the innermost while checking template-parameter-lists.
+  std::reverse(NestedTypes.begin(), NestedTypes.end());
+
+  // C++0x [temp.expl.spec]p17:
+  //   A member or a member template may be nested within many
+  //   enclosing class templates. In an explicit specialization for
+  //   such a member, the member declaration shall be preceded by a
+  //   template<> for each enclosing class template that is
+  //   explicitly specialized.
+  bool SawNonEmptyTemplateParameterList = false;
+
+  auto CheckExplicitSpecialization = [&](SourceRange Range, bool Recovery) {
+    if (SawNonEmptyTemplateParameterList) {
+      if (!SuppressDiagnostic)
+        Diag(DeclLoc, diag::err_specialize_member_of_template)
+          << !Recovery << Range;
+      Invalid = true;
+      IsMemberSpecialization = false;
+      return true;
+    }
+
+    return false;
+  };
+
+  auto DiagnoseMissingExplicitSpecialization = [&] (SourceRange Range) {
+    // Check that we can have an explicit specialization here.
+    if (CheckExplicitSpecialization(Range, true))
+      return true;
+
+    // We don't have a template header, but we should.
+    SourceLocation ExpectedTemplateLoc;
+    if (!ParamLists.empty())
+      ExpectedTemplateLoc = ParamLists[0]->getTemplateLoc();
+    else
+      ExpectedTemplateLoc = DeclStartLoc;
+
+    if (!SuppressDiagnostic)
+      Diag(DeclLoc, diag::err_template_spec_needs_header)
+        << Range
+        << FixItHint::CreateInsertion(ExpectedTemplateLoc, "template<> ");
+    return false;
+  };
+
+  unsigned ParamIdx = 0;
+  for (unsigned TypeIdx = 0, NumTypes = NestedTypes.size(); TypeIdx != NumTypes;
+       ++TypeIdx) {
+    T = NestedTypes[TypeIdx];
+
+    // Whether we expect a 'template<>' header.
+    bool NeedEmptyTemplateHeader = false;
+
+    // Whether we expect a template header with parameters.
+    bool NeedNonemptyTemplateHeader = false;
+
+    // For a dependent type, the set of template parameters that we
+    // expect to see.
+    TemplateParameterList *ExpectedTemplateParams = nullptr;
+
+    // C++0x [temp.expl.spec]p15:
+    //   A member or a member template may be nested within many enclosing
+    //   class templates. In an explicit specialization for such a member, the
+    //   member declaration shall be preceded by a template<> for each
+    //   enclosing class template that is explicitly specialized.
+    if (CXXRecordDecl *Record = T->getAsCXXRecordDecl()) {
+      if (ClassTemplatePartialSpecializationDecl *Partial
+            = dyn_cast<ClassTemplatePartialSpecializationDecl>(Record)) {
+        ExpectedTemplateParams = Partial->getTemplateParameters();
+        NeedNonemptyTemplateHeader = true;
+      } else if (Record->isDependentType()) {
+        if (Record->getDescribedClassTemplate()) {
+          ExpectedTemplateParams = Record->getDescribedClassTemplate()
+                                                      ->getTemplateParameters();
+          NeedNonemptyTemplateHeader = true;
+        }
+      } else if (ClassTemplateSpecializationDecl *Spec
+                     = dyn_cast<ClassTemplateSpecializationDecl>(Record)) {
+        // C++0x [temp.expl.spec]p4:
+        //   Members of an explicitly specialized class template are defined
+        //   in the same manner as members of normal classes, and not using
+        //   the template<> syntax.
+        if (Spec->getSpecializationKind() != TSK_ExplicitSpecialization)
+          NeedEmptyTemplateHeader = true;
+        else
+          continue;
+      } else if (Record->getTemplateSpecializationKind()) {
+        if (Record->getTemplateSpecializationKind()
+                                                != TSK_ExplicitSpecialization &&
+            TypeIdx == NumTypes - 1)
+          IsMemberSpecialization = true;
+
+        continue;
+      }
+    } else if (const TemplateSpecializationType *TST
+                                     = T->getAs<TemplateSpecializationType>()) {
+      if (TemplateDecl *Template = TST->getTemplateName().getAsTemplateDecl()) {
+        ExpectedTemplateParams = Template->getTemplateParameters();
+        NeedNonemptyTemplateHeader = true;
+      }
+    } else if (T->getAs<DependentTemplateSpecializationType>()) {
+      // FIXME:  We actually could/should check the template arguments here
+      // against the corresponding template parameter list.
+      NeedNonemptyTemplateHeader = false;
+    }
+
+    // C++ [temp.expl.spec]p16:
+    //   In an explicit specialization declaration for a member of a class
+    //   template or a member template that ap- pears in namespace scope, the
+    //   member template and some of its enclosing class templates may remain
+    //   unspecialized, except that the declaration shall not explicitly
+    //   specialize a class member template if its en- closing class templates
+    //   are not explicitly specialized as well.
+    if (ParamIdx < ParamLists.size()) {
+      if (ParamLists[ParamIdx]->size() == 0) {
+        if (CheckExplicitSpecialization(ParamLists[ParamIdx]->getSourceRange(),
+                                        false))
+          return nullptr;
+      } else
+        SawNonEmptyTemplateParameterList = true;
+    }
+
+    if (NeedEmptyTemplateHeader) {
+      // If we're on the last of the types, and we need a 'template<>' header
+      // here, then it's a member specialization.
+      if (TypeIdx == NumTypes - 1)
+        IsMemberSpecialization = true;
+
+      if (ParamIdx < ParamLists.size()) {
+        if (ParamLists[ParamIdx]->size() > 0) {
+          // The header has template parameters when it shouldn't. Complain.
+          if (!SuppressDiagnostic)
+            Diag(ParamLists[ParamIdx]->getTemplateLoc(),
+                 diag::err_template_param_list_matches_nontemplate)
+              << T
+              << SourceRange(ParamLists[ParamIdx]->getLAngleLoc(),
+                             ParamLists[ParamIdx]->getRAngleLoc())
+              << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
+          Invalid = true;
+          return nullptr;
+        }
+
+        // Consume this template header.
+        ++ParamIdx;
+        continue;
+      }
+
+      if (!IsFriend)
+        if (DiagnoseMissingExplicitSpecialization(
+                getRangeOfTypeInNestedNameSpecifier(Context, T, SS)))
+          return nullptr;
+
+      continue;
+    }
+
+    if (NeedNonemptyTemplateHeader) {
+      // In friend declarations we can have template-ids which don't
+      // depend on the corresponding template parameter lists.  But
+      // assume that empty parameter lists are supposed to match this
+      // template-id.
+      if (IsFriend && T->isDependentType()) {
+        if (ParamIdx < ParamLists.size() &&
+            DependsOnTemplateParameters(T, ParamLists[ParamIdx]))
+          ExpectedTemplateParams = nullptr;
+        else
+          continue;
+      }
+
+      if (ParamIdx < ParamLists.size()) {
+        // Check the template parameter list, if we can.
+        if (ExpectedTemplateParams &&
+            !TemplateParameterListsAreEqual(ParamLists[ParamIdx],
+                                            ExpectedTemplateParams,
+                                            !SuppressDiagnostic, TPL_TemplateMatch))
+          Invalid = true;
+
+        if (!Invalid &&
+            CheckTemplateParameterList(ParamLists[ParamIdx], nullptr,
+                                       TPC_ClassTemplateMember))
+          Invalid = true;
+
+        ++ParamIdx;
+        continue;
+      }
+
+      if (!SuppressDiagnostic)
+        Diag(DeclLoc, diag::err_template_spec_needs_template_parameters)
+          << T
+          << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
+      Invalid = true;
+      continue;
+    }
+  }
+
+  // If there were at least as many template-ids as there were template
+  // parameter lists, then there are no template parameter lists remaining for
+  // the declaration itself.
+  if (ParamIdx >= ParamLists.size()) {
+    if (TemplateId && !IsFriend) {
+      // We don't have a template header for the declaration itself, but we
+      // should.
+      DiagnoseMissingExplicitSpecialization(SourceRange(TemplateId->LAngleLoc,
+                                                        TemplateId->RAngleLoc));
+
+      // Fabricate an empty template parameter list for the invented header.
+      return TemplateParameterList::Create(Context, SourceLocation(),
+                                           SourceLocation(), None,
+                                           SourceLocation(), nullptr);
+    }
+
+    return nullptr;
+  }
+
+  // If there were too many template parameter lists, complain about that now.
+  if (ParamIdx < ParamLists.size() - 1) {
+    bool HasAnyExplicitSpecHeader = false;
+    bool AllExplicitSpecHeaders = true;
+    for (unsigned I = ParamIdx, E = ParamLists.size() - 1; I != E; ++I) {
+      if (ParamLists[I]->size() == 0)
+        HasAnyExplicitSpecHeader = true;
+      else
+        AllExplicitSpecHeaders = false;
+    }
+
+    if (!SuppressDiagnostic)
+      Diag(ParamLists[ParamIdx]->getTemplateLoc(),
+           AllExplicitSpecHeaders ? diag::warn_template_spec_extra_headers
+                                  : diag::err_template_spec_extra_headers)
+          << SourceRange(ParamLists[ParamIdx]->getTemplateLoc(),
+                         ParamLists[ParamLists.size() - 2]->getRAngleLoc());
+
+    // If there was a specialization somewhere, such that 'template<>' is
+    // not required, and there were any 'template<>' headers, note where the
+    // specialization occurred.
+    if (ExplicitSpecLoc.isValid() && HasAnyExplicitSpecHeader &&
+        !SuppressDiagnostic)
+      Diag(ExplicitSpecLoc,
+           diag::note_explicit_template_spec_does_not_need_header)
+        << NestedTypes.back();
+
+    // We have a template parameter list with no corresponding scope, which
+    // means that the resulting template declaration can't be instantiated
+    // properly (we'll end up with dependent nodes when we shouldn't).
+    if (!AllExplicitSpecHeaders)
+      Invalid = true;
+  }
+
+  // C++ [temp.expl.spec]p16:
+  //   In an explicit specialization declaration for a member of a class
+  //   template or a member template that ap- pears in namespace scope, the
+  //   member template and some of its enclosing class templates may remain
+  //   unspecialized, except that the declaration shall not explicitly
+  //   specialize a class member template if its en- closing class templates
+  //   are not explicitly specialized as well.
+  if (ParamLists.back()->size() == 0 &&
+      CheckExplicitSpecialization(ParamLists[ParamIdx]->getSourceRange(),
+                                  false))
+    return nullptr;
+
+  // Return the last template parameter list, which corresponds to the
+  // entity being declared.
+  return ParamLists.back();
+}
+
+void Sema::NoteAllFoundTemplates(TemplateName Name) {
+  if (TemplateDecl *Template = Name.getAsTemplateDecl()) {
+    Diag(Template->getLocation(), diag::note_template_declared_here)
+        << (isa<FunctionTemplateDecl>(Template)
+                ? 0
+                : isa<ClassTemplateDecl>(Template)
+                      ? 1
+                      : isa<VarTemplateDecl>(Template)
+                            ? 2
+                            : isa<TypeAliasTemplateDecl>(Template) ? 3 : 4)
+        << Template->getDeclName();
+    return;
+  }
+
+  if (OverloadedTemplateStorage *OST = Name.getAsOverloadedTemplate()) {
+    for (OverloadedTemplateStorage::iterator I = OST->begin(),
+                                          IEnd = OST->end();
+         I != IEnd; ++I)
+      Diag((*I)->getLocation(), diag::note_template_declared_here)
+        << 0 << (*I)->getDeclName();
+
+    return;
+  }
+}
+
+static QualType
+checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
+                           const SmallVectorImpl<TemplateArgument> &Converted,
+                           SourceLocation TemplateLoc,
+                           TemplateArgumentListInfo &TemplateArgs) {
+  ASTContext &Context = SemaRef.getASTContext();
+  switch (BTD->getBuiltinTemplateKind()) {
+  case BTK__make_integer_seq: {
+    // Specializations of __make_integer_seq<S, T, N> are treated like
+    // S<T, 0, ..., N-1>.
+
+    // C++14 [inteseq.intseq]p1:
+    //   T shall be an integer type.
+    if (!Converted[1].getAsType()->isIntegralType(Context)) {
+      SemaRef.Diag(TemplateArgs[1].getLocation(),
+                   diag::err_integer_sequence_integral_element_type);
+      return QualType();
+    }
+
+    // C++14 [inteseq.make]p1:
+    //   If N is negative the program is ill-formed.
+    TemplateArgument NumArgsArg = Converted[2];
+    llvm::APSInt NumArgs = NumArgsArg.getAsIntegral();
+    if (NumArgs < 0) {
+      SemaRef.Diag(TemplateArgs[2].getLocation(),
+                   diag::err_integer_sequence_negative_length);
+      return QualType();
+    }
+
+    QualType ArgTy = NumArgsArg.getIntegralType();
+    TemplateArgumentListInfo SyntheticTemplateArgs;
+    // The type argument gets reused as the first template argument in the
+    // synthetic template argument list.
+    SyntheticTemplateArgs.addArgument(TemplateArgs[1]);
+    // Expand N into 0 ... N-1.
+    for (llvm::APSInt I(NumArgs.getBitWidth(), NumArgs.isUnsigned());
+         I < NumArgs; ++I) {
+      TemplateArgument TA(Context, I, ArgTy);
+      SyntheticTemplateArgs.addArgument(SemaRef.getTrivialTemplateArgumentLoc(
+          TA, ArgTy, TemplateArgs[2].getLocation()));
+    }
+    // The first template argument will be reused as the template decl that
+    // our synthetic template arguments will be applied to.
+    return SemaRef.CheckTemplateIdType(Converted[0].getAsTemplate(),
+                                       TemplateLoc, SyntheticTemplateArgs);
+  }
+
+  case BTK__type_pack_element:
+    // Specializations of
+    //    __type_pack_element<Index, T_1, ..., T_N>
+    // are treated like T_Index.
+    assert(Converted.size() == 2 &&
+      "__type_pack_element should be given an index and a parameter pack");
+
+    // If the Index is out of bounds, the program is ill-formed.
+    TemplateArgument IndexArg = Converted[0], Ts = Converted[1];
+    llvm::APSInt Index = IndexArg.getAsIntegral();
+    assert(Index >= 0 && "the index used with __type_pack_element should be of "
+                         "type std::size_t, and hence be non-negative");
+    if (Index >= Ts.pack_size()) {
+      SemaRef.Diag(TemplateArgs[0].getLocation(),
+                   diag::err_type_pack_element_out_of_bounds);
+      return QualType();
+    }
+
+    // We simply return the type at index `Index`.
+    auto Nth = std::next(Ts.pack_begin(), Index.getExtValue());
+    return Nth->getAsType();
+  }
+  llvm_unreachable("unexpected BuiltinTemplateDecl!");
+}
+
+/// Determine whether this alias template is "enable_if_t".
+/// libc++ >=14 uses "__enable_if_t" in C++11 mode.
+static bool isEnableIfAliasTemplate(TypeAliasTemplateDecl *AliasTemplate) {
+  return AliasTemplate->getName().equals("enable_if_t") ||
+         AliasTemplate->getName().equals("__enable_if_t");
+}
+
+/// Collect all of the separable terms in the given condition, which
+/// might be a conjunction.
+///
+/// FIXME: The right answer is to convert the logical expression into
+/// disjunctive normal form, so we can find the first failed term
+/// within each possible clause.
+static void collectConjunctionTerms(Expr *Clause,
+                                    SmallVectorImpl<Expr *> &Terms) {
+  if (auto BinOp = dyn_cast<BinaryOperator>(Clause->IgnoreParenImpCasts())) {
+    if (BinOp->getOpcode() == BO_LAnd) {
+      collectConjunctionTerms(BinOp->getLHS(), Terms);
+      collectConjunctionTerms(BinOp->getRHS(), Terms);
+    }
+
+    return;
+  }
+
+  Terms.push_back(Clause);
+}
+
+// The ranges-v3 library uses an odd pattern of a top-level "||" with
+// a left-hand side that is value-dependent but never true. Identify
+// the idiom and ignore that term.
+static Expr *lookThroughRangesV3Condition(Preprocessor &PP, Expr *Cond) {
+  // Top-level '||'.
+  auto *BinOp = dyn_cast<BinaryOperator>(Cond->IgnoreParenImpCasts());
+  if (!BinOp) return Cond;
+
+  if (BinOp->getOpcode() != BO_LOr) return Cond;
+
+  // With an inner '==' that has a literal on the right-hand side.
+  Expr *LHS = BinOp->getLHS();
+  auto *InnerBinOp = dyn_cast<BinaryOperator>(LHS->IgnoreParenImpCasts());
+  if (!InnerBinOp) return Cond;
+
+  if (InnerBinOp->getOpcode() != BO_EQ ||
+      !isa<IntegerLiteral>(InnerBinOp->getRHS()))
+    return Cond;
+
+  // If the inner binary operation came from a macro expansion named
+  // CONCEPT_REQUIRES or CONCEPT_REQUIRES_, return the right-hand side
+  // of the '||', which is the real, user-provided condition.
+  SourceLocation Loc = InnerBinOp->getExprLoc();
+  if (!Loc.isMacroID()) return Cond;
+
+  StringRef MacroName = PP.getImmediateMacroName(Loc);
+  if (MacroName == "CONCEPT_REQUIRES" || MacroName == "CONCEPT_REQUIRES_")
+    return BinOp->getRHS();
+
+  return Cond;
+}
+
+namespace {
+
+// A PrinterHelper that prints more helpful diagnostics for some sub-expressions
+// within failing boolean expression, such as substituting template parameters
+// for actual types.
+class FailedBooleanConditionPrinterHelper : public PrinterHelper {
+public:
+  explicit FailedBooleanConditionPrinterHelper(const PrintingPolicy &P)
+      : Policy(P) {}
+
+  bool handledStmt(Stmt *E, raw_ostream &OS) override {
+    const auto *DR = dyn_cast<DeclRefExpr>(E);
+    if (DR && DR->getQualifier()) {
+      // If this is a qualified name, expand the template arguments in nested
+      // qualifiers.
+      DR->getQualifier()->print(OS, Policy, true);
+      // Then print the decl itself.
+      const ValueDecl *VD = DR->getDecl();
+      OS << VD->getName();
+      if (const auto *IV = dyn_cast<VarTemplateSpecializationDecl>(VD)) {
+        // This is a template variable, print the expanded template arguments.
+        printTemplateArgumentList(
+            OS, IV->getTemplateArgs().asArray(), Policy,
+            IV->getSpecializedTemplate()->getTemplateParameters());
+      }
+      return true;
+    }
+    return false;
+  }
+
+private:
+  const PrintingPolicy Policy;
+};
+
+} // end anonymous namespace
+
+std::pair<Expr *, std::string>
+Sema::findFailedBooleanCondition(Expr *Cond) {
+  Cond = lookThroughRangesV3Condition(PP, Cond);
+
+  // Separate out all of the terms in a conjunction.
+  SmallVector<Expr *, 4> Terms;
+  collectConjunctionTerms(Cond, Terms);
+
+  // Determine which term failed.
+  Expr *FailedCond = nullptr;
+  for (Expr *Term : Terms) {
+    Expr *TermAsWritten = Term->IgnoreParenImpCasts();
+
+    // Literals are uninteresting.
+    if (isa<CXXBoolLiteralExpr>(TermAsWritten) ||
+        isa<IntegerLiteral>(TermAsWritten))
+      continue;
+
+    // The initialization of the parameter from the argument is
+    // a constant-evaluated context.
+    EnterExpressionEvaluationContext ConstantEvaluated(
+      *this, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+
+    bool Succeeded;
+    if (Term->EvaluateAsBooleanCondition(Succeeded, Context) &&
+        !Succeeded) {
+      FailedCond = TermAsWritten;
+      break;
+    }
+  }
+  if (!FailedCond)
+    FailedCond = Cond->IgnoreParenImpCasts();
+
+  std::string Description;
+  {
+    llvm::raw_string_ostream Out(Description);
+    PrintingPolicy Policy = getPrintingPolicy();
+    Policy.PrintCanonicalTypes = true;
+    FailedBooleanConditionPrinterHelper Helper(Policy);
+    FailedCond->printPretty(Out, &Helper, Policy, 0, "\n", nullptr);
+  }
+  return { FailedCond, Description };
+}
+
+QualType Sema::CheckTemplateIdType(TemplateName Name,
+                                   SourceLocation TemplateLoc,
+                                   TemplateArgumentListInfo &TemplateArgs) {
+  DependentTemplateName *DTN
+    = Name.getUnderlying().getAsDependentTemplateName();
+  if (DTN && DTN->isIdentifier())
+    // When building a template-id where the template-name is dependent,
+    // assume the template is a type template. Either our assumption is
+    // correct, or the code is ill-formed and will be diagnosed when the
+    // dependent name is substituted.
+    return Context.getDependentTemplateSpecializationType(ETK_None,
+                                                          DTN->getQualifier(),
+                                                          DTN->getIdentifier(),
+                                                          TemplateArgs);
+
+  if (Name.getAsAssumedTemplateName() &&
+      resolveAssumedTemplateNameAsType(/*Scope*/nullptr, Name, TemplateLoc))
+    return QualType();
+
+  TemplateDecl *Template = Name.getAsTemplateDecl();
+  if (!Template || isa<FunctionTemplateDecl>(Template) ||
+      isa<VarTemplateDecl>(Template) || isa<ConceptDecl>(Template)) {
+    // We might have a substituted template template parameter pack. If so,
+    // build a template specialization type for it.
+    if (Name.getAsSubstTemplateTemplateParmPack())
+      return Context.getTemplateSpecializationType(Name, TemplateArgs);
+
+    Diag(TemplateLoc, diag::err_template_id_not_a_type)
+      << Name;
+    NoteAllFoundTemplates(Name);
+    return QualType();
+  }
+
+  // Check that the template argument list is well-formed for this
+  // template.
+  SmallVector<TemplateArgument, 4> Converted;
+  if (CheckTemplateArgumentList(Template, TemplateLoc, TemplateArgs,
+                                false, Converted,
+                                /*UpdateArgsWithConversions=*/true))
+    return QualType();
+
+  QualType CanonType;
+
+  if (TypeAliasTemplateDecl *AliasTemplate =
+          dyn_cast<TypeAliasTemplateDecl>(Template)) {
+
+    // Find the canonical type for this type alias template specialization.
+    TypeAliasDecl *Pattern = AliasTemplate->getTemplatedDecl();
+    if (Pattern->isInvalidDecl())
+      return QualType();
+
+    TemplateArgumentList StackTemplateArgs(TemplateArgumentList::OnStack,
+                                           Converted);
+
+    // Only substitute for the innermost template argument list.
+    MultiLevelTemplateArgumentList TemplateArgLists;
+    TemplateArgLists.addOuterTemplateArguments(&StackTemplateArgs);
+    TemplateArgLists.addOuterRetainedLevels(
+        AliasTemplate->getTemplateParameters()->getDepth());
+
+    LocalInstantiationScope Scope(*this);
+    InstantiatingTemplate Inst(*this, TemplateLoc, Template);
+    if (Inst.isInvalid())
+      return QualType();
+
+    CanonType = SubstType(Pattern->getUnderlyingType(),
+                          TemplateArgLists, AliasTemplate->getLocation(),
+                          AliasTemplate->getDeclName());
+    if (CanonType.isNull()) {
+      // If this was enable_if and we failed to find the nested type
+      // within enable_if in a SFINAE context, dig out the specific
+      // enable_if condition that failed and present that instead.
+      if (isEnableIfAliasTemplate(AliasTemplate)) {
+        if (auto DeductionInfo = isSFINAEContext()) {
+          if (*DeductionInfo &&
+              (*DeductionInfo)->hasSFINAEDiagnostic() &&
+              (*DeductionInfo)->peekSFINAEDiagnostic().second.getDiagID() ==
+                diag::err_typename_nested_not_found_enable_if &&
+              TemplateArgs[0].getArgument().getKind()
+                == TemplateArgument::Expression) {
+            Expr *FailedCond;
+            std::string FailedDescription;
+            std::tie(FailedCond, FailedDescription) =
+              findFailedBooleanCondition(TemplateArgs[0].getSourceExpression());
+
+            // Remove the old SFINAE diagnostic.
+            PartialDiagnosticAt OldDiag =
+              {SourceLocation(), PartialDiagnostic::NullDiagnostic()};
+            (*DeductionInfo)->takeSFINAEDiagnostic(OldDiag);
+
+            // Add a new SFINAE diagnostic specifying which condition
+            // failed.
+            (*DeductionInfo)->addSFINAEDiagnostic(
+              OldDiag.first,
+              PDiag(diag::err_typename_nested_not_found_requirement)
+                << FailedDescription
+                << FailedCond->getSourceRange());
+          }
+        }
+      }
+
+      return QualType();
+    }
+  } else if (Name.isDependent() ||
+             TemplateSpecializationType::anyDependentTemplateArguments(
+                 TemplateArgs, Converted)) {
+    // This class template specialization is a dependent
+    // type. Therefore, its canonical type is another class template
+    // specialization type that contains all of the converted
+    // arguments in canonical form. This ensures that, e.g., A<T> and
+    // A<T, T> have identical types when A is declared as:
+    //
+    //   template<typename T, typename U = T> struct A;
+    CanonType = Context.getCanonicalTemplateSpecializationType(Name, Converted);
+
+    // This might work out to be a current instantiation, in which
+    // case the canonical type needs to be the InjectedClassNameType.
+    //
+    // TODO: in theory this could be a simple hashtable lookup; most
+    // changes to CurContext don't change the set of current
+    // instantiations.
+    if (isa<ClassTemplateDecl>(Template)) {
+      for (DeclContext *Ctx = CurContext; Ctx; Ctx = Ctx->getLookupParent()) {
+        // If we get out to a namespace, we're done.
+        if (Ctx->isFileContext()) break;
+
+        // If this isn't a record, keep looking.
+        CXXRecordDecl *Record = dyn_cast<CXXRecordDecl>(Ctx);
+        if (!Record) continue;
+
+        // Look for one of the two cases with InjectedClassNameTypes
+        // and check whether it's the same template.
+        if (!isa<ClassTemplatePartialSpecializationDecl>(Record) &&
+            !Record->getDescribedClassTemplate())
+          continue;
+
+        // Fetch the injected class name type and check whether its
+        // injected type is equal to the type we just built.
+        QualType ICNT = Context.getTypeDeclType(Record);
+        QualType Injected = cast<InjectedClassNameType>(ICNT)
+          ->getInjectedSpecializationType();
+
+        if (CanonType != Injected->getCanonicalTypeInternal())
+          continue;
+
+        // If so, the canonical type of this TST is the injected
+        // class name type of the record we just found.
+        assert(ICNT.isCanonical());
+        CanonType = ICNT;
+        break;
+      }
+    }
+  } else if (ClassTemplateDecl *ClassTemplate
+               = dyn_cast<ClassTemplateDecl>(Template)) {
+    // Find the class template specialization declaration that
+    // corresponds to these arguments.
+    void *InsertPos = nullptr;
+    ClassTemplateSpecializationDecl *Decl
+      = ClassTemplate->findSpecialization(Converted, InsertPos);
+    if (!Decl) {
+      // This is the first time we have referenced this class template
+      // specialization. Create the canonical declaration and add it to
+      // the set of specializations.
+      Decl = ClassTemplateSpecializationDecl::Create(
+          Context, ClassTemplate->getTemplatedDecl()->getTagKind(),
+          ClassTemplate->getDeclContext(),
+          ClassTemplate->getTemplatedDecl()->getBeginLoc(),
+          ClassTemplate->getLocation(), ClassTemplate, Converted, nullptr);
+      ClassTemplate->AddSpecialization(Decl, InsertPos);
+      if (ClassTemplate->isOutOfLine())
+        Decl->setLexicalDeclContext(ClassTemplate->getLexicalDeclContext());
+    }
+
+    if (Decl->getSpecializationKind() == TSK_Undeclared &&
+        ClassTemplate->getTemplatedDecl()->hasAttrs()) {
+      InstantiatingTemplate Inst(*this, TemplateLoc, Decl);
+      if (!Inst.isInvalid()) {
+        MultiLevelTemplateArgumentList TemplateArgLists;
+        TemplateArgLists.addOuterTemplateArguments(Converted);
+        InstantiateAttrsForDecl(TemplateArgLists,
+                                ClassTemplate->getTemplatedDecl(), Decl);
+      }
+    }
+
+    // Diagnose uses of this specialization.
+    (void)DiagnoseUseOfDecl(Decl, TemplateLoc);
+
+    CanonType = Context.getTypeDeclType(Decl);
+    assert(isa<RecordType>(CanonType) &&
+           "type of non-dependent specialization is not a RecordType");
+  } else if (auto *BTD = dyn_cast<BuiltinTemplateDecl>(Template)) {
+    CanonType = checkBuiltinTemplateIdType(*this, BTD, Converted, TemplateLoc,
+                                           TemplateArgs);
+  }
+
+  // Build the fully-sugared type for this class template
+  // specialization, which refers back to the class template
+  // specialization we created or found.
+  return Context.getTemplateSpecializationType(Name, TemplateArgs, CanonType);
+}
+
+void Sema::ActOnUndeclaredTypeTemplateName(Scope *S, TemplateTy &ParsedName,
+                                           TemplateNameKind &TNK,
+                                           SourceLocation NameLoc,
+                                           IdentifierInfo *&II) {
+  assert(TNK == TNK_Undeclared_template && "not an undeclared template name");
+
+  TemplateName Name = ParsedName.get();
+  auto *ATN = Name.getAsAssumedTemplateName();
+  assert(ATN && "not an assumed template name");
+  II = ATN->getDeclName().getAsIdentifierInfo();
+
+  if (!resolveAssumedTemplateNameAsType(S, Name, NameLoc, /*Diagnose*/false)) {
+    // Resolved to a type template name.
+    ParsedName = TemplateTy::make(Name);
+    TNK = TNK_Type_template;
+  }
+}
+
+bool Sema::resolveAssumedTemplateNameAsType(Scope *S, TemplateName &Name,
+                                            SourceLocation NameLoc,
+                                            bool Diagnose) {
+  // We assumed this undeclared identifier to be an (ADL-only) function
+  // template name, but it was used in a context where a type was required.
+  // Try to typo-correct it now.
+  AssumedTemplateStorage *ATN = Name.getAsAssumedTemplateName();
+  assert(ATN && "not an assumed template name");
+
+  LookupResult R(*this, ATN->getDeclName(), NameLoc, LookupOrdinaryName);
+  struct CandidateCallback : CorrectionCandidateCallback {
+    bool ValidateCandidate(const TypoCorrection &TC) override {
+      return TC.getCorrectionDecl() &&
+             getAsTypeTemplateDecl(TC.getCorrectionDecl());
+    }
+    std::unique_ptr<CorrectionCandidateCallback> clone() override {
+      return std::make_unique<CandidateCallback>(*this);
+    }
+  } FilterCCC;
+
+  TypoCorrection Corrected =
+      CorrectTypo(R.getLookupNameInfo(), R.getLookupKind(), S, nullptr,
+                  FilterCCC, CTK_ErrorRecovery);
+  if (Corrected && Corrected.getFoundDecl()) {
+    diagnoseTypo(Corrected, PDiag(diag::err_no_template_suggest)
+                                << ATN->getDeclName());
+    Name = TemplateName(Corrected.getCorrectionDeclAs<TemplateDecl>());
+    return false;
+  }
+
+  if (Diagnose)
+    Diag(R.getNameLoc(), diag::err_no_template) << R.getLookupName();
+  return true;
+}
+
+TypeResult Sema::ActOnTemplateIdType(
+    Scope *S, CXXScopeSpec &SS, SourceLocation TemplateKWLoc,
+    TemplateTy TemplateD, IdentifierInfo *TemplateII,
+    SourceLocation TemplateIILoc, SourceLocation LAngleLoc,
+    ASTTemplateArgsPtr TemplateArgsIn, SourceLocation RAngleLoc,
+    bool IsCtorOrDtorName, bool IsClassName) {
+  if (SS.isInvalid())
+    return true;
+
+  if (!IsCtorOrDtorName && !IsClassName && SS.isSet()) {
+    DeclContext *LookupCtx = computeDeclContext(SS, /*EnteringContext*/false);
+
+    // C++ [temp.res]p3:
+    //   A qualified-id that refers to a type and in which the
+    //   nested-name-specifier depends on a template-parameter (14.6.2)
+    //   shall be prefixed by the keyword typename to indicate that the
+    //   qualified-id denotes a type, forming an
+    //   elaborated-type-specifier (7.1.5.3).
+    if (!LookupCtx && isDependentScopeSpecifier(SS)) {
+      Diag(SS.getBeginLoc(), diag::err_typename_missing_template)
+        << SS.getScopeRep() << TemplateII->getName();
+      // Recover as if 'typename' were specified.
+      // FIXME: This is not quite correct recovery as we don't transform SS
+      // into the corresponding dependent form (and we don't diagnose missing
+      // 'template' keywords within SS as a result).
+      return ActOnTypenameType(nullptr, SourceLocation(), SS, TemplateKWLoc,
+                               TemplateD, TemplateII, TemplateIILoc, LAngleLoc,
+                               TemplateArgsIn, RAngleLoc);
+    }
+
+    // Per C++ [class.qual]p2, if the template-id was an injected-class-name,
+    // it's not actually allowed to be used as a type in most cases. Because
+    // we annotate it before we know whether it's valid, we have to check for
+    // this case here.
+    auto *LookupRD = dyn_cast_or_null<CXXRecordDecl>(LookupCtx);
+    if (LookupRD && LookupRD->getIdentifier() == TemplateII) {
+      Diag(TemplateIILoc,
+           TemplateKWLoc.isInvalid()
+               ? diag::err_out_of_line_qualified_id_type_names_constructor
+               : diag::ext_out_of_line_qualified_id_type_names_constructor)
+        << TemplateII << 0 /*injected-class-name used as template name*/
+        << 1 /*if any keyword was present, it was 'template'*/;
+    }
+  }
+
+  TemplateName Template = TemplateD.get();
+  if (Template.getAsAssumedTemplateName() &&
+      resolveAssumedTemplateNameAsType(S, Template, TemplateIILoc))
+    return true;
+
+  // Translate the parser's template argument list in our AST format.
+  TemplateArgumentListInfo TemplateArgs(LAngleLoc, RAngleLoc);
+  translateTemplateArguments(TemplateArgsIn, TemplateArgs);
+
+  if (DependentTemplateName *DTN = Template.getAsDependentTemplateName()) {
+    QualType T
+      = Context.getDependentTemplateSpecializationType(ETK_None,
+                                                       DTN->getQualifier(),
+                                                       DTN->getIdentifier(),
+                                                       TemplateArgs);
+    // Build type-source information.
+    TypeLocBuilder TLB;
+    DependentTemplateSpecializationTypeLoc SpecTL
+      = TLB.push<DependentTemplateSpecializationTypeLoc>(T);
+    SpecTL.setElaboratedKeywordLoc(SourceLocation());
+    SpecTL.setQualifierLoc(SS.getWithLocInContext(Context));
+    SpecTL.setTemplateKeywordLoc(TemplateKWLoc);
+    SpecTL.setTemplateNameLoc(TemplateIILoc);
+    SpecTL.setLAngleLoc(LAngleLoc);
+    SpecTL.setRAngleLoc(RAngleLoc);
+    for (unsigned I = 0, N = SpecTL.getNumArgs(); I != N; ++I)
+      SpecTL.setArgLocInfo(I, TemplateArgs[I].getLocInfo());
+    return CreateParsedType(T, TLB.getTypeSourceInfo(Context, T));
+  }
+
+  QualType Result = CheckTemplateIdType(Template, TemplateIILoc, TemplateArgs);
+  if (Result.isNull())
+    return true;
+
+  // Build type-source information.
+  TypeLocBuilder TLB;
+  TemplateSpecializationTypeLoc SpecTL
+    = TLB.push<TemplateSpecializationTypeLoc>(Result);
+  SpecTL.setTemplateKeywordLoc(TemplateKWLoc);
+  SpecTL.setTemplateNameLoc(TemplateIILoc);
+  SpecTL.setLAngleLoc(LAngleLoc);
+  SpecTL.setRAngleLoc(RAngleLoc);
+  for (unsigned i = 0, e = SpecTL.getNumArgs(); i != e; ++i)
+    SpecTL.setArgLocInfo(i, TemplateArgs[i].getLocInfo());
+
+  // NOTE: avoid constructing an ElaboratedTypeLoc if this is a
+  // constructor or destructor name (in such a case, the scope specifier
+  // will be attached to the enclosing Decl or Expr node).
+  if (SS.isNotEmpty() && !IsCtorOrDtorName) {
+    // Create an elaborated-type-specifier containing the nested-name-specifier.
+    Result = Context.getElaboratedType(ETK_None, SS.getScopeRep(), Result);
+    ElaboratedTypeLoc ElabTL = TLB.push<ElaboratedTypeLoc>(Result);
+    ElabTL.setElaboratedKeywordLoc(SourceLocation());
+    ElabTL.setQualifierLoc(SS.getWithLocInContext(Context));
+  }
+
+  return CreateParsedType(Result, TLB.getTypeSourceInfo(Context, Result));
+}
+
+TypeResult Sema::ActOnTagTemplateIdType(TagUseKind TUK,
+                                        TypeSpecifierType TagSpec,
+                                        SourceLocation TagLoc,
+                                        CXXScopeSpec &SS,
+                                        SourceLocation TemplateKWLoc,
+                                        TemplateTy TemplateD,
+                                        SourceLocation TemplateLoc,
+                                        SourceLocation LAngleLoc,
+                                        ASTTemplateArgsPtr TemplateArgsIn,
+                                        SourceLocation RAngleLoc) {
+  if (SS.isInvalid())
+    return TypeResult(true);
+
+  TemplateName Template = TemplateD.get();
+
+  // Translate the parser's template argument list in our AST format.
+  TemplateArgumentListInfo TemplateArgs(LAngleLoc, RAngleLoc);
+  translateTemplateArguments(TemplateArgsIn, TemplateArgs);
+
+  // Determine the tag kind
+  TagTypeKind TagKind = TypeWithKeyword::getTagTypeKindForTypeSpec(TagSpec);
+  ElaboratedTypeKeyword Keyword
+    = TypeWithKeyword::getKeywordForTagTypeKind(TagKind);
+
+  if (DependentTemplateName *DTN = Template.getAsDependentTemplateName()) {
+    QualType T = Context.getDependentTemplateSpecializationType(Keyword,
+                                                          DTN->getQualifier(),
+                                                          DTN->getIdentifier(),
+                                                                TemplateArgs);
+
+    // Build type-source information.
+    TypeLocBuilder TLB;
+    DependentTemplateSpecializationTypeLoc SpecTL
+      = TLB.push<DependentTemplateSpecializationTypeLoc>(T);
+    SpecTL.setElaboratedKeywordLoc(TagLoc);
+    SpecTL.setQualifierLoc(SS.getWithLocInContext(Context));
+    SpecTL.setTemplateKeywordLoc(TemplateKWLoc);
+    SpecTL.setTemplateNameLoc(TemplateLoc);
+    SpecTL.setLAngleLoc(LAngleLoc);
+    SpecTL.setRAngleLoc(RAngleLoc);
+    for (unsigned I = 0, N = SpecTL.getNumArgs(); I != N; ++I)
+      SpecTL.setArgLocInfo(I, TemplateArgs[I].getLocInfo());
+    return CreateParsedType(T, TLB.getTypeSourceInfo(Context, T));
+  }
+
+  if (TypeAliasTemplateDecl *TAT =
+        dyn_cast_or_null<TypeAliasTemplateDecl>(Template.getAsTemplateDecl())) {
+    // C++0x [dcl.type.elab]p2:
+    //   If the identifier resolves to a typedef-name or the simple-template-id
+    //   resolves to an alias template specialization, the
+    //   elaborated-type-specifier is ill-formed.
+    Diag(TemplateLoc, diag::err_tag_reference_non_tag)
+        << TAT << NTK_TypeAliasTemplate << TagKind;
+    Diag(TAT->getLocation(), diag::note_declared_at);
+  }
+
+  QualType Result = CheckTemplateIdType(Template, TemplateLoc, TemplateArgs);
+  if (Result.isNull())
+    return TypeResult(true);
+
+  // Check the tag kind
+  if (const RecordType *RT = Result->getAs<RecordType>()) {
+    RecordDecl *D = RT->getDecl();
+
+    IdentifierInfo *Id = D->getIdentifier();
+    assert(Id && "templated class must have an identifier");
+
+    if (!isAcceptableTagRedeclaration(D, TagKind, TUK == TUK_Definition,
+                                      TagLoc, Id)) {
+      Diag(TagLoc, diag::err_use_with_wrong_tag)
+        << Result
+        << FixItHint::CreateReplacement(SourceRange(TagLoc), D->getKindName());
+      Diag(D->getLocation(), diag::note_previous_use);
+    }
+  }
+
+  // Provide source-location information for the template specialization.
+  TypeLocBuilder TLB;
+  TemplateSpecializationTypeLoc SpecTL
+    = TLB.push<TemplateSpecializationTypeLoc>(Result);
+  SpecTL.setTemplateKeywordLoc(TemplateKWLoc);
+  SpecTL.setTemplateNameLoc(TemplateLoc);
+  SpecTL.setLAngleLoc(LAngleLoc);
+  SpecTL.setRAngleLoc(RAngleLoc);
+  for (unsigned i = 0, e = SpecTL.getNumArgs(); i != e; ++i)
+    SpecTL.setArgLocInfo(i, TemplateArgs[i].getLocInfo());
+
+  // Construct an elaborated type containing the nested-name-specifier (if any)
+  // and tag keyword.
+  Result = Context.getElaboratedType(Keyword, SS.getScopeRep(), Result);
+  ElaboratedTypeLoc ElabTL = TLB.push<ElaboratedTypeLoc>(Result);
+  ElabTL.setElaboratedKeywordLoc(TagLoc);
+  ElabTL.setQualifierLoc(SS.getWithLocInContext(Context));
+  return CreateParsedType(Result, TLB.getTypeSourceInfo(Context, Result));
+}
+
+static bool CheckTemplateSpecializationScope(Sema &S, NamedDecl *Specialized,
+                                             NamedDecl *PrevDecl,
+                                             SourceLocation Loc,
+                                             bool IsPartialSpecialization);
+
+static TemplateSpecializationKind getTemplateSpecializationKind(Decl *D);
+
+static bool isTemplateArgumentTemplateParameter(
+    const TemplateArgument &Arg, unsigned Depth, unsigned Index) {
+  switch (Arg.getKind()) {
+  case TemplateArgument::Null:
+  case TemplateArgument::NullPtr:
+  case TemplateArgument::Integral:
+  case TemplateArgument::Declaration:
+  case TemplateArgument::Pack:
+  case TemplateArgument::TemplateExpansion:
+    return false;
+
+  case TemplateArgument::Type: {
+    QualType Type = Arg.getAsType();
+    const TemplateTypeParmType *TPT =
+        Arg.getAsType()->getAs<TemplateTypeParmType>();
+    return TPT && !Type.hasQualifiers() &&
+           TPT->getDepth() == Depth && TPT->getIndex() == Index;
+  }
+
+  case TemplateArgument::Expression: {
+    DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(Arg.getAsExpr());
+    if (!DRE || !DRE->getDecl())
+      return false;
+    const NonTypeTemplateParmDecl *NTTP =
+        dyn_cast<NonTypeTemplateParmDecl>(DRE->getDecl());
+    return NTTP && NTTP->getDepth() == Depth && NTTP->getIndex() == Index;
+  }
+
+  case TemplateArgument::Template:
+    const TemplateTemplateParmDecl *TTP =
+        dyn_cast_or_null<TemplateTemplateParmDecl>(
+            Arg.getAsTemplateOrTemplatePattern().getAsTemplateDecl());
+    return TTP && TTP->getDepth() == Depth && TTP->getIndex() == Index;
+  }
+  llvm_unreachable("unexpected kind of template argument");
+}
+
+static bool isSameAsPrimaryTemplate(TemplateParameterList *Params,
+                                    ArrayRef<TemplateArgument> Args) {
+  if (Params->size() != Args.size())
+    return false;
+
+  unsigned Depth = Params->getDepth();
+
+  for (unsigned I = 0, N = Args.size(); I != N; ++I) {
+    TemplateArgument Arg = Args[I];
+
+    // If the parameter is a pack expansion, the argument must be a pack
+    // whose only element is a pack expansion.
+    if (Params->getParam(I)->isParameterPack()) {
+      if (Arg.getKind() != TemplateArgument::Pack || Arg.pack_size() != 1 ||
+          !Arg.pack_begin()->isPackExpansion())
+        return false;
+      Arg = Arg.pack_begin()->getPackExpansionPattern();
+    }
+
+    if (!isTemplateArgumentTemplateParameter(Arg, Depth, I))
+      return false;
+  }
+
+  return true;
+}
+
+template<typename PartialSpecDecl>
+static void checkMoreSpecializedThanPrimary(Sema &S, PartialSpecDecl *Partial) {
+  if (Partial->getDeclContext()->isDependentContext())
+    return;
+
+  // FIXME: Get the TDK from deduction in order to provide better diagnostics
+  // for non-substitution-failure issues?
+  TemplateDeductionInfo Info(Partial->getLocation());
+  if (S.isMoreSpecializedThanPrimary(Partial, Info))
+    return;
+
+  auto *Template = Partial->getSpecializedTemplate();
+  S.Diag(Partial->getLocation(),
+         diag::ext_partial_spec_not_more_specialized_than_primary)
+      << isa<VarTemplateDecl>(Template);
+
+  if (Info.hasSFINAEDiagnostic()) {
+    PartialDiagnosticAt Diag = {SourceLocation(),
+                                PartialDiagnostic::NullDiagnostic()};
+    Info.takeSFINAEDiagnostic(Diag);
+    SmallString<128> SFINAEArgString;
+    Diag.second.EmitToString(S.getDiagnostics(), SFINAEArgString);
+    S.Diag(Diag.first,
+           diag::note_partial_spec_not_more_specialized_than_primary)
+      << SFINAEArgString;
+  }
+
+  S.Diag(Template->getLocation(), diag::note_template_decl_here);
+  SmallVector<const Expr *, 3> PartialAC, TemplateAC;
+  Template->getAssociatedConstraints(TemplateAC);
+  Partial->getAssociatedConstraints(PartialAC);
+  S.MaybeEmitAmbiguousAtomicConstraintsDiagnostic(Partial, PartialAC, Template,
+                                                  TemplateAC);
+}
+
+static void
+noteNonDeducibleParameters(Sema &S, TemplateParameterList *TemplateParams,
+                           const llvm::SmallBitVector &DeducibleParams) {
+  for (unsigned I = 0, N = DeducibleParams.size(); I != N; ++I) {
+    if (!DeducibleParams[I]) {
+      NamedDecl *Param = TemplateParams->getParam(I);
+      if (Param->getDeclName())
+        S.Diag(Param->getLocation(), diag::note_non_deducible_parameter)
+            << Param->getDeclName();
+      else
+        S.Diag(Param->getLocation(), diag::note_non_deducible_parameter)
+            << "(anonymous)";
+    }
+  }
+}
+
+
+template<typename PartialSpecDecl>
+static void checkTemplatePartialSpecialization(Sema &S,
+                                               PartialSpecDecl *Partial) {
+  // C++1z [temp.class.spec]p8: (DR1495)
+  //   - The specialization shall be more specialized than the primary
+  //     template (14.5.5.2).
+  checkMoreSpecializedThanPrimary(S, Partial);
+
+  // C++ [temp.class.spec]p8: (DR1315)
+  //   - Each template-parameter shall appear at least once in the
+  //     template-id outside a non-deduced context.
+  // C++1z [temp.class.spec.match]p3 (P0127R2)
+  //   If the template arguments of a partial specialization cannot be
+  //   deduced because of the structure of its template-parameter-list
+  //   and the template-id, the program is ill-formed.
+  auto *TemplateParams = Partial->getTemplateParameters();
+  llvm::SmallBitVector DeducibleParams(TemplateParams->size());
+  S.MarkUsedTemplateParameters(Partial->getTemplateArgs(), true,
+                               TemplateParams->getDepth(), DeducibleParams);
+
+  if (!DeducibleParams.all()) {
+    unsigned NumNonDeducible = DeducibleParams.size() - DeducibleParams.count();
+    S.Diag(Partial->getLocation(), diag::ext_partial_specs_not_deducible)
+      << isa<VarTemplatePartialSpecializationDecl>(Partial)
+      << (NumNonDeducible > 1)
+      << SourceRange(Partial->getLocation(),
+                     Partial->getTemplateArgsAsWritten()->RAngleLoc);
+    noteNonDeducibleParameters(S, TemplateParams, DeducibleParams);
+  }
+}
+
+void Sema::CheckTemplatePartialSpecialization(
+    ClassTemplatePartialSpecializationDecl *Partial) {
+  checkTemplatePartialSpecialization(*this, Partial);
+}
+
+void Sema::CheckTemplatePartialSpecialization(
+    VarTemplatePartialSpecializationDecl *Partial) {
+  checkTemplatePartialSpecialization(*this, Partial);
+}
+
+void Sema::CheckDeductionGuideTemplate(FunctionTemplateDecl *TD) {
+  // C++1z [temp.param]p11:
+  //   A template parameter of a deduction guide template that does not have a
+  //   default-argument shall be deducible from the parameter-type-list of the
+  //   deduction guide template.
+  auto *TemplateParams = TD->getTemplateParameters();
+  llvm::SmallBitVector DeducibleParams(TemplateParams->size());
+  MarkDeducedTemplateParameters(TD, DeducibleParams);
+  for (unsigned I = 0; I != TemplateParams->size(); ++I) {
+    // A parameter pack is deducible (to an empty pack).
+    auto *Param = TemplateParams->getParam(I);
+    if (Param->isParameterPack() || hasVisibleDefaultArgument(Param))
+      DeducibleParams[I] = true;
+  }
+
+  if (!DeducibleParams.all()) {
+    unsigned NumNonDeducible = DeducibleParams.size() - DeducibleParams.count();
+    Diag(TD->getLocation(), diag::err_deduction_guide_template_not_deducible)
+      << (NumNonDeducible > 1);
+    noteNonDeducibleParameters(*this, TemplateParams, DeducibleParams);
+  }
+}
+
+DeclResult Sema::ActOnVarTemplateSpecialization(
+    Scope *S, Declarator &D, TypeSourceInfo *DI, SourceLocation TemplateKWLoc,
+    TemplateParameterList *TemplateParams, StorageClass SC,
+    bool IsPartialSpecialization) {
+  // D must be variable template id.
+  assert(D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId &&
+         "Variable template specialization is declared with a template it.");
+
+  TemplateIdAnnotation *TemplateId = D.getName().TemplateId;
+  TemplateArgumentListInfo TemplateArgs =
+      makeTemplateArgumentListInfo(*this, *TemplateId);
+  SourceLocation TemplateNameLoc = D.getIdentifierLoc();
+  SourceLocation LAngleLoc = TemplateId->LAngleLoc;
+  SourceLocation RAngleLoc = TemplateId->RAngleLoc;
+
+  TemplateName Name = TemplateId->Template.get();
+
+  // The template-id must name a variable template.
+  VarTemplateDecl *VarTemplate =
+      dyn_cast_or_null<VarTemplateDecl>(Name.getAsTemplateDecl());
+  if (!VarTemplate) {
+    NamedDecl *FnTemplate;
+    if (auto *OTS = Name.getAsOverloadedTemplate())
+      FnTemplate = *OTS->begin();
+    else
+      FnTemplate = dyn_cast_or_null<FunctionTemplateDecl>(Name.getAsTemplateDecl());
+    if (FnTemplate)
+      return Diag(D.getIdentifierLoc(), diag::err_var_spec_no_template_but_method)
+               << FnTemplate->getDeclName();
+    return Diag(D.getIdentifierLoc(), diag::err_var_spec_no_template)
+             << IsPartialSpecialization;
+  }
+
+  // Check for unexpanded parameter packs in any of the template arguments.
+  for (unsigned I = 0, N = TemplateArgs.size(); I != N; ++I)
+    if (DiagnoseUnexpandedParameterPack(TemplateArgs[I],
+                                        UPPC_PartialSpecialization))
+      return true;
+
+  // Check that the template argument list is well-formed for this
+  // template.
+  SmallVector<TemplateArgument, 4> Converted;
+  if (CheckTemplateArgumentList(VarTemplate, TemplateNameLoc, TemplateArgs,
+                                false, Converted,
+                                /*UpdateArgsWithConversions=*/true))
+    return true;
+
+  // Find the variable template (partial) specialization declaration that
+  // corresponds to these arguments.
+  if (IsPartialSpecialization) {
+    if (CheckTemplatePartialSpecializationArgs(TemplateNameLoc, VarTemplate,
+                                               TemplateArgs.size(), Converted))
+      return true;
+
+    // FIXME: Move these checks to CheckTemplatePartialSpecializationArgs so we
+    // also do them during instantiation.
+    if (!Name.isDependent() &&
+        !TemplateSpecializationType::anyDependentTemplateArguments(TemplateArgs,
+                                                                   Converted)) {
+      Diag(TemplateNameLoc, diag::err_partial_spec_fully_specialized)
+          << VarTemplate->getDeclName();
+      IsPartialSpecialization = false;
+    }
+
+    if (isSameAsPrimaryTemplate(VarTemplate->getTemplateParameters(),
+                                Converted) &&
+        (!Context.getLangOpts().CPlusPlus20 ||
+         !TemplateParams->hasAssociatedConstraints())) {
+      // C++ [temp.class.spec]p9b3:
+      //
+      //   -- The argument list of the specialization shall not be identical
+      //      to the implicit argument list of the primary template.
+      Diag(TemplateNameLoc, diag::err_partial_spec_args_match_primary_template)
+        << /*variable template*/ 1
+        << /*is definition*/(SC != SC_Extern && !CurContext->isRecord())
+        << FixItHint::CreateRemoval(SourceRange(LAngleLoc, RAngleLoc));
+      // FIXME: Recover from this by treating the declaration as a redeclaration
+      // of the primary template.
+      return true;
+    }
+  }
+
+  void *InsertPos = nullptr;
+  VarTemplateSpecializationDecl *PrevDecl = nullptr;
+
+  if (IsPartialSpecialization)
+    PrevDecl = VarTemplate->findPartialSpecialization(Converted, TemplateParams,
+                                                      InsertPos);
+  else
+    PrevDecl = VarTemplate->findSpecialization(Converted, InsertPos);
+
+  VarTemplateSpecializationDecl *Specialization = nullptr;
+
+  // Check whether we can declare a variable template specialization in
+  // the current scope.
+  if (CheckTemplateSpecializationScope(*this, VarTemplate, PrevDecl,
+                                       TemplateNameLoc,
+                                       IsPartialSpecialization))
+    return true;
+
+  if (PrevDecl && PrevDecl->getSpecializationKind() == TSK_Undeclared) {
+    // Since the only prior variable template specialization with these
+    // arguments was referenced but not declared,  reuse that
+    // declaration node as our own, updating its source location and
+    // the list of outer template parameters to reflect our new declaration.
+    Specialization = PrevDecl;
+    Specialization->setLocation(TemplateNameLoc);
+    PrevDecl = nullptr;
+  } else if (IsPartialSpecialization) {
+    // Create a new class template partial specialization declaration node.
+    VarTemplatePartialSpecializationDecl *PrevPartial =
+        cast_or_null<VarTemplatePartialSpecializationDecl>(PrevDecl);
+    VarTemplatePartialSpecializationDecl *Partial =
+        VarTemplatePartialSpecializationDecl::Create(
+            Context, VarTemplate->getDeclContext(), TemplateKWLoc,
+            TemplateNameLoc, TemplateParams, VarTemplate, DI->getType(), DI, SC,
+            Converted, TemplateArgs);
+
+    if (!PrevPartial)
+      VarTemplate->AddPartialSpecialization(Partial, InsertPos);
+    Specialization = Partial;
+
+    // If we are providing an explicit specialization of a member variable
+    // template specialization, make a note of that.
+    if (PrevPartial && PrevPartial->getInstantiatedFromMember())
+      PrevPartial->setMemberSpecialization();
+
+    CheckTemplatePartialSpecialization(Partial);
+  } else {
+    // Create a new class template specialization declaration node for
+    // this explicit specialization or friend declaration.
+    Specialization = VarTemplateSpecializationDecl::Create(
+        Context, VarTemplate->getDeclContext(), TemplateKWLoc, TemplateNameLoc,
+        VarTemplate, DI->getType(), DI, SC, Converted);
+    Specialization->setTemplateArgsInfo(TemplateArgs);
+
+    if (!PrevDecl)
+      VarTemplate->AddSpecialization(Specialization, InsertPos);
+  }
+
+  // C++ [temp.expl.spec]p6:
+  //   If a template, a member template or the member of a class template is
+  //   explicitly specialized then that specialization shall be declared
+  //   before the first use of that specialization that would cause an implicit
+  //   instantiation to take place, in every translation unit in which such a
+  //   use occurs; no diagnostic is required.
+  if (PrevDecl && PrevDecl->getPointOfInstantiation().isValid()) {
+    bool Okay = false;
+    for (Decl *Prev = PrevDecl; Prev; Prev = Prev->getPreviousDecl()) {
+      // Is there any previous explicit specialization declaration?
+      if (getTemplateSpecializationKind(Prev) == TSK_ExplicitSpecialization) {
+        Okay = true;
+        break;
+      }
+    }
+
+    if (!Okay) {
+      SourceRange Range(TemplateNameLoc, RAngleLoc);
+      Diag(TemplateNameLoc, diag::err_specialization_after_instantiation)
+          << Name << Range;
+
+      Diag(PrevDecl->getPointOfInstantiation(),
+           diag::note_instantiation_required_here)
+          << (PrevDecl->getTemplateSpecializationKind() !=
+              TSK_ImplicitInstantiation);
+      return true;
+    }
+  }
+
+  Specialization->setTemplateKeywordLoc(TemplateKWLoc);
+  Specialization->setLexicalDeclContext(CurContext);
+
+  // Add the specialization into its lexical context, so that it can
+  // be seen when iterating through the list of declarations in that
+  // context. However, specializations are not found by name lookup.
+  CurContext->addDecl(Specialization);
+
+  // Note that this is an explicit specialization.
+  Specialization->setSpecializationKind(TSK_ExplicitSpecialization);
+
+  if (PrevDecl) {
+    // Check that this isn't a redefinition of this specialization,
+    // merging with previous declarations.
+    LookupResult PrevSpec(*this, GetNameForDeclarator(D), LookupOrdinaryName,
+                          forRedeclarationInCurContext());
+    PrevSpec.addDecl(PrevDecl);
+    D.setRedeclaration(CheckVariableDeclaration(Specialization, PrevSpec));
+  } else if (Specialization->isStaticDataMember() &&
+             Specialization->isOutOfLine()) {
+    Specialization->setAccess(VarTemplate->getAccess());
+  }
+
+  return Specialization;
+}
+
+namespace {
+/// A partial specialization whose template arguments have matched
+/// a given template-id.
+struct PartialSpecMatchResult {
+  VarTemplatePartialSpecializationDecl *Partial;
+  TemplateArgumentList *Args;
+};
+} // end anonymous namespace
+
+DeclResult
+Sema::CheckVarTemplateId(VarTemplateDecl *Template, SourceLocation TemplateLoc,
+                         SourceLocation TemplateNameLoc,
+                         const TemplateArgumentListInfo &TemplateArgs) {
+  assert(Template && "A variable template id without template?");
+
+  // Check that the template argument list is well-formed for this template.
+  SmallVector<TemplateArgument, 4> Converted;
+  if (CheckTemplateArgumentList(
+          Template, TemplateNameLoc,
+          const_cast<TemplateArgumentListInfo &>(TemplateArgs), false,
+          Converted, /*UpdateArgsWithConversions=*/true))
+    return true;
+
+  // Produce a placeholder value if the specialization is dependent.
+  if (Template->getDeclContext()->isDependentContext() ||
+      TemplateSpecializationType::anyDependentTemplateArguments(TemplateArgs,
+                                                                Converted))
+    return DeclResult();
+
+  // Find the variable template specialization declaration that
+  // corresponds to these arguments.
+  void *InsertPos = nullptr;
+  if (VarTemplateSpecializationDecl *Spec = Template->findSpecialization(
+          Converted, InsertPos)) {
+    checkSpecializationVisibility(TemplateNameLoc, Spec);
+    // If we already have a variable template specialization, return it.
+    return Spec;
+  }
+
+  // This is the first time we have referenced this variable template
+  // specialization. Create the canonical declaration and add it to
+  // the set of specializations, based on the closest partial specialization
+  // that it represents. That is,
+  VarDecl *InstantiationPattern = Template->getTemplatedDecl();
+  TemplateArgumentList TemplateArgList(TemplateArgumentList::OnStack,
+                                       Converted);
+  TemplateArgumentList *InstantiationArgs = &TemplateArgList;
+  bool AmbiguousPartialSpec = false;
+  typedef PartialSpecMatchResult MatchResult;
+  SmallVector<MatchResult, 4> Matched;
+  SourceLocation PointOfInstantiation = TemplateNameLoc;
+  TemplateSpecCandidateSet FailedCandidates(PointOfInstantiation,
+                                            /*ForTakingAddress=*/false);
+
+  // 1. Attempt to find the closest partial specialization that this
+  // specializes, if any.
+  // TODO: Unify with InstantiateClassTemplateSpecialization()?
+  //       Perhaps better after unification of DeduceTemplateArguments() and
+  //       getMoreSpecializedPartialSpecialization().
+  SmallVector<VarTemplatePartialSpecializationDecl *, 4> PartialSpecs;
+  Template->getPartialSpecializations(PartialSpecs);
+
+  for (unsigned I = 0, N = PartialSpecs.size(); I != N; ++I) {
+    VarTemplatePartialSpecializationDecl *Partial = PartialSpecs[I];
+    TemplateDeductionInfo Info(FailedCandidates.getLocation());
+
+    if (TemplateDeductionResult Result =
+            DeduceTemplateArguments(Partial, TemplateArgList, Info)) {
+      // Store the failed-deduction information for use in diagnostics, later.
+      // TODO: Actually use the failed-deduction info?
+      FailedCandidates.addCandidate().set(
+          DeclAccessPair::make(Template, AS_public), Partial,
+          MakeDeductionFailureInfo(Context, Result, Info));
+      (void)Result;
+    } else {
+      Matched.push_back(PartialSpecMatchResult());
+      Matched.back().Partial = Partial;
+      Matched.back().Args = Info.take();
+    }
+  }
+
+  if (Matched.size() >= 1) {
+    SmallVector<MatchResult, 4>::iterator Best = Matched.begin();
+    if (Matched.size() == 1) {
+      //   -- If exactly one matching specialization is found, the
+      //      instantiation is generated from that specialization.
+      // We don't need to do anything for this.
+    } else {
+      //   -- If more than one matching specialization is found, the
+      //      partial order rules (14.5.4.2) are used to determine
+      //      whether one of the specializations is more specialized
+      //      than the others. If none of the specializations is more
+      //      specialized than all of the other matching
+      //      specializations, then the use of the variable template is
+      //      ambiguous and the program is ill-formed.
+      for (SmallVector<MatchResult, 4>::iterator P = Best + 1,
+                                                 PEnd = Matched.end();
+           P != PEnd; ++P) {
+        if (getMoreSpecializedPartialSpecialization(P->Partial, Best->Partial,
+                                                    PointOfInstantiation) ==
+            P->Partial)
+          Best = P;
+      }
+
+      // Determine if the best partial specialization is more specialized than
+      // the others.
+      for (SmallVector<MatchResult, 4>::iterator P = Matched.begin(),
+                                                 PEnd = Matched.end();
+           P != PEnd; ++P) {
+        if (P != Best && getMoreSpecializedPartialSpecialization(
+                             P->Partial, Best->Partial,
+                             PointOfInstantiation) != Best->Partial) {
+          AmbiguousPartialSpec = true;
+          break;
+        }
+      }
+    }
+
+    // Instantiate using the best variable template partial specialization.
+    InstantiationPattern = Best->Partial;
+    InstantiationArgs = Best->Args;
+  } else {
+    //   -- If no match is found, the instantiation is generated
+    //      from the primary template.
+    // InstantiationPattern = Template->getTemplatedDecl();
+  }
+
+  // 2. Create the canonical declaration.
+  // Note that we do not instantiate a definition until we see an odr-use
+  // in DoMarkVarDeclReferenced().
+  // FIXME: LateAttrs et al.?
+  VarTemplateSpecializationDecl *Decl = BuildVarTemplateInstantiation(
+      Template, InstantiationPattern, *InstantiationArgs, TemplateArgs,
+      Converted, TemplateNameLoc /*, LateAttrs, StartingScope*/);
+  if (!Decl)
+    return true;
+
+  if (AmbiguousPartialSpec) {
+    // Partial ordering did not produce a clear winner. Complain.
+    Decl->setInvalidDecl();
+    Diag(PointOfInstantiation, diag::err_partial_spec_ordering_ambiguous)
+        << Decl;
+
+    // Print the matching partial specializations.
+    for (MatchResult P : Matched)
+      Diag(P.Partial->getLocation(), diag::note_partial_spec_match)
+          << getTemplateArgumentBindingsText(P.Partial->getTemplateParameters(),
+                                             *P.Args);
+    return true;
+  }
+
+  if (VarTemplatePartialSpecializationDecl *D =
+          dyn_cast<VarTemplatePartialSpecializationDecl>(InstantiationPattern))
+    Decl->setInstantiationOf(D, InstantiationArgs);
+
+  checkSpecializationVisibility(TemplateNameLoc, Decl);
+
+  assert(Decl && "No variable template specialization?");
+  return Decl;
+}
+
+ExprResult
+Sema::CheckVarTemplateId(const CXXScopeSpec &SS,
+                         const DeclarationNameInfo &NameInfo,
+                         VarTemplateDecl *Template, SourceLocation TemplateLoc,
+                         const TemplateArgumentListInfo *TemplateArgs) {
+
+  DeclResult Decl = CheckVarTemplateId(Template, TemplateLoc, NameInfo.getLoc(),
+                                       *TemplateArgs);
+  if (Decl.isInvalid())
+    return ExprError();
+
+  if (!Decl.get())
+    return ExprResult();
+
+  VarDecl *Var = cast<VarDecl>(Decl.get());
+  if (!Var->getTemplateSpecializationKind())
+    Var->setTemplateSpecializationKind(TSK_ImplicitInstantiation,
+                                       NameInfo.getLoc());
+
+  // Build an ordinary singleton decl ref.
+  return BuildDeclarationNameExpr(SS, NameInfo, Var,
+                                  /*FoundD=*/nullptr, TemplateArgs);
+}
+
+void Sema::diagnoseMissingTemplateArguments(TemplateName Name,
+                                            SourceLocation Loc) {
+  Diag(Loc, diag::err_template_missing_args)
+    << (int)getTemplateNameKindForDiagnostics(Name) << Name;
+  if (TemplateDecl *TD = Name.getAsTemplateDecl()) {
+    Diag(TD->getLocation(), diag::note_template_decl_here)
+      << TD->getTemplateParameters()->getSourceRange();
+  }
+}
+
+ExprResult
+Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
+                             SourceLocation TemplateKWLoc,
+                             const DeclarationNameInfo &ConceptNameInfo,
+                             NamedDecl *FoundDecl,
+                             ConceptDecl *NamedConcept,
+                             const TemplateArgumentListInfo *TemplateArgs) {
+  assert(NamedConcept && "A concept template id without a template?");
+
+  llvm::SmallVector<TemplateArgument, 4> Converted;
+  if (CheckTemplateArgumentList(NamedConcept, ConceptNameInfo.getLoc(),
+                           const_cast<TemplateArgumentListInfo&>(*TemplateArgs),
+                                /*PartialTemplateArgs=*/false, Converted,
+                                /*UpdateArgsWithConversions=*/false))
+    return ExprError();
+
+  ConstraintSatisfaction Satisfaction;
+  bool AreArgsDependent =
+      TemplateSpecializationType::anyDependentTemplateArguments(*TemplateArgs,
+                                                                Converted);
+  if (!AreArgsDependent &&
+      CheckConstraintSatisfaction(
+          NamedConcept, {NamedConcept->getConstraintExpr()}, Converted,
+          SourceRange(SS.isSet() ? SS.getBeginLoc() : ConceptNameInfo.getLoc(),
+                      TemplateArgs->getRAngleLoc()),
+          Satisfaction))
+    return ExprError();
+
+  return ConceptSpecializationExpr::Create(Context,
+      SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc{},
+      TemplateKWLoc, ConceptNameInfo, FoundDecl, NamedConcept,
+      ASTTemplateArgumentListInfo::Create(Context, *TemplateArgs), Converted,
+      AreArgsDependent ? nullptr : &Satisfaction);
+}
+
+ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
+                                     SourceLocation TemplateKWLoc,
+                                     LookupResult &R,
+                                     bool RequiresADL,
+                                 const TemplateArgumentListInfo *TemplateArgs) {
+  // FIXME: Can we do any checking at this point? I guess we could check the
+  // template arguments that we have against the template name, if the template
+  // name refers to a single template. That's not a terribly common case,
+  // though.
+  // foo<int> could identify a single function unambiguously
+  // This approach does NOT work, since f<int>(1);
+  // gets resolved prior to resorting to overload resolution
+  // i.e., template<class T> void f(double);
+  //       vs template<class T, class U> void f(U);
+
+  // These should be filtered out by our callers.
+  assert(!R.isAmbiguous() && "ambiguous lookup when building templateid");
+
+  // Non-function templates require a template argument list.
+  if (auto *TD = R.getAsSingle<TemplateDecl>()) {
+    if (!TemplateArgs && !isa<FunctionTemplateDecl>(TD)) {
+      diagnoseMissingTemplateArguments(TemplateName(TD), R.getNameLoc());
+      return ExprError();
+    }
+  }
+
+  // In C++1y, check variable template ids.
+  if (R.getAsSingle<VarTemplateDecl>()) {
+    ExprResult Res = CheckVarTemplateId(SS, R.getLookupNameInfo(),
+                                        R.getAsSingle<VarTemplateDecl>(),
+                                        TemplateKWLoc, TemplateArgs);
+    if (Res.isInvalid() || Res.isUsable())
+      return Res;
+    // Result is dependent. Carry on to build an UnresolvedLookupEpxr.
+  }
+
+  if (R.getAsSingle<ConceptDecl>()) {
+    return CheckConceptTemplateId(SS, TemplateKWLoc, R.getLookupNameInfo(),
+                                  R.getFoundDecl(),
+                                  R.getAsSingle<ConceptDecl>(), TemplateArgs);
+  }
+
+  // We don't want lookup warnings at this point.
+  R.suppressDiagnostics();
+
+  UnresolvedLookupExpr *ULE
+    = UnresolvedLookupExpr::Create(Context, R.getNamingClass(),
+                                   SS.getWithLocInContext(Context),
+                                   TemplateKWLoc,
+                                   R.getLookupNameInfo(),
+                                   RequiresADL, TemplateArgs,
+                                   R.begin(), R.end());
+
+  return ULE;
+}
+
+// We actually only call this from template instantiation.
+ExprResult
+Sema::BuildQualifiedTemplateIdExpr(CXXScopeSpec &SS,
+                                   SourceLocation TemplateKWLoc,
+                                   const DeclarationNameInfo &NameInfo,
+                             const TemplateArgumentListInfo *TemplateArgs) {
+
+  assert(TemplateArgs || TemplateKWLoc.isValid());
+  DeclContext *DC;
+  if (!(DC = computeDeclContext(SS, false)) ||
+      DC->isDependentContext() ||
+      RequireCompleteDeclContext(SS, DC))
+    return BuildDependentDeclRefExpr(SS, TemplateKWLoc, NameInfo, TemplateArgs);
+
+  bool MemberOfUnknownSpecialization;
+  LookupResult R(*this, NameInfo, LookupOrdinaryName);
+  if (LookupTemplateName(R, (Scope *)nullptr, SS, QualType(),
+                         /*Entering*/false, MemberOfUnknownSpecialization,
+                         TemplateKWLoc))
+    return ExprError();
+
+  if (R.isAmbiguous())
+    return ExprError();
+
+  if (R.empty()) {
+    Diag(NameInfo.getLoc(), diag::err_no_member)
+      << NameInfo.getName() << DC << SS.getRange();
+    return ExprError();
+  }
+
+  if (ClassTemplateDecl *Temp = R.getAsSingle<ClassTemplateDecl>()) {
+    Diag(NameInfo.getLoc(), diag::err_template_kw_refers_to_class_template)
+      << SS.getScopeRep()
+      << NameInfo.getName().getAsString() << SS.getRange();
+    Diag(Temp->getLocation(), diag::note_referenced_class_template);
+    return ExprError();
+  }
+
+  return BuildTemplateIdExpr(SS, TemplateKWLoc, R, /*ADL*/ false, TemplateArgs);
+}
+
+/// Form a template name from a name that is syntactically required to name a
+/// template, either due to use of the 'template' keyword or because a name in
+/// this syntactic context is assumed to name a template (C++ [temp.names]p2-4).
+///
+/// This action forms a template name given the name of the template and its
+/// optional scope specifier. This is used when the 'template' keyword is used
+/// or when the parsing context unambiguously treats a following '<' as
+/// introducing a template argument list. Note that this may produce a
+/// non-dependent template name if we can perform the lookup now and identify
+/// the named template.
+///
+/// For example, given "x.MetaFun::template apply", the scope specifier
+/// \p SS will be "MetaFun::", \p TemplateKWLoc contains the location
+/// of the "template" keyword, and "apply" is the \p Name.
+TemplateNameKind Sema::ActOnTemplateName(Scope *S,
+                                         CXXScopeSpec &SS,
+                                         SourceLocation TemplateKWLoc,
+                                         const UnqualifiedId &Name,
+                                         ParsedType ObjectType,
+                                         bool EnteringContext,
+                                         TemplateTy &Result,
+                                         bool AllowInjectedClassName) {
+  if (TemplateKWLoc.isValid() && S && !S->getTemplateParamParent())
+    Diag(TemplateKWLoc,
+         getLangOpts().CPlusPlus11 ?
+           diag::warn_cxx98_compat_template_outside_of_template :
+           diag::ext_template_outside_of_template)
+      << FixItHint::CreateRemoval(TemplateKWLoc);
+
+  if (SS.isInvalid())
+    return TNK_Non_template;
+
+  // Figure out where isTemplateName is going to look.
+  DeclContext *LookupCtx = nullptr;
+  if (SS.isNotEmpty())
+    LookupCtx = computeDeclContext(SS, EnteringContext);
+  else if (ObjectType)
+    LookupCtx = computeDeclContext(GetTypeFromParser(ObjectType));
+
+  // C++0x [temp.names]p5:
+  //   If a name prefixed by the keyword template is not the name of
+  //   a template, the program is ill-formed. [Note: the keyword
+  //   template may not be applied to non-template members of class
+  //   templates. -end note ] [ Note: as is the case with the
+  //   typename prefix, the template prefix is allowed in cases
+  //   where it is not strictly necessary; i.e., when the
+  //   nested-name-specifier or the expression on the left of the ->
+  //   or . is not dependent on a template-parameter, or the use
+  //   does not appear in the scope of a template. -end note]
+  //
+  // Note: C++03 was more strict here, because it banned the use of
+  // the "template" keyword prior to a template-name that was not a
+  // dependent name. C++ DR468 relaxed this requirement (the
+  // "template" keyword is now permitted). We follow the C++0x
+  // rules, even in C++03 mode with a warning, retroactively applying the DR.
+  bool MemberOfUnknownSpecialization;
+  TemplateNameKind TNK = isTemplateName(S, SS, TemplateKWLoc.isValid(), Name,
+                                        ObjectType, EnteringContext, Result,
+                                        MemberOfUnknownSpecialization);
+  if (TNK != TNK_Non_template) {
+    // We resolved this to a (non-dependent) template name. Return it.
+    auto *LookupRD = dyn_cast_or_null<CXXRecordDecl>(LookupCtx);
+    if (!AllowInjectedClassName && SS.isNotEmpty() && LookupRD &&
+        Name.getKind() == UnqualifiedIdKind::IK_Identifier &&
+        Name.Identifier && LookupRD->getIdentifier() == Name.Identifier) {
+      // C++14 [class.qual]p2:
+      //   In a lookup in which function names are not ignored and the
+      //   nested-name-specifier nominates a class C, if the name specified
+      //   [...] is the injected-class-name of C, [...] the name is instead
+      //   considered to name the constructor
+      //
+      // We don't get here if naming the constructor would be valid, so we
+      // just reject immediately and recover by treating the
+      // injected-class-name as naming the template.
+      Diag(Name.getBeginLoc(),
+           diag::ext_out_of_line_qualified_id_type_names_constructor)
+          << Name.Identifier
+          << 0 /*injected-class-name used as template name*/
+          << TemplateKWLoc.isValid();
+    }
+    return TNK;
+  }
+
+  if (!MemberOfUnknownSpecialization) {
+    // Didn't find a template name, and the lookup wasn't dependent.
+    // Do the lookup again to determine if this is a "nothing found" case or
+    // a "not a template" case. FIXME: Refactor isTemplateName so we don't
+    // need to do this.
+    DeclarationNameInfo DNI = GetNameFromUnqualifiedId(Name);
+    LookupResult R(*this, DNI.getName(), Name.getBeginLoc(),
+                   LookupOrdinaryName);
+    bool MOUS;
+    // Tell LookupTemplateName that we require a template so that it diagnoses
+    // cases where it finds a non-template.
+    RequiredTemplateKind RTK = TemplateKWLoc.isValid()
+                                   ? RequiredTemplateKind(TemplateKWLoc)
+                                   : TemplateNameIsRequired;
+    if (!LookupTemplateName(R, S, SS, ObjectType.get(), EnteringContext, MOUS,
+                            RTK, nullptr, /*AllowTypoCorrection=*/false) &&
+        !R.isAmbiguous()) {
+      if (LookupCtx)
+        Diag(Name.getBeginLoc(), diag::err_no_member)
+            << DNI.getName() << LookupCtx << SS.getRange();
+      else
+        Diag(Name.getBeginLoc(), diag::err_undeclared_use)
+            << DNI.getName() << SS.getRange();
+    }
+    return TNK_Non_template;
+  }
+
+  NestedNameSpecifier *Qualifier = SS.getScopeRep();
+
+  switch (Name.getKind()) {
+  case UnqualifiedIdKind::IK_Identifier:
+    Result = TemplateTy::make(
+        Context.getDependentTemplateName(Qualifier, Name.Identifier));
+    return TNK_Dependent_template_name;
+
+  case UnqualifiedIdKind::IK_OperatorFunctionId:
+    Result = TemplateTy::make(Context.getDependentTemplateName(
+        Qualifier, Name.OperatorFunctionId.Operator));
+    return TNK_Function_template;
+
+  case UnqualifiedIdKind::IK_LiteralOperatorId:
+    // This is a kind of template name, but can never occur in a dependent
+    // scope (literal operators can only be declared at namespace scope).
+    break;
+
+  default:
+    break;
+  }
+
+  // This name cannot possibly name a dependent template. Diagnose this now
+  // rather than building a dependent template name that can never be valid.
+  Diag(Name.getBeginLoc(),
+       diag::err_template_kw_refers_to_dependent_non_template)
+      << GetNameFromUnqualifiedId(Name).getName() << Name.getSourceRange()
+      << TemplateKWLoc.isValid() << TemplateKWLoc;
+  return TNK_Non_template;
+}
+
+bool Sema::CheckTemplateTypeArgument(TemplateTypeParmDecl *Param,
+                                     TemplateArgumentLoc &AL,
+                          SmallVectorImpl<TemplateArgument> &Converted) {
+  const TemplateArgument &Arg = AL.getArgument();
+  QualType ArgType;
+  TypeSourceInfo *TSI = nullptr;
+
+  // Check template type parameter.
+  switch(Arg.getKind()) {
+  case TemplateArgument::Type:
+    // C++ [temp.arg.type]p1:
+    //   A template-argument for a template-parameter which is a
+    //   type shall be a type-id.
+    ArgType = Arg.getAsType();
+    TSI = AL.getTypeSourceInfo();
+    break;
+  case TemplateArgument::Template:
+  case TemplateArgument::TemplateExpansion: {
+    // We have a template type parameter but the template argument
+    // is a template without any arguments.
+    SourceRange SR = AL.getSourceRange();
+    TemplateName Name = Arg.getAsTemplateOrTemplatePattern();
+    diagnoseMissingTemplateArguments(Name, SR.getEnd());
+    return true;
+  }
+  case TemplateArgument::Expression: {
+    // We have a template type parameter but the template argument is an
+    // expression; see if maybe it is missing the "typename" keyword.
+    CXXScopeSpec SS;
+    DeclarationNameInfo NameInfo;
+
+   if (DependentScopeDeclRefExpr *ArgExpr =
+               dyn_cast<DependentScopeDeclRefExpr>(Arg.getAsExpr())) {
+      SS.Adopt(ArgExpr->getQualifierLoc());
+      NameInfo = ArgExpr->getNameInfo();
+    } else if (CXXDependentScopeMemberExpr *ArgExpr =
+               dyn_cast<CXXDependentScopeMemberExpr>(Arg.getAsExpr())) {
+      if (ArgExpr->isImplicitAccess()) {
+        SS.Adopt(ArgExpr->getQualifierLoc());
+        NameInfo = ArgExpr->getMemberNameInfo();
+      }
+    }
+
+    if (auto *II = NameInfo.getName().getAsIdentifierInfo()) {
+      LookupResult Result(*this, NameInfo, LookupOrdinaryName);
+      LookupParsedName(Result, CurScope, &SS);
+
+      if (Result.getAsSingle<TypeDecl>() ||
+          Result.getResultKind() ==
+              LookupResult::NotFoundInCurrentInstantiation) {
+        assert(SS.getScopeRep() && "dependent scope expr must has a scope!");
+        // Suggest that the user add 'typename' before the NNS.
+        SourceLocation Loc = AL.getSourceRange().getBegin();
+        Diag(Loc, getLangOpts().MSVCCompat
+                      ? diag::ext_ms_template_type_arg_missing_typename
+                      : diag::err_template_arg_must_be_type_suggest)
+            << FixItHint::CreateInsertion(Loc, "typename ");
+        Diag(Param->getLocation(), diag::note_template_param_here);
+
+        // Recover by synthesizing a type using the location information that we
+        // already have.
+        ArgType =
+            Context.getDependentNameType(ETK_Typename, SS.getScopeRep(), II);
+        TypeLocBuilder TLB;
+        DependentNameTypeLoc TL = TLB.push<DependentNameTypeLoc>(ArgType);
+        TL.setElaboratedKeywordLoc(SourceLocation(/*synthesized*/));
+        TL.setQualifierLoc(SS.getWithLocInContext(Context));
+        TL.setNameLoc(NameInfo.getLoc());
+        TSI = TLB.getTypeSourceInfo(Context, ArgType);
+
+        // Overwrite our input TemplateArgumentLoc so that we can recover
+        // properly.
+        AL = TemplateArgumentLoc(TemplateArgument(ArgType),
+                                 TemplateArgumentLocInfo(TSI));
+
+        break;
+      }
+    }
+    // fallthrough
+    LLVM_FALLTHROUGH;
+  }
+  default: {
+    // We have a template type parameter but the template argument
+    // is not a type.
+    SourceRange SR = AL.getSourceRange();
+    Diag(SR.getBegin(), diag::err_template_arg_must_be_type) << SR;
+    Diag(Param->getLocation(), diag::note_template_param_here);
+
+    return true;
+  }
+  }
+
+  if (CheckTemplateArgument(TSI))
+    return true;
+
+  // Add the converted template type argument.
+  ArgType = Context.getCanonicalType(ArgType);
+
+  // Objective-C ARC:
+  //   If an explicitly-specified template argument type is a lifetime type
+  //   with no lifetime qualifier, the __strong lifetime qualifier is inferred.
+  if (getLangOpts().ObjCAutoRefCount &&
+      ArgType->isObjCLifetimeType() &&
+      !ArgType.getObjCLifetime()) {
+    Qualifiers Qs;
+    Qs.setObjCLifetime(Qualifiers::OCL_Strong);
+    ArgType = Context.getQualifiedType(ArgType, Qs);
+  }
+
+  Converted.push_back(TemplateArgument(ArgType));
+  return false;
+}
+
+/// Substitute template arguments into the default template argument for
+/// the given template type parameter.
+///
+/// \param SemaRef the semantic analysis object for which we are performing
+/// the substitution.
+///
+/// \param Template the template that we are synthesizing template arguments
+/// for.
+///
+/// \param TemplateLoc the location of the template name that started the
+/// template-id we are checking.
+///
+/// \param RAngleLoc the location of the right angle bracket ('>') that
+/// terminates the template-id.
+///
+/// \param Param the template template parameter whose default we are
+/// substituting into.
+///
+/// \param Converted the list of template arguments provided for template
+/// parameters that precede \p Param in the template parameter list.
+/// \returns the substituted template argument, or NULL if an error occurred.
+static TypeSourceInfo *
+SubstDefaultTemplateArgument(Sema &SemaRef,
+                             TemplateDecl *Template,
+                             SourceLocation TemplateLoc,
+                             SourceLocation RAngleLoc,
+                             TemplateTypeParmDecl *Param,
+                             SmallVectorImpl<TemplateArgument> &Converted) {
+  TypeSourceInfo *ArgType = Param->getDefaultArgumentInfo();
+
+  // If the argument type is dependent, instantiate it now based
+  // on the previously-computed template arguments.
+  if (ArgType->getType()->isInstantiationDependentType()) {
+    Sema::InstantiatingTemplate Inst(SemaRef, TemplateLoc,
+                                     Param, Template, Converted,
+                                     SourceRange(TemplateLoc, RAngleLoc));
+    if (Inst.isInvalid())
+      return nullptr;
+
+    TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack, Converted);
+
+    // Only substitute for the innermost template argument list.
+    MultiLevelTemplateArgumentList TemplateArgLists;
+    TemplateArgLists.addOuterTemplateArguments(&TemplateArgs);
+    for (unsigned i = 0, e = Param->getDepth(); i != e; ++i)
+      TemplateArgLists.addOuterTemplateArguments(None);
+
+    bool ForLambdaCallOperator = false;
+    if (const auto *Rec = dyn_cast<CXXRecordDecl>(Template->getDeclContext()))
+      ForLambdaCallOperator = Rec->isLambda();
+    Sema::ContextRAII SavedContext(SemaRef, Template->getDeclContext(),
+                                   !ForLambdaCallOperator);
+    ArgType =
+        SemaRef.SubstType(ArgType, TemplateArgLists,
+                          Param->getDefaultArgumentLoc(), Param->getDeclName());
+  }
+
+  return ArgType;
+}
+
+/// Substitute template arguments into the default template argument for
+/// the given non-type template parameter.
+///
+/// \param SemaRef the semantic analysis object for which we are performing
+/// the substitution.
+///
+/// \param Template the template that we are synthesizing template arguments
+/// for.
+///
+/// \param TemplateLoc the location of the template name that started the
+/// template-id we are checking.
+///
+/// \param RAngleLoc the location of the right angle bracket ('>') that
+/// terminates the template-id.
+///
+/// \param Param the non-type template parameter whose default we are
+/// substituting into.
+///
+/// \param Converted the list of template arguments provided for template
+/// parameters that precede \p Param in the template parameter list.
+///
+/// \returns the substituted template argument, or NULL if an error occurred.
+static ExprResult
+SubstDefaultTemplateArgument(Sema &SemaRef,
+                             TemplateDecl *Template,
+                             SourceLocation TemplateLoc,
+                             SourceLocation RAngleLoc,
+                             NonTypeTemplateParmDecl *Param,
+                        SmallVectorImpl<TemplateArgument> &Converted) {
+  Sema::InstantiatingTemplate Inst(SemaRef, TemplateLoc,
+                                   Param, Template, Converted,
+                                   SourceRange(TemplateLoc, RAngleLoc));
+  if (Inst.isInvalid())
+    return ExprError();
+
+  TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack, Converted);
+
+  // Only substitute for the innermost template argument list.
+  MultiLevelTemplateArgumentList TemplateArgLists;
+  TemplateArgLists.addOuterTemplateArguments(&TemplateArgs);
+  for (unsigned i = 0, e = Param->getDepth(); i != e; ++i)
+    TemplateArgLists.addOuterTemplateArguments(None);
+
+  Sema::ContextRAII SavedContext(SemaRef, Template->getDeclContext());
+  EnterExpressionEvaluationContext ConstantEvaluated(
+      SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+  return SemaRef.SubstExpr(Param->getDefaultArgument(), TemplateArgLists);
+}
+
+/// Substitute template arguments into the default template argument for
+/// the given template template parameter.
+///
+/// \param SemaRef the semantic analysis object for which we are performing
+/// the substitution.
+///
+/// \param Template the template that we are synthesizing template arguments
+/// for.
+///
+/// \param TemplateLoc the location of the template name that started the
+/// template-id we are checking.
+///
+/// \param RAngleLoc the location of the right angle bracket ('>') that
+/// terminates the template-id.
+///
+/// \param Param the template template parameter whose default we are
+/// substituting into.
+///
+/// \param Converted the list of template arguments provided for template
+/// parameters that precede \p Param in the template parameter list.
+///
+/// \param QualifierLoc Will be set to the nested-name-specifier (with
+/// source-location information) that precedes the template name.
+///
+/// \returns the substituted template argument, or NULL if an error occurred.
+static TemplateName
+SubstDefaultTemplateArgument(Sema &SemaRef,
+                             TemplateDecl *Template,
+                             SourceLocation TemplateLoc,
+                             SourceLocation RAngleLoc,
+                             TemplateTemplateParmDecl *Param,
+                       SmallVectorImpl<TemplateArgument> &Converted,
+                             NestedNameSpecifierLoc &QualifierLoc) {
+  Sema::InstantiatingTemplate Inst(
+      SemaRef, TemplateLoc, TemplateParameter(Param), Template, Converted,
+      SourceRange(TemplateLoc, RAngleLoc));
+  if (Inst.isInvalid())
+    return TemplateName();
+
+  TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack, Converted);
+
+  // Only substitute for the innermost template argument list.
+  MultiLevelTemplateArgumentList TemplateArgLists;
+  TemplateArgLists.addOuterTemplateArguments(&TemplateArgs);
+  for (unsigned i = 0, e = Param->getDepth(); i != e; ++i)
+    TemplateArgLists.addOuterTemplateArguments(None);
+
+  Sema::ContextRAII SavedContext(SemaRef, Template->getDeclContext());
+  // Substitute into the nested-name-specifier first,
+  QualifierLoc = Param->getDefaultArgument().getTemplateQualifierLoc();
+  if (QualifierLoc) {
+    QualifierLoc =
+        SemaRef.SubstNestedNameSpecifierLoc(QualifierLoc, TemplateArgLists);
+    if (!QualifierLoc)
+      return TemplateName();
+  }
+
+  return SemaRef.SubstTemplateName(
+             QualifierLoc,
+             Param->getDefaultArgument().getArgument().getAsTemplate(),
+             Param->getDefaultArgument().getTemplateNameLoc(),
+             TemplateArgLists);
+}
+
+/// If the given template parameter has a default template
+/// argument, substitute into that default template argument and
+/// return the corresponding template argument.
+TemplateArgumentLoc
+Sema::SubstDefaultTemplateArgumentIfAvailable(TemplateDecl *Template,
+                                              SourceLocation TemplateLoc,
+                                              SourceLocation RAngleLoc,
+                                              Decl *Param,
+                                              SmallVectorImpl<TemplateArgument>
+                                                &Converted,
+                                              bool &HasDefaultArg) {
+  HasDefaultArg = false;
+
+  if (TemplateTypeParmDecl *TypeParm = dyn_cast<TemplateTypeParmDecl>(Param)) {
+    if (!hasVisibleDefaultArgument(TypeParm))
+      return TemplateArgumentLoc();
+
+    HasDefaultArg = true;
+    TypeSourceInfo *DI = SubstDefaultTemplateArgument(*this, Template,
+                                                      TemplateLoc,
+                                                      RAngleLoc,
+                                                      TypeParm,
+                                                      Converted);
+    if (DI)
+      return TemplateArgumentLoc(TemplateArgument(DI->getType()), DI);
+
+    return TemplateArgumentLoc();
+  }
+
+  if (NonTypeTemplateParmDecl *NonTypeParm
+        = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
+    if (!hasVisibleDefaultArgument(NonTypeParm))
+      return TemplateArgumentLoc();
+
+    HasDefaultArg = true;
+    ExprResult Arg = SubstDefaultTemplateArgument(*this, Template,
+                                                  TemplateLoc,
+                                                  RAngleLoc,
+                                                  NonTypeParm,
+                                                  Converted);
+    if (Arg.isInvalid())
+      return TemplateArgumentLoc();
+
+    Expr *ArgE = Arg.getAs<Expr>();
+    return TemplateArgumentLoc(TemplateArgument(ArgE), ArgE);
+  }
+
+  TemplateTemplateParmDecl *TempTempParm
+    = cast<TemplateTemplateParmDecl>(Param);
+  if (!hasVisibleDefaultArgument(TempTempParm))
+    return TemplateArgumentLoc();
+
+  HasDefaultArg = true;
+  NestedNameSpecifierLoc QualifierLoc;
+  TemplateName TName = SubstDefaultTemplateArgument(*this, Template,
+                                                    TemplateLoc,
+                                                    RAngleLoc,
+                                                    TempTempParm,
+                                                    Converted,
+                                                    QualifierLoc);
+  if (TName.isNull())
+    return TemplateArgumentLoc();
+
+  return TemplateArgumentLoc(
+      Context, TemplateArgument(TName),
+      TempTempParm->getDefaultArgument().getTemplateQualifierLoc(),
+      TempTempParm->getDefaultArgument().getTemplateNameLoc());
+}
+
+/// Convert a template-argument that we parsed as a type into a template, if
+/// possible. C++ permits injected-class-names to perform dual service as
+/// template template arguments and as template type arguments.
+static TemplateArgumentLoc
+convertTypeTemplateArgumentToTemplate(ASTContext &Context, TypeLoc TLoc) {
+  // Extract and step over any surrounding nested-name-specifier.
+  NestedNameSpecifierLoc QualLoc;
+  if (auto ETLoc = TLoc.getAs<ElaboratedTypeLoc>()) {
+    if (ETLoc.getTypePtr()->getKeyword() != ETK_None)
+      return TemplateArgumentLoc();
+
+    QualLoc = ETLoc.getQualifierLoc();
+    TLoc = ETLoc.getNamedTypeLoc();
+  }
+  // If this type was written as an injected-class-name, it can be used as a
+  // template template argument.
+  if (auto InjLoc = TLoc.getAs<InjectedClassNameTypeLoc>())
+    return TemplateArgumentLoc(Context, InjLoc.getTypePtr()->getTemplateName(),
+                               QualLoc, InjLoc.getNameLoc());
+
+  // If this type was written as an injected-class-name, it may have been
+  // converted to a RecordType during instantiation. If the RecordType is
+  // *not* wrapped in a TemplateSpecializationType and denotes a class
+  // template specialization, it must have come from an injected-class-name.
+  if (auto RecLoc = TLoc.getAs<RecordTypeLoc>())
+    if (auto *CTSD =
+            dyn_cast<ClassTemplateSpecializationDecl>(RecLoc.getDecl()))
+      return TemplateArgumentLoc(Context,
+                                 TemplateName(CTSD->getSpecializedTemplate()),
+                                 QualLoc, RecLoc.getNameLoc());
+
+  return TemplateArgumentLoc();
+}
+
+/// Check that the given template argument corresponds to the given
+/// template parameter.
+///
+/// \param Param The template parameter against which the argument will be
+/// checked.
+///
+/// \param Arg The template argument, which may be updated due to conversions.
+///
+/// \param Template The template in which the template argument resides.
+///
+/// \param TemplateLoc The location of the template name for the template
+/// whose argument list we're matching.
+///
+/// \param RAngleLoc The location of the right angle bracket ('>') that closes
+/// the template argument list.
+///
+/// \param ArgumentPackIndex The index into the argument pack where this
+/// argument will be placed. Only valid if the parameter is a parameter pack.
+///
+/// \param Converted The checked, converted argument will be added to the
+/// end of this small vector.
+///
+/// \param CTAK Describes how we arrived at this particular template argument:
+/// explicitly written, deduced, etc.
+///
+/// \returns true on error, false otherwise.
+bool Sema::CheckTemplateArgument(NamedDecl *Param,
+                                 TemplateArgumentLoc &Arg,
+                                 NamedDecl *Template,
+                                 SourceLocation TemplateLoc,
+                                 SourceLocation RAngleLoc,
+                                 unsigned ArgumentPackIndex,
+                            SmallVectorImpl<TemplateArgument> &Converted,
+                                 CheckTemplateArgumentKind CTAK) {
+  // Check template type parameters.
+  if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(Param))
+    return CheckTemplateTypeArgument(TTP, Arg, Converted);
+
+  // Check non-type template parameters.
+  if (NonTypeTemplateParmDecl *NTTP =dyn_cast<NonTypeTemplateParmDecl>(Param)) {
+    // Do substitution on the type of the non-type template parameter
+    // with the template arguments we've seen thus far.  But if the
+    // template has a dependent context then we cannot substitute yet.
+    QualType NTTPType = NTTP->getType();
+    if (NTTP->isParameterPack() && NTTP->isExpandedParameterPack())
+      NTTPType = NTTP->getExpansionType(ArgumentPackIndex);
+
+    if (NTTPType->isInstantiationDependentType() &&
+        !isa<TemplateTemplateParmDecl>(Template) &&
+        !Template->getDeclContext()->isDependentContext()) {
+      // Do substitution on the type of the non-type template parameter.
+      InstantiatingTemplate Inst(*this, TemplateLoc, Template,
+                                 NTTP, Converted,
+                                 SourceRange(TemplateLoc, RAngleLoc));
+      if (Inst.isInvalid())
+        return true;
+
+      TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack,
+                                        Converted);
+
+      // If the parameter is a pack expansion, expand this slice of the pack.
+      if (auto *PET = NTTPType->getAs<PackExpansionType>()) {
+        Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(*this,
+                                                           ArgumentPackIndex);
+        NTTPType = SubstType(PET->getPattern(),
+                             MultiLevelTemplateArgumentList(TemplateArgs),
+                             NTTP->getLocation(),
+                             NTTP->getDeclName());
+      } else {
+        NTTPType = SubstType(NTTPType,
+                             MultiLevelTemplateArgumentList(TemplateArgs),
+                             NTTP->getLocation(),
+                             NTTP->getDeclName());
+      }
+
+      // If that worked, check the non-type template parameter type
+      // for validity.
+      if (!NTTPType.isNull())
+        NTTPType = CheckNonTypeTemplateParameterType(NTTPType,
+                                                     NTTP->getLocation());
+      if (NTTPType.isNull())
+        return true;
+    }
+
+    switch (Arg.getArgument().getKind()) {
+    case TemplateArgument::Null:
+      llvm_unreachable("Should never see a NULL template argument here");
+
+    case TemplateArgument::Expression: {
+      TemplateArgument Result;
+      unsigned CurSFINAEErrors = NumSFINAEErrors;
+      ExprResult Res =
+        CheckTemplateArgument(NTTP, NTTPType, Arg.getArgument().getAsExpr(),
+                              Result, CTAK);
+      if (Res.isInvalid())
+        return true;
+      // If the current template argument causes an error, give up now.
+      if (CurSFINAEErrors < NumSFINAEErrors)
+        return true;
+
+      // If the resulting expression is new, then use it in place of the
+      // old expression in the template argument.
+      if (Res.get() != Arg.getArgument().getAsExpr()) {
+        TemplateArgument TA(Res.get());
+        Arg = TemplateArgumentLoc(TA, Res.get());
+      }
+
+      Converted.push_back(Result);
+      break;
+    }
+
+    case TemplateArgument::Declaration:
+    case TemplateArgument::Integral:
+    case TemplateArgument::NullPtr:
+      // We've already checked this template argument, so just copy
+      // it to the list of converted arguments.
+      Converted.push_back(Arg.getArgument());
+      break;
+
+    case TemplateArgument::Template:
+    case TemplateArgument::TemplateExpansion:
+      // We were given a template template argument. It may not be ill-formed;
+      // see below.
+      if (DependentTemplateName *DTN
+            = Arg.getArgument().getAsTemplateOrTemplatePattern()
+                                              .getAsDependentTemplateName()) {
+        // We have a template argument such as \c T::template X, which we
+        // parsed as a template template argument. However, since we now
+        // know that we need a non-type template argument, convert this
+        // template name into an expression.
+
+        DeclarationNameInfo NameInfo(DTN->getIdentifier(),
+                                     Arg.getTemplateNameLoc());
+
+        CXXScopeSpec SS;
+        SS.Adopt(Arg.getTemplateQualifierLoc());
+        // FIXME: the template-template arg was a DependentTemplateName,
+        // so it was provided with a template keyword. However, its source
+        // location is not stored in the template argument structure.
+        SourceLocation TemplateKWLoc;
+        ExprResult E = DependentScopeDeclRefExpr::Create(
+            Context, SS.getWithLocInContext(Context), TemplateKWLoc, NameInfo,
+            nullptr);
+
+        // If we parsed the template argument as a pack expansion, create a
+        // pack expansion expression.
+        if (Arg.getArgument().getKind() == TemplateArgument::TemplateExpansion){
+          E = ActOnPackExpansion(E.get(), Arg.getTemplateEllipsisLoc());
+          if (E.isInvalid())
+            return true;
+        }
+
+        TemplateArgument Result;
+        E = CheckTemplateArgument(NTTP, NTTPType, E.get(), Result);
+        if (E.isInvalid())
+          return true;
+
+        Converted.push_back(Result);
+        break;
+      }
+
+      // We have a template argument that actually does refer to a class
+      // template, alias template, or template template parameter, and
+      // therefore cannot be a non-type template argument.
+      Diag(Arg.getLocation(), diag::err_template_arg_must_be_expr)
+        << Arg.getSourceRange();
+
+      Diag(Param->getLocation(), diag::note_template_param_here);
+      return true;
+
+    case TemplateArgument::Type: {
+      // We have a non-type template parameter but the template
+      // argument is a type.
+
+      // C++ [temp.arg]p2:
+      //   In a template-argument, an ambiguity between a type-id and
+      //   an expression is resolved to a type-id, regardless of the
+      //   form of the corresponding template-parameter.
+      //
+      // We warn specifically about this case, since it can be rather
+      // confusing for users.
+      QualType T = Arg.getArgument().getAsType();
+      SourceRange SR = Arg.getSourceRange();
+      if (T->isFunctionType())
+        Diag(SR.getBegin(), diag::err_template_arg_nontype_ambig) << SR << T;
+      else
+        Diag(SR.getBegin(), diag::err_template_arg_must_be_expr) << SR;
+      Diag(Param->getLocation(), diag::note_template_param_here);
+      return true;
+    }
+
+    case TemplateArgument::Pack:
+      llvm_unreachable("Caller must expand template argument packs");
+    }
+
+    return false;
+  }
+
+
+  // Check template template parameters.
+  TemplateTemplateParmDecl *TempParm = cast<TemplateTemplateParmDecl>(Param);
+
+  TemplateParameterList *Params = TempParm->getTemplateParameters();
+  if (TempParm->isExpandedParameterPack())
+    Params = TempParm->getExpansionTemplateParameters(ArgumentPackIndex);
+
+  // Substitute into the template parameter list of the template
+  // template parameter, since previously-supplied template arguments
+  // may appear within the template template parameter.
+  //
+  // FIXME: Skip this if the parameters aren't instantiation-dependent.
+  {
+    // Set up a template instantiation context.
+    LocalInstantiationScope Scope(*this);
+    InstantiatingTemplate Inst(*this, TemplateLoc, Template,
+                               TempParm, Converted,
+                               SourceRange(TemplateLoc, RAngleLoc));
+    if (Inst.isInvalid())
+      return true;
+
+    TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack, Converted);
+    Params = SubstTemplateParams(Params, CurContext,
+                                 MultiLevelTemplateArgumentList(TemplateArgs));
+    if (!Params)
+      return true;
+  }
+
+  // C++1z [temp.local]p1: (DR1004)
+  //   When [the injected-class-name] is used [...] as a template-argument for
+  //   a template template-parameter [...] it refers to the class template
+  //   itself.
+  if (Arg.getArgument().getKind() == TemplateArgument::Type) {
+    TemplateArgumentLoc ConvertedArg = convertTypeTemplateArgumentToTemplate(
+        Context, Arg.getTypeSourceInfo()->getTypeLoc());
+    if (!ConvertedArg.getArgument().isNull())
+      Arg = ConvertedArg;
+  }
+
+  switch (Arg.getArgument().getKind()) {
+  case TemplateArgument::Null:
+    llvm_unreachable("Should never see a NULL template argument here");
+
+  case TemplateArgument::Template:
+  case TemplateArgument::TemplateExpansion:
+    if (CheckTemplateTemplateArgument(TempParm, Params, Arg))
+      return true;
+
+    Converted.push_back(Arg.getArgument());
+    break;
+
+  case TemplateArgument::Expression:
+  case TemplateArgument::Type:
+    // We have a template template parameter but the template
+    // argument does not refer to a template.
+    Diag(Arg.getLocation(), diag::err_template_arg_must_be_template)
+      << getLangOpts().CPlusPlus11;
+    return true;
+
+  case TemplateArgument::Declaration:
+    llvm_unreachable("Declaration argument with template template parameter");
+  case TemplateArgument::Integral:
+    llvm_unreachable("Integral argument with template template parameter");
+  case TemplateArgument::NullPtr:
+    llvm_unreachable("Null pointer argument with template template parameter");
+
+  case TemplateArgument::Pack:
+    llvm_unreachable("Caller must expand template argument packs");
+  }
+
+  return false;
+}
+
+/// Diagnose a missing template argument.
+template<typename TemplateParmDecl>
+static bool diagnoseMissingArgument(Sema &S, SourceLocation Loc,
+                                    TemplateDecl *TD,
+                                    const TemplateParmDecl *D,
+                                    TemplateArgumentListInfo &Args) {
+  // Dig out the most recent declaration of the template parameter; there may be
+  // declarations of the template that are more recent than TD.
+  D = cast<TemplateParmDecl>(cast<TemplateDecl>(TD->getMostRecentDecl())
+                                 ->getTemplateParameters()
+                                 ->getParam(D->getIndex()));
+
+  // If there's a default argument that's not visible, diagnose that we're
+  // missing a module import.
+  llvm::SmallVector<Module*, 8> Modules;
+  if (D->hasDefaultArgument() && !S.hasVisibleDefaultArgument(D, &Modules)) {
+    S.diagnoseMissingImport(Loc, cast<NamedDecl>(TD),
+                            D->getDefaultArgumentLoc(), Modules,
+                            Sema::MissingImportKind::DefaultArgument,
+                            /*Recover*/true);
+    return true;
+  }
+
+  // FIXME: If there's a more recent default argument that *is* visible,
+  // diagnose that it was declared too late.
+
+  TemplateParameterList *Params = TD->getTemplateParameters();
+
+  S.Diag(Loc, diag::err_template_arg_list_different_arity)
+    << /*not enough args*/0
+    << (int)S.getTemplateNameKindForDiagnostics(TemplateName(TD))
+    << TD;
+  S.Diag(TD->getLocation(), diag::note_template_decl_here)
+    << Params->getSourceRange();
+  return true;
+}
+
+/// Check that the given template argument list is well-formed
+/// for specializing the given template.
+bool Sema::CheckTemplateArgumentList(
+    TemplateDecl *Template, SourceLocation TemplateLoc,
+    TemplateArgumentListInfo &TemplateArgs, bool PartialTemplateArgs,
+    SmallVectorImpl<TemplateArgument> &Converted,
+    bool UpdateArgsWithConversions, bool *ConstraintsNotSatisfied) {
+
+  if (ConstraintsNotSatisfied)
+    *ConstraintsNotSatisfied = false;
+
+  // Make a copy of the template arguments for processing.  Only make the
+  // changes at the end when successful in matching the arguments to the
+  // template.
+  TemplateArgumentListInfo NewArgs = TemplateArgs;
+
+  // Make sure we get the template parameter list from the most
+  // recent declaration, since that is the only one that is guaranteed to
+  // have all the default template argument information.
+  TemplateParameterList *Params =
+      cast<TemplateDecl>(Template->getMostRecentDecl())
+          ->getTemplateParameters();
+
+  SourceLocation RAngleLoc = NewArgs.getRAngleLoc();
+
+  // C++ [temp.arg]p1:
+  //   [...] The type and form of each template-argument specified in
+  //   a template-id shall match the type and form specified for the
+  //   corresponding parameter declared by the template in its
+  //   template-parameter-list.
+  bool isTemplateTemplateParameter = isa<TemplateTemplateParmDecl>(Template);
+  SmallVector<TemplateArgument, 2> ArgumentPack;
+  unsigned ArgIdx = 0, NumArgs = NewArgs.size();
+  LocalInstantiationScope InstScope(*this, true);
+  for (TemplateParameterList::iterator Param = Params->begin(),
+                                       ParamEnd = Params->end();
+       Param != ParamEnd; /* increment in loop */) {
+    // If we have an expanded parameter pack, make sure we don't have too
+    // many arguments.
+    if (Optional<unsigned> Expansions = getExpandedPackSize(*Param)) {
+      if (*Expansions == ArgumentPack.size()) {
+        // We're done with this parameter pack. Pack up its arguments and add
+        // them to the list.
+        Converted.push_back(
+            TemplateArgument::CreatePackCopy(Context, ArgumentPack));
+        ArgumentPack.clear();
+
+        // This argument is assigned to the next parameter.
+        ++Param;
+        continue;
+      } else if (ArgIdx == NumArgs && !PartialTemplateArgs) {
+        // Not enough arguments for this parameter pack.
+        Diag(TemplateLoc, diag::err_template_arg_list_different_arity)
+          << /*not enough args*/0
+          << (int)getTemplateNameKindForDiagnostics(TemplateName(Template))
+          << Template;
+        Diag(Template->getLocation(), diag::note_template_decl_here)
+          << Params->getSourceRange();
+        return true;
+      }
+    }
+
+    if (ArgIdx < NumArgs) {
+      // Check the template argument we were given.
+      if (CheckTemplateArgument(*Param, NewArgs[ArgIdx], Template,
+                                TemplateLoc, RAngleLoc,
+                                ArgumentPack.size(), Converted))
+        return true;
+
+      bool PackExpansionIntoNonPack =
+          NewArgs[ArgIdx].getArgument().isPackExpansion() &&
+          (!(*Param)->isTemplateParameterPack() || getExpandedPackSize(*Param));
+      if (PackExpansionIntoNonPack && (isa<TypeAliasTemplateDecl>(Template) ||
+                                       isa<ConceptDecl>(Template))) {
+        // Core issue 1430: we have a pack expansion as an argument to an
+        // alias template, and it's not part of a parameter pack. This
+        // can't be canonicalized, so reject it now.
+        // As for concepts - we cannot normalize constraints where this
+        // situation exists.
+        Diag(NewArgs[ArgIdx].getLocation(),
+             diag::err_template_expansion_into_fixed_list)
+          << (isa<ConceptDecl>(Template) ? 1 : 0)
+          << NewArgs[ArgIdx].getSourceRange();
+        Diag((*Param)->getLocation(), diag::note_template_param_here);
+        return true;
+      }
+
+      // We're now done with this argument.
+      ++ArgIdx;
+
+      if ((*Param)->isTemplateParameterPack()) {
+        // The template parameter was a template parameter pack, so take the
+        // deduced argument and place it on the argument pack. Note that we
+        // stay on the same template parameter so that we can deduce more
+        // arguments.
+        ArgumentPack.push_back(Converted.pop_back_val());
+      } else {
+        // Move to the next template parameter.
+        ++Param;
+      }
+
+      // If we just saw a pack expansion into a non-pack, then directly convert
+      // the remaining arguments, because we don't know what parameters they'll
+      // match up with.
+      if (PackExpansionIntoNonPack) {
+        if (!ArgumentPack.empty()) {
+          // If we were part way through filling in an expanded parameter pack,
+          // fall back to just producing individual arguments.
+          Converted.insert(Converted.end(),
+                           ArgumentPack.begin(), ArgumentPack.end());
+          ArgumentPack.clear();
+        }
+
+        while (ArgIdx < NumArgs) {
+          Converted.push_back(NewArgs[ArgIdx].getArgument());
+          ++ArgIdx;
+        }
+
+        return false;
+      }
+
+      continue;
+    }
+
+    // If we're checking a partial template argument list, we're done.
+    if (PartialTemplateArgs) {
+      if ((*Param)->isTemplateParameterPack() && !ArgumentPack.empty())
+        Converted.push_back(
+            TemplateArgument::CreatePackCopy(Context, ArgumentPack));
+      return false;
+    }
+
+    // If we have a template parameter pack with no more corresponding
+    // arguments, just break out now and we'll fill in the argument pack below.
+    if ((*Param)->isTemplateParameterPack()) {
+      assert(!getExpandedPackSize(*Param) &&
+             "Should have dealt with this already");
+
+      // A non-expanded parameter pack before the end of the parameter list
+      // only occurs for an ill-formed template parameter list, unless we've
+      // got a partial argument list for a function template, so just bail out.
+      if (Param + 1 != ParamEnd)
+        return true;
+
+      Converted.push_back(
+          TemplateArgument::CreatePackCopy(Context, ArgumentPack));
+      ArgumentPack.clear();
+
+      ++Param;
+      continue;
+    }
+
+    // Check whether we have a default argument.
+    TemplateArgumentLoc Arg;
+
+    // Retrieve the default template argument from the template
+    // parameter. For each kind of template parameter, we substitute the
+    // template arguments provided thus far and any "outer" template arguments
+    // (when the template parameter was part of a nested template) into
+    // the default argument.
+    if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(*Param)) {
+      if (!hasVisibleDefaultArgument(TTP))
+        return diagnoseMissingArgument(*this, TemplateLoc, Template, TTP,
+                                       NewArgs);
+
+      TypeSourceInfo *ArgType = SubstDefaultTemplateArgument(*this,
+                                                             Template,
+                                                             TemplateLoc,
+                                                             RAngleLoc,
+                                                             TTP,
+                                                             Converted);
+      if (!ArgType)
+        return true;
+
+      Arg = TemplateArgumentLoc(TemplateArgument(ArgType->getType()),
+                                ArgType);
+    } else if (NonTypeTemplateParmDecl *NTTP
+                 = dyn_cast<NonTypeTemplateParmDecl>(*Param)) {
+      if (!hasVisibleDefaultArgument(NTTP))
+        return diagnoseMissingArgument(*this, TemplateLoc, Template, NTTP,
+                                       NewArgs);
+
+      ExprResult E = SubstDefaultTemplateArgument(*this, Template,
+                                                              TemplateLoc,
+                                                              RAngleLoc,
+                                                              NTTP,
+                                                              Converted);
+      if (E.isInvalid())
+        return true;
+
+      Expr *Ex = E.getAs<Expr>();
+      Arg = TemplateArgumentLoc(TemplateArgument(Ex), Ex);
+    } else {
+      TemplateTemplateParmDecl *TempParm
+        = cast<TemplateTemplateParmDecl>(*Param);
+
+      if (!hasVisibleDefaultArgument(TempParm))
+        return diagnoseMissingArgument(*this, TemplateLoc, Template, TempParm,
+                                       NewArgs);
+
+      NestedNameSpecifierLoc QualifierLoc;
+      TemplateName Name = SubstDefaultTemplateArgument(*this, Template,
+                                                       TemplateLoc,
+                                                       RAngleLoc,
+                                                       TempParm,
+                                                       Converted,
+                                                       QualifierLoc);
+      if (Name.isNull())
+        return true;
+
+      Arg = TemplateArgumentLoc(
+          Context, TemplateArgument(Name), QualifierLoc,
+          TempParm->getDefaultArgument().getTemplateNameLoc());
+    }
+
+    // Introduce an instantiation record that describes where we are using
+    // the default template argument. We're not actually instantiating a
+    // template here, we just create this object to put a note into the
+    // context stack.
+    InstantiatingTemplate Inst(*this, RAngleLoc, Template, *Param, Converted,
+                               SourceRange(TemplateLoc, RAngleLoc));
+    if (Inst.isInvalid())
+      return true;
+
+    // Check the default template argument.
+    if (CheckTemplateArgument(*Param, Arg, Template, TemplateLoc,
+                              RAngleLoc, 0, Converted))
+      return true;
+
+    // Core issue 150 (assumed resolution): if this is a template template
+    // parameter, keep track of the default template arguments from the
+    // template definition.
+    if (isTemplateTemplateParameter)
+      NewArgs.addArgument(Arg);
+
+    // Move to the next template parameter and argument.
+    ++Param;
+    ++ArgIdx;
+  }
+
+  // If we're performing a partial argument substitution, allow any trailing
+  // pack expansions; they might be empty. This can happen even if
+  // PartialTemplateArgs is false (the list of arguments is complete but
+  // still dependent).
+  if (ArgIdx < NumArgs && CurrentInstantiationScope &&
+      CurrentInstantiationScope->getPartiallySubstitutedPack()) {
+    while (ArgIdx < NumArgs && NewArgs[ArgIdx].getArgument().isPackExpansion())
+      Converted.push_back(NewArgs[ArgIdx++].getArgument());
+  }
+
+  // If we have any leftover arguments, then there were too many arguments.
+  // Complain and fail.
+  if (ArgIdx < NumArgs) {
+    Diag(TemplateLoc, diag::err_template_arg_list_different_arity)
+        << /*too many args*/1
+        << (int)getTemplateNameKindForDiagnostics(TemplateName(Template))
+        << Template
+        << SourceRange(NewArgs[ArgIdx].getLocation(), NewArgs.getRAngleLoc());
+    Diag(Template->getLocation(), diag::note_template_decl_here)
+        << Params->getSourceRange();
+    return true;
+  }
+
+  // No problems found with the new argument list, propagate changes back
+  // to caller.
+  if (UpdateArgsWithConversions)
+    TemplateArgs = std::move(NewArgs);
+
+  if (!PartialTemplateArgs &&
+      EnsureTemplateArgumentListConstraints(
+        Template, Converted, SourceRange(TemplateLoc,
+                                         TemplateArgs.getRAngleLoc()))) {
+    if (ConstraintsNotSatisfied)
+      *ConstraintsNotSatisfied = true;
+    return true;
+  }
+
+  return false;
+}
+
+namespace {
+  class UnnamedLocalNoLinkageFinder
+    : public TypeVisitor<UnnamedLocalNoLinkageFinder, bool>
+  {
+    Sema &S;
+    SourceRange SR;
+
+    typedef TypeVisitor<UnnamedLocalNoLinkageFinder, bool> inherited;
+
+  public:
+    UnnamedLocalNoLinkageFinder(Sema &S, SourceRange SR) : S(S), SR(SR) { }
+
+    bool Visit(QualType T) {
+      return T.isNull() ? false : inherited::Visit(T.getTypePtr());
+    }
+
+#define TYPE(Class, Parent) \
+    bool Visit##Class##Type(const Class##Type *);
+#define ABSTRACT_TYPE(Class, Parent) \
+    bool Visit##Class##Type(const Class##Type *) { return false; }
+#define NON_CANONICAL_TYPE(Class, Parent) \
+    bool Visit##Class##Type(const Class##Type *) { return false; }
+#include "clang/AST/TypeNodes.inc"
+
+    bool VisitTagDecl(const TagDecl *Tag);
+    bool VisitNestedNameSpecifier(NestedNameSpecifier *NNS);
+  };
+} // end anonymous namespace
+
+bool UnnamedLocalNoLinkageFinder::VisitBuiltinType(const BuiltinType*) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitComplexType(const ComplexType* T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitPointerType(const PointerType* T) {
+  return Visit(T->getPointeeType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitBlockPointerType(
+                                                    const BlockPointerType* T) {
+  return Visit(T->getPointeeType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitLValueReferenceType(
+                                                const LValueReferenceType* T) {
+  return Visit(T->getPointeeType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitRValueReferenceType(
+                                                const RValueReferenceType* T) {
+  return Visit(T->getPointeeType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitMemberPointerType(
+                                                  const MemberPointerType* T) {
+  return Visit(T->getPointeeType()) || Visit(QualType(T->getClass(), 0));
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitConstantArrayType(
+                                                  const ConstantArrayType* T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitIncompleteArrayType(
+                                                 const IncompleteArrayType* T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitVariableArrayType(
+                                                   const VariableArrayType* T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitDependentSizedArrayType(
+                                            const DependentSizedArrayType* T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitDependentSizedExtVectorType(
+                                         const DependentSizedExtVectorType* T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitDependentSizedMatrixType(
+    const DependentSizedMatrixType *T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitDependentAddressSpaceType(
+    const DependentAddressSpaceType *T) {
+  return Visit(T->getPointeeType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitVectorType(const VectorType* T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitDependentVectorType(
+    const DependentVectorType *T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitExtVectorType(const ExtVectorType* T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitConstantMatrixType(
+    const ConstantMatrixType *T) {
+  return Visit(T->getElementType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitFunctionProtoType(
+                                                  const FunctionProtoType* T) {
+  for (const auto &A : T->param_types()) {
+    if (Visit(A))
+      return true;
+  }
+
+  return Visit(T->getReturnType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitFunctionNoProtoType(
+                                               const FunctionNoProtoType* T) {
+  return Visit(T->getReturnType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitUnresolvedUsingType(
+                                                  const UnresolvedUsingType*) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitTypeOfExprType(const TypeOfExprType*) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitTypeOfType(const TypeOfType* T) {
+  return Visit(T->getUnderlyingType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitDecltypeType(const DecltypeType*) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitUnaryTransformType(
+                                                    const UnaryTransformType*) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitAutoType(const AutoType *T) {
+  return Visit(T->getDeducedType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitDeducedTemplateSpecializationType(
+    const DeducedTemplateSpecializationType *T) {
+  return Visit(T->getDeducedType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitRecordType(const RecordType* T) {
+  return VisitTagDecl(T->getDecl());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitEnumType(const EnumType* T) {
+  return VisitTagDecl(T->getDecl());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitTemplateTypeParmType(
+                                                 const TemplateTypeParmType*) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitSubstTemplateTypeParmPackType(
+                                        const SubstTemplateTypeParmPackType *) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitTemplateSpecializationType(
+                                            const TemplateSpecializationType*) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitInjectedClassNameType(
+                                              const InjectedClassNameType* T) {
+  return VisitTagDecl(T->getDecl());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitDependentNameType(
+                                                   const DependentNameType* T) {
+  return VisitNestedNameSpecifier(T->getQualifier());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitDependentTemplateSpecializationType(
+                                 const DependentTemplateSpecializationType* T) {
+  if (auto *Q = T->getQualifier())
+    return VisitNestedNameSpecifier(Q);
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitPackExpansionType(
+                                                   const PackExpansionType* T) {
+  return Visit(T->getPattern());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitObjCObjectType(const ObjCObjectType *) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitObjCInterfaceType(
+                                                   const ObjCInterfaceType *) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitObjCObjectPointerType(
+                                                const ObjCObjectPointerType *) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitAtomicType(const AtomicType* T) {
+  return Visit(T->getValueType());
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitPipeType(const PipeType* T) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitBitIntType(const BitIntType *T) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitDependentBitIntType(
+    const DependentBitIntType *T) {
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitTagDecl(const TagDecl *Tag) {
+  if (Tag->getDeclContext()->isFunctionOrMethod()) {
+    S.Diag(SR.getBegin(),
+           S.getLangOpts().CPlusPlus11 ?
+             diag::warn_cxx98_compat_template_arg_local_type :
+             diag::ext_template_arg_local_type)
+      << S.Context.getTypeDeclType(Tag) << SR;
+    return true;
+  }
+
+  if (!Tag->hasNameForLinkage()) {
+    S.Diag(SR.getBegin(),
+           S.getLangOpts().CPlusPlus11 ?
+             diag::warn_cxx98_compat_template_arg_unnamed_type :
+             diag::ext_template_arg_unnamed_type) << SR;
+    S.Diag(Tag->getLocation(), diag::note_template_unnamed_type_here);
+    return true;
+  }
+
+  return false;
+}
+
+bool UnnamedLocalNoLinkageFinder::VisitNestedNameSpecifier(
+                                                    NestedNameSpecifier *NNS) {
+  assert(NNS);
+  if (NNS->getPrefix() && VisitNestedNameSpecifier(NNS->getPrefix()))
+    return true;
+
+  switch (NNS->getKind()) {
+  case NestedNameSpecifier::Identifier:
+  case NestedNameSpecifier::Namespace:
+  case NestedNameSpecifier::NamespaceAlias:
+  case NestedNameSpecifier::Global:
+  case NestedNameSpecifier::Super:
+    return false;
+
+  case NestedNameSpecifier::TypeSpec:
+  case NestedNameSpecifier::TypeSpecWithTemplate:
+    return Visit(QualType(NNS->getAsType(), 0));
+  }
+  llvm_unreachable("Invalid NestedNameSpecifier::Kind!");
+}
+
+/// Check a template argument against its corresponding
+/// template type parameter.
+///
+/// This routine implements the semantics of C++ [temp.arg.type]. It
+/// returns true if an error occurred, and false otherwise.
+bool Sema::CheckTemplateArgument(TypeSourceInfo *ArgInfo) {
+  assert(ArgInfo && "invalid TypeSourceInfo");
+  QualType Arg = ArgInfo->getType();
+  SourceRange SR = ArgInfo->getTypeLoc().getSourceRange();
+
+  if (Arg->isVariablyModifiedType()) {
+    return Diag(SR.getBegin(), diag::err_variably_modified_template_arg) << Arg;
+  } else if (Context.hasSameUnqualifiedType(Arg, Context.OverloadTy)) {
+    return Diag(SR.getBegin(), diag::err_template_arg_overload_type) << SR;
+  }
+
+  // C++03 [temp.arg.type]p2:
+  //   A local type, a type with no linkage, an unnamed type or a type
+  //   compounded from any of these types shall not be used as a
+  //   template-argument for a template type-parameter.
+  //
+  // C++11 allows these, and even in C++03 we allow them as an extension with
+  // a warning.
+  if (LangOpts.CPlusPlus11 || Arg->hasUnnamedOrLocalType()) {
+    UnnamedLocalNoLinkageFinder Finder(*this, SR);
+    (void)Finder.Visit(Context.getCanonicalType(Arg));
+  }
+
+  return false;
+}
+
+enum NullPointerValueKind {
+  NPV_NotNullPointer,
+  NPV_NullPointer,
+  NPV_Error
+};
+
+/// Determine whether the given template argument is a null pointer
+/// value of the appropriate type.
+static NullPointerValueKind
+isNullPointerValueTemplateArgument(Sema &S, NonTypeTemplateParmDecl *Param,
+                                   QualType ParamType, Expr *Arg,
+                                   Decl *Entity = nullptr) {
+  if (Arg->isValueDependent() || Arg->isTypeDependent())
+    return NPV_NotNullPointer;
+
+  // dllimport'd entities aren't constant but are available inside of template
+  // arguments.
+  if (Entity && Entity->hasAttr<DLLImportAttr>())
+    return NPV_NotNullPointer;
+
+  if (!S.isCompleteType(Arg->getExprLoc(), ParamType))
+    llvm_unreachable(
+        "Incomplete parameter type in isNullPointerValueTemplateArgument!");
+
+  if (!S.getLangOpts().CPlusPlus11)
+    return NPV_NotNullPointer;
+
+  // Determine whether we have a constant expression.
+  ExprResult ArgRV = S.DefaultFunctionArrayConversion(Arg);
+  if (ArgRV.isInvalid())
+    return NPV_Error;
+  Arg = ArgRV.get();
+
+  Expr::EvalResult EvalResult;
+  SmallVector<PartialDiagnosticAt, 8> Notes;
+  EvalResult.Diag = &Notes;
+  if (!Arg->EvaluateAsRValue(EvalResult, S.Context) ||
+      EvalResult.HasSideEffects) {
+    SourceLocation DiagLoc = Arg->getExprLoc();
+
+    // If our only note is the usual "invalid subexpression" note, just point
+    // the caret at its location rather than producing an essentially
+    // redundant note.
+    if (Notes.size() == 1 && Notes[0].second.getDiagID() ==
+        diag::note_invalid_subexpr_in_const_expr) {
+      DiagLoc = Notes[0].first;
+      Notes.clear();
+    }
+
+    S.Diag(DiagLoc, diag::err_template_arg_not_address_constant)
+      << Arg->getType() << Arg->getSourceRange();
+    for (unsigned I = 0, N = Notes.size(); I != N; ++I)
+      S.Diag(Notes[I].first, Notes[I].second);
+
+    S.Diag(Param->getLocation(), diag::note_template_param_here);
+    return NPV_Error;
+  }
+
+  // C++11 [temp.arg.nontype]p1:
+  //   - an address constant expression of type std::nullptr_t
+  if (Arg->getType()->isNullPtrType())
+    return NPV_NullPointer;
+
+  //   - a constant expression that evaluates to a null pointer value (4.10); or
+  //   - a constant expression that evaluates to a null member pointer value
+  //     (4.11); or
+  if ((EvalResult.Val.isLValue() && !EvalResult.Val.getLValueBase()) ||
+      (EvalResult.Val.isMemberPointer() &&
+       !EvalResult.Val.getMemberPointerDecl())) {
+    // If our expression has an appropriate type, we've succeeded.
+    bool ObjCLifetimeConversion;
+    if (S.Context.hasSameUnqualifiedType(Arg->getType(), ParamType) ||
+        S.IsQualificationConversion(Arg->getType(), ParamType, false,
+                                     ObjCLifetimeConversion))
+      return NPV_NullPointer;
+
+    // The types didn't match, but we know we got a null pointer; complain,
+    // then recover as if the types were correct.
+    S.Diag(Arg->getExprLoc(), diag::err_template_arg_wrongtype_null_constant)
+      << Arg->getType() << ParamType << Arg->getSourceRange();
+    S.Diag(Param->getLocation(), diag::note_template_param_here);
+    return NPV_NullPointer;
+  }
+
+  // If we don't have a null pointer value, but we do have a NULL pointer
+  // constant, suggest a cast to the appropriate type.
+  if (Arg->isNullPointerConstant(S.Context, Expr::NPC_NeverValueDependent)) {
+    std::string Code = "static_cast<" + ParamType.getAsString() + ">(";
+    S.Diag(Arg->getExprLoc(), diag::err_template_arg_untyped_null_constant)
+        << ParamType << FixItHint::CreateInsertion(Arg->getBeginLoc(), Code)
+        << FixItHint::CreateInsertion(S.getLocForEndOfToken(Arg->getEndLoc()),
+                                      ")");
+    S.Diag(Param->getLocation(), diag::note_template_param_here);
+    return NPV_NullPointer;
+  }
+
+  // FIXME: If we ever want to support general, address-constant expressions
+  // as non-type template arguments, we should return the ExprResult here to
+  // be interpreted by the caller.
+  return NPV_NotNullPointer;
+}
+
+/// Checks whether the given template argument is compatible with its
+/// template parameter.
+static bool CheckTemplateArgumentIsCompatibleWithParameter(
+    Sema &S, NonTypeTemplateParmDecl *Param, QualType ParamType, Expr *ArgIn,
+    Expr *Arg, QualType ArgType) {
+  bool ObjCLifetimeConversion;
+  if (ParamType->isPointerType() &&
+      !ParamType->castAs<PointerType>()->getPointeeType()->isFunctionType() &&
+      S.IsQualificationConversion(ArgType, ParamType, false,
+                                  ObjCLifetimeConversion)) {
+    // For pointer-to-object types, qualification conversions are
+    // permitted.
+  } else {
+    if (const ReferenceType *ParamRef = ParamType->getAs<ReferenceType>()) {
+      if (!ParamRef->getPointeeType()->isFunctionType()) {
+        // C++ [temp.arg.nontype]p5b3:
+        //   For a non-type template-parameter of type reference to
+        //   object, no conversions apply. The type referred to by the
+        //   reference may be more cv-qualified than the (otherwise
+        //   identical) type of the template- argument. The
+        //   template-parameter is bound directly to the
+        //   template-argument, which shall be an lvalue.
+
+        // FIXME: Other qualifiers?
+        unsigned ParamQuals = ParamRef->getPointeeType().getCVRQualifiers();
+        unsigned ArgQuals = ArgType.getCVRQualifiers();
+
+        if ((ParamQuals | ArgQuals) != ParamQuals) {
+          S.Diag(Arg->getBeginLoc(),
+                 diag::err_template_arg_ref_bind_ignores_quals)
+              << ParamType << Arg->getType() << Arg->getSourceRange();
+          S.Diag(Param->getLocation(), diag::note_template_param_here);
+          return true;
+        }
+      }
+    }
+
+    // At this point, the template argument refers to an object or
+    // function with external linkage. We now need to check whether the
+    // argument and parameter types are compatible.
+    if (!S.Context.hasSameUnqualifiedType(ArgType,
+                                          ParamType.getNonReferenceType())) {
+      // We can't perform this conversion or binding.
+      if (ParamType->isReferenceType())
+        S.Diag(Arg->getBeginLoc(), diag::err_template_arg_no_ref_bind)
+            << ParamType << ArgIn->getType() << Arg->getSourceRange();
+      else
+        S.Diag(Arg->getBeginLoc(), diag::err_template_arg_not_convertible)
+            << ArgIn->getType() << ParamType << Arg->getSourceRange();
+      S.Diag(Param->getLocation(), diag::note_template_param_here);
+      return true;
+    }
+  }
+
+  return false;
+}
+
+/// Checks whether the given template argument is the address
+/// of an object or function according to C++ [temp.arg.nontype]p1.
+static bool
+CheckTemplateArgumentAddressOfObjectOrFunction(Sema &S,
+                                               NonTypeTemplateParmDecl *Param,
+                                               QualType ParamType,
+                                               Expr *ArgIn,
+                                               TemplateArgument &Converted) {
+  bool Invalid = false;
+  Expr *Arg = ArgIn;
+  QualType ArgType = Arg->getType();
+
+  bool AddressTaken = false;
+  SourceLocation AddrOpLoc;
+  if (S.getLangOpts().MicrosoftExt) {
+    // Microsoft Visual C++ strips all casts, allows an arbitrary number of
+    // dereference and address-of operators.
+    Arg = Arg->IgnoreParenCasts();
+
+    bool ExtWarnMSTemplateArg = false;
+    UnaryOperatorKind FirstOpKind;
+    SourceLocation FirstOpLoc;
+    while (UnaryOperator *UnOp = dyn_cast<UnaryOperator>(Arg)) {
+      UnaryOperatorKind UnOpKind = UnOp->getOpcode();
+      if (UnOpKind == UO_Deref)
+        ExtWarnMSTemplateArg = true;
+      if (UnOpKind == UO_AddrOf || UnOpKind == UO_Deref) {
+        Arg = UnOp->getSubExpr()->IgnoreParenCasts();
+        if (!AddrOpLoc.isValid()) {
+          FirstOpKind = UnOpKind;
+          FirstOpLoc = UnOp->getOperatorLoc();
+        }
+      } else
+        break;
+    }
+    if (FirstOpLoc.isValid()) {
+      if (ExtWarnMSTemplateArg)
+        S.Diag(ArgIn->getBeginLoc(), diag::ext_ms_deref_template_argument)
+            << ArgIn->getSourceRange();
+
+      if (FirstOpKind == UO_AddrOf)
+        AddressTaken = true;
+      else if (Arg->getType()->isPointerType()) {
+        // We cannot let pointers get dereferenced here, that is obviously not a
+        // constant expression.
+        assert(FirstOpKind == UO_Deref);
+        S.Diag(Arg->getBeginLoc(), diag::err_template_arg_not_decl_ref)
+            << Arg->getSourceRange();
+      }
+    }
+  } else {
+    // See through any implicit casts we added to fix the type.
+    Arg = Arg->IgnoreImpCasts();
+
+    // C++ [temp.arg.nontype]p1:
+    //
+    //   A template-argument for a non-type, non-template
+    //   template-parameter shall be one of: [...]
+    //
+    //     -- the address of an object or function with external
+    //        linkage, including function templates and function
+    //        template-ids but excluding non-static class members,
+    //        expressed as & id-expression where the & is optional if
+    //        the name refers to a function or array, or if the
+    //        corresponding template-parameter is a reference; or
+
+    // In C++98/03 mode, give an extension warning on any extra parentheses.
+    // See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#773
+    bool ExtraParens = false;
+    while (ParenExpr *Parens = dyn_cast<ParenExpr>(Arg)) {
+      if (!Invalid && !ExtraParens) {
+        S.Diag(Arg->getBeginLoc(),
+               S.getLangOpts().CPlusPlus11
+                   ? diag::warn_cxx98_compat_template_arg_extra_parens
+                   : diag::ext_template_arg_extra_parens)
+            << Arg->getSourceRange();
+        ExtraParens = true;
+      }
+
+      Arg = Parens->getSubExpr();
+    }
+
+    while (SubstNonTypeTemplateParmExpr *subst =
+               dyn_cast<SubstNonTypeTemplateParmExpr>(Arg))
+      Arg = subst->getReplacement()->IgnoreImpCasts();
+
+    if (UnaryOperator *UnOp = dyn_cast<UnaryOperator>(Arg)) {
+      if (UnOp->getOpcode() == UO_AddrOf) {
+        Arg = UnOp->getSubExpr();
+        AddressTaken = true;
+        AddrOpLoc = UnOp->getOperatorLoc();
+      }
+    }
+
+    while (SubstNonTypeTemplateParmExpr *subst =
+               dyn_cast<SubstNonTypeTemplateParmExpr>(Arg))
+      Arg = subst->getReplacement()->IgnoreImpCasts();
+  }
+
+  ValueDecl *Entity = nullptr;
+  if (DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(Arg))
+    Entity = DRE->getDecl();
+  else if (CXXUuidofExpr *CUE = dyn_cast<CXXUuidofExpr>(Arg))
+    Entity = CUE->getGuidDecl();
+
+  // If our parameter has pointer type, check for a null template value.
+  if (ParamType->isPointerType() || ParamType->isNullPtrType()) {
+    switch (isNullPointerValueTemplateArgument(S, Param, ParamType, ArgIn,
+                                               Entity)) {
+    case NPV_NullPointer:
+      S.Diag(Arg->getExprLoc(), diag::warn_cxx98_compat_template_arg_null);
+      Converted = TemplateArgument(S.Context.getCanonicalType(ParamType),
+                                   /*isNullPtr=*/true);
+      return false;
+
+    case NPV_Error:
+      return true;
+
+    case NPV_NotNullPointer:
+      break;
+    }
+  }
+
+  // Stop checking the precise nature of the argument if it is value dependent,
+  // it should be checked when instantiated.
+  if (Arg->isValueDependent()) {
+    Converted = TemplateArgument(ArgIn);
+    return false;
+  }
+
+  if (!Entity) {
+    S.Diag(Arg->getBeginLoc(), diag::err_template_arg_not_decl_ref)
+        << Arg->getSourceRange();
+    S.Diag(Param->getLocation(), diag::note_template_param_here);
+    return true;
+  }
+
+  // Cannot refer to non-static data members
+  if (isa<FieldDecl>(Entity) || isa<IndirectFieldDecl>(Entity)) {
+    S.Diag(Arg->getBeginLoc(), diag::err_template_arg_field)
+        << Entity << Arg->getSourceRange();
+    S.Diag(Param->getLocation(), diag::note_template_param_here);
+    return true;
+  }
+
+  // Cannot refer to non-static member functions
+  if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Entity)) {
+    if (!Method->isStatic()) {
+      S.Diag(Arg->getBeginLoc(), diag::err_template_arg_method)
+          << Method << Arg->getSourceRange();
+      S.Diag(Param->getLocation(), diag::note_template_param_here);
+      return true;
+    }
+  }
+
+  FunctionDecl *Func = dyn_cast<FunctionDecl>(Entity);
+  VarDecl *Var = dyn_cast<VarDecl>(Entity);
+  MSGuidDecl *Guid = dyn_cast<MSGuidDecl>(Entity);
+
+  // A non-type template argument must refer to an object or function.
+  if (!Func && !Var && !Guid) {
+    // We found something, but we don't know specifically what it is.
+    S.Diag(Arg->getBeginLoc(), diag::err_template_arg_not_object_or_func)
+        << Arg->getSourceRange();
+    S.Diag(Entity->getLocation(), diag::note_template_arg_refers_here);
+    return true;
+  }
+
+  // Address / reference template args must have external linkage in C++98.
+  if (Entity->getFormalLinkage() == InternalLinkage) {
+    S.Diag(Arg->getBeginLoc(),
+           S.getLangOpts().CPlusPlus11
+               ? diag::warn_cxx98_compat_template_arg_object_internal
+               : diag::ext_template_arg_object_internal)
+        << !Func << Entity << Arg->getSourceRange();
+    S.Diag(Entity->getLocation(), diag::note_template_arg_internal_object)
+      << !Func;
+  } else if (!Entity->hasLinkage()) {
+    S.Diag(Arg->getBeginLoc(), diag::err_template_arg_object_no_linkage)
+        << !Func << Entity << Arg->getSourceRange();
+    S.Diag(Entity->getLocation(), diag::note_template_arg_internal_object)
+      << !Func;
+    return true;
+  }
+
+  if (Var) {
+    // A value of reference type is not an object.
+    if (Var->getType()->isReferenceType()) {
+      S.Diag(Arg->getBeginLoc(), diag::err_template_arg_reference_var)
+          << Var->getType() << Arg->getSourceRange();
+      S.Diag(Param->getLocation(), diag::note_template_param_here);
+      return true;
+    }
+
+    // A template argument must have static storage duration.
+    if (Var->getTLSKind()) {
+      S.Diag(Arg->getBeginLoc(), diag::err_template_arg_thread_local)
+          << Arg->getSourceRange();
+      S.Diag(Var->getLocation(), diag::note_template_arg_refers_here);
+      return true;
+    }
+  }
+
+  if (AddressTaken && ParamType->isReferenceType()) {
+    // If we originally had an address-of operator, but the
+    // parameter has reference type, complain and (if things look
+    // like they will work) drop the address-of operator.
+    if (!S.Context.hasSameUnqualifiedType(Entity->getType(),
+                                          ParamType.getNonReferenceType())) {
+      S.Diag(AddrOpLoc, diag::err_template_arg_address_of_non_pointer)
+        << ParamType;
+      S.Diag(Param->getLocation(), diag::note_template_param_here);
+      return true;
+    }
+
+    S.Diag(AddrOpLoc, diag::err_template_arg_address_of_non_pointer)
+      << ParamType
+      << FixItHint::CreateRemoval(AddrOpLoc);
+    S.Diag(Param->getLocation(), diag::note_template_param_here);
+
+    ArgType = Entity->getType();
+  }
+
+  // If the template parameter has pointer type, either we must have taken the
+  // address or the argument must decay to a pointer.
+  if (!AddressTaken && ParamType->isPointerType()) {
+    if (Func) {
+      // Function-to-pointer decay.
+      ArgType = S.Context.getPointerType(Func->getType());
+    } else if (Entity->getType()->isArrayType()) {
+      // Array-to-pointer decay.
+      ArgType = S.Context.getArrayDecayedType(Entity->getType());
+    } else {
+      // If the template parameter has pointer type but the address of
+      // this object was not taken, complain and (possibly) recover by
+      // taking the address of the entity.
+      ArgType = S.Context.getPointerType(Entity->getType());
+      if (!S.Context.hasSameUnqualifiedType(ArgType, ParamType)) {
+        S.Diag(Arg->getBeginLoc(), diag::err_template_arg_not_address_of)
+          << ParamType;
+        S.Diag(Param->getLocation(), diag::note_template_param_here);
+        return true;
+      }
+
+      S.Diag(Arg->getBeginLoc(), diag::err_template_arg_not_address_of)
+        << ParamType << FixItHint::CreateInsertion(Arg->getBeginLoc(), "&");
+
+      S.Diag(Param->getLocation(), diag::note_template_param_here);
+    }
+  }
+
+  if (CheckTemplateArgumentIsCompatibleWithParameter(S, Param, ParamType, ArgIn,
+                                                     Arg, ArgType))
+    return true;
+
+  // Create the template argument.
+  Converted = TemplateArgument(cast<ValueDecl>(Entity->getCanonicalDecl()),
+                               S.Context.getCanonicalType(ParamType));
+  S.MarkAnyDeclReferenced(Arg->getBeginLoc(), Entity, false);
+  return false;
+}
+
+/// Checks whether the given template argument is a pointer to
+/// member constant according to C++ [temp.arg.nontype]p1.
+static bool CheckTemplateArgumentPointerToMember(Sema &S,
+                                                 NonTypeTemplateParmDecl *Param,
+                                                 QualType ParamType,
+                                                 Expr *&ResultArg,
+                                                 TemplateArgument &Converted) {
+  bool Invalid = false;
+
+  Expr *Arg = ResultArg;
+  bool ObjCLifetimeConversion;
+
+  // C++ [temp.arg.nontype]p1:
+  //
+  //   A template-argument for a non-type, non-template
+  //   template-parameter shall be one of: [...]
+  //
+  //     -- a pointer to member expressed as described in 5.3.1.
+  DeclRefExpr *DRE = nullptr;
+
+  // In C++98/03 mode, give an extension warning on any extra parentheses.
+  // See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#773
+  bool ExtraParens = false;
+  while (ParenExpr *Parens = dyn_cast<ParenExpr>(Arg)) {
+    if (!Invalid && !ExtraParens) {
+      S.Diag(Arg->getBeginLoc(),
+             S.getLangOpts().CPlusPlus11
+                 ? diag::warn_cxx98_compat_template_arg_extra_parens
+                 : diag::ext_template_arg_extra_parens)
+          << Arg->getSourceRange();
+      ExtraParens = true;
+    }
+
+    Arg = Parens->getSubExpr();
+  }
+
+  while (SubstNonTypeTemplateParmExpr *subst =
+           dyn_cast<SubstNonTypeTemplateParmExpr>(Arg))
+    Arg = subst->getReplacement()->IgnoreImpCasts();
+
+  // A pointer-to-member constant written &Class::member.
+  if (UnaryOperator *UnOp = dyn_cast<UnaryOperator>(Arg)) {
+    if (UnOp->getOpcode() == UO_AddrOf) {
+      DRE = dyn_cast<DeclRefExpr>(UnOp->getSubExpr());
+      if (DRE && !DRE->getQualifier())
+        DRE = nullptr;
+    }
+  }
+  // A constant of pointer-to-member type.
+  else if ((DRE = dyn_cast<DeclRefExpr>(Arg))) {
+    ValueDecl *VD = DRE->getDecl();
+    if (VD->getType()->isMemberPointerType()) {
+      if (isa<NonTypeTemplateParmDecl>(VD)) {
+        if (Arg->isTypeDependent() || Arg->isValueDependent()) {
+          Converted = TemplateArgument(Arg);
+        } else {
+          VD = cast<ValueDecl>(VD->getCanonicalDecl());
+          Converted = TemplateArgument(VD, ParamType);
+        }
+        return Invalid;
+      }
+    }
+
+    DRE = nullptr;
+  }
+
+  ValueDecl *Entity = DRE ? DRE->getDecl() : nullptr;
+
+  // Check for a null pointer value.
+  switch (isNullPointerValueTemplateArgument(S, Param, ParamType, ResultArg,
+                                             Entity)) {
+  case NPV_Error:
+    return true;
+  case NPV_NullPointer:
+    S.Diag(ResultArg->getExprLoc(), diag::warn_cxx98_compat_template_arg_null);
+    Converted = TemplateArgument(S.Context.getCanonicalType(ParamType),
+                                 /*isNullPtr*/true);
+    return false;
+  case NPV_NotNullPointer:
+    break;
+  }
+
+  if (S.IsQualificationConversion(ResultArg->getType(),
+                                  ParamType.getNonReferenceType(), false,
+                                  ObjCLifetimeConversion)) {
+    ResultArg = S.ImpCastExprToType(ResultArg, ParamType, CK_NoOp,
+                                    ResultArg->getValueKind())
+                    .get();
+  } else if (!S.Context.hasSameUnqualifiedType(
+                 ResultArg->getType(), ParamType.getNonReferenceType())) {
+    // We can't perform this conversion.
+    S.Diag(ResultArg->getBeginLoc(), diag::err_template_arg_not_convertible)
+        << ResultArg->getType() << ParamType << ResultArg->getSourceRange();
+    S.Diag(Param->getLocation(), diag::note_template_param_here);
+    return true;
+  }
+
+  if (!DRE)
+    return S.Diag(Arg->getBeginLoc(),
+                  diag::err_template_arg_not_pointer_to_member_form)
+           << Arg->getSourceRange();
+
+  if (isa<FieldDecl>(DRE->getDecl()) ||
+      isa<IndirectFieldDecl>(DRE->getDecl()) ||
+      isa<CXXMethodDecl>(DRE->getDecl())) {
+    assert((isa<FieldDecl>(DRE->getDecl()) ||
+            isa<IndirectFieldDecl>(DRE->getDecl()) ||
+            !cast<CXXMethodDecl>(DRE->getDecl())->isStatic()) &&
+           "Only non-static member pointers can make it here");
+
+    // Okay: this is the address of a non-static member, and therefore
+    // a member pointer constant.
+    if (Arg->isTypeDependent() || Arg->isValueDependent()) {
+      Converted = TemplateArgument(Arg);
+    } else {
+      ValueDecl *D = cast<ValueDecl>(DRE->getDecl()->getCanonicalDecl());
+      Converted = TemplateArgument(D, S.Context.getCanonicalType(ParamType));
+    }
+    return Invalid;
+  }
+
+  // We found something else, but we don't know specifically what it is.
+  S.Diag(Arg->getBeginLoc(), diag::err_template_arg_not_pointer_to_member_form)
+      << Arg->getSourceRange();
+  S.Diag(DRE->getDecl()->getLocation(), diag::note_template_arg_refers_here);
+  return true;
+}
+
+/// Check a template argument against its corresponding
+/// non-type template parameter.
+///
+/// This routine implements the semantics of C++ [temp.arg.nontype].
+/// If an error occurred, it returns ExprError(); otherwise, it
+/// returns the converted template argument. \p ParamType is the
+/// type of the non-type template parameter after it has been instantiated.
+ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
+                                       QualType ParamType, Expr *Arg,
+                                       TemplateArgument &Converted,
+                                       CheckTemplateArgumentKind CTAK) {
+  SourceLocation StartLoc = Arg->getBeginLoc();
+
+  // If the parameter type somehow involves auto, deduce the type now.
+  DeducedType *DeducedT = ParamType->getContainedDeducedType();
+  if (getLangOpts().CPlusPlus17 && DeducedT && !DeducedT->isDeduced()) {
+    // During template argument deduction, we allow 'decltype(auto)' to
+    // match an arbitrary dependent argument.
+    // FIXME: The language rules don't say what happens in this case.
+    // FIXME: We get an opaque dependent type out of decltype(auto) if the
+    // expression is merely instantiation-dependent; is this enough?
+    if (CTAK == CTAK_Deduced && Arg->isTypeDependent()) {
+      auto *AT = dyn_cast<AutoType>(DeducedT);
+      if (AT && AT->isDecltypeAuto()) {
+        Converted = TemplateArgument(Arg);
+        return Arg;
+      }
+    }
+
+    // When checking a deduced template argument, deduce from its type even if
+    // the type is dependent, in order to check the types of non-type template
+    // arguments line up properly in partial ordering.
+    Optional<unsigned> Depth = Param->getDepth() + 1;
+    Expr *DeductionArg = Arg;
+    if (auto *PE = dyn_cast<PackExpansionExpr>(DeductionArg))
+      DeductionArg = PE->getPattern();
+    TypeSourceInfo *TSI =
+        Context.getTrivialTypeSourceInfo(ParamType, Param->getLocation());
+    if (isa<DeducedTemplateSpecializationType>(DeducedT)) {
+      InitializedEntity Entity =
+          InitializedEntity::InitializeTemplateParameter(ParamType, Param);
+      InitializationKind Kind = InitializationKind::CreateForInit(
+          DeductionArg->getBeginLoc(), /*DirectInit*/false, DeductionArg);
+      Expr *Inits[1] = {DeductionArg};
+      ParamType =
+          DeduceTemplateSpecializationFromInitializer(TSI, Entity, Kind, Inits);
+      if (ParamType.isNull())
+        return ExprError();
+    } else if (DeduceAutoType(
+                   TSI, DeductionArg, ParamType, Depth,
+                   // We do not check constraints right now because the
+                   // immediately-declared constraint of the auto type is also
+                   // an associated constraint, and will be checked along with
+                   // the other associated constraints after checking the
+                   // template argument list.
+                   /*IgnoreConstraints=*/true) == DAR_Failed) {
+      Diag(Arg->getExprLoc(),
+           diag::err_non_type_template_parm_type_deduction_failure)
+        << Param->getDeclName() << Param->getType() << Arg->getType()
+        << Arg->getSourceRange();
+      Diag(Param->getLocation(), diag::note_template_param_here);
+      return ExprError();
+    }
+    // CheckNonTypeTemplateParameterType will produce a diagnostic if there's
+    // an error. The error message normally references the parameter
+    // declaration, but here we'll pass the argument location because that's
+    // where the parameter type is deduced.
+    ParamType = CheckNonTypeTemplateParameterType(ParamType, Arg->getExprLoc());
+    if (ParamType.isNull()) {
+      Diag(Param->getLocation(), diag::note_template_param_here);
+      return ExprError();
+    }
+  }
+
+  // We should have already dropped all cv-qualifiers by now.
+  assert(!ParamType.hasQualifiers() &&
+         "non-type template parameter type cannot be qualified");
+
+  // FIXME: When Param is a reference, should we check that Arg is an lvalue?
+  if (CTAK == CTAK_Deduced &&
+      (ParamType->isReferenceType()
+           ? !Context.hasSameType(ParamType.getNonReferenceType(),
+                                  Arg->getType())
+           : !Context.hasSameUnqualifiedType(ParamType, Arg->getType()))) {
+    // FIXME: If either type is dependent, we skip the check. This isn't
+    // correct, since during deduction we're supposed to have replaced each
+    // template parameter with some unique (non-dependent) placeholder.
+    // FIXME: If the argument type contains 'auto', we carry on and fail the
+    // type check in order to force specific types to be more specialized than
+    // 'auto'. It's not clear how partial ordering with 'auto' is supposed to
+    // work. Similarly for CTAD, when comparing 'A<x>' against 'A'.
+    if ((ParamType->isDependentType() || Arg->isTypeDependent()) &&
+        !Arg->getType()->getContainedDeducedType()) {
+      Converted = TemplateArgument(Arg);
+      return Arg;
+    }
+    // FIXME: This attempts to implement C++ [temp.deduct.type]p17. Per DR1770,
+    // we should actually be checking the type of the template argument in P,
+    // not the type of the template argument deduced from A, against the
+    // template parameter type.
+    Diag(StartLoc, diag::err_deduced_non_type_template_arg_type_mismatch)
+      << Arg->getType()
+      << ParamType.getUnqualifiedType();
+    Diag(Param->getLocation(), diag::note_template_param_here);
+    return ExprError();
+  }
+
+  // If either the parameter has a dependent type or the argument is
+  // type-dependent, there's nothing we can check now. The argument only
+  // contains an unexpanded pack during partial ordering, and there's
+  // nothing more we can check in that case.
+  if (ParamType->isDependentType() || Arg->isTypeDependent() ||
+      Arg->containsUnexpandedParameterPack()) {
+    // Force the argument to the type of the parameter to maintain invariants.
+    auto *PE = dyn_cast<PackExpansionExpr>(Arg);
+    if (PE)
+      Arg = PE->getPattern();
+    ExprResult E = ImpCastExprToType(
+        Arg, ParamType.getNonLValueExprType(Context), CK_Dependent,
+        ParamType->isLValueReferenceType()   ? VK_LValue
+        : ParamType->isRValueReferenceType() ? VK_XValue
+                                             : VK_PRValue);
+    if (E.isInvalid())
+      return ExprError();
+    if (PE) {
+      // Recreate a pack expansion if we unwrapped one.
+      E = new (Context)
+          PackExpansionExpr(E.get()->getType(), E.get(), PE->getEllipsisLoc(),
+                            PE->getNumExpansions());
+    }
+    Converted = TemplateArgument(E.get());
+    return E;
+  }
+
+  // The initialization of the parameter from the argument is
+  // a constant-evaluated context.
+  EnterExpressionEvaluationContext ConstantEvaluated(
+      *this, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+
+  if (getLangOpts().CPlusPlus17) {
+    QualType CanonParamType = Context.getCanonicalType(ParamType);
+
+    // Avoid making a copy when initializing a template parameter of class type
+    // from a template parameter object of the same type. This is going beyond
+    // the standard, but is required for soundness: in
+    //   template<A a> struct X { X *p; X<a> *q; };
+    // ... we need p and q to have the same type.
+    //
+    // Similarly, don't inject a call to a copy constructor when initializing
+    // from a template parameter of the same type.
+    Expr *InnerArg = Arg->IgnoreParenImpCasts();
+    if (ParamType->isRecordType() && isa<DeclRefExpr>(InnerArg) &&
+        Context.hasSameUnqualifiedType(ParamType, InnerArg->getType())) {
+      NamedDecl *ND = cast<DeclRefExpr>(InnerArg)->getDecl();
+      if (auto *TPO = dyn_cast<TemplateParamObjectDecl>(ND)) {
+        Converted = TemplateArgument(TPO, CanonParamType);
+        return Arg;
+      }
+      if (isa<NonTypeTemplateParmDecl>(ND)) {
+        Converted = TemplateArgument(Arg);
+        return Arg;
+      }
+    }
+
+    // C++17 [temp.arg.nontype]p1:
+    //   A template-argument for a non-type template parameter shall be
+    //   a converted constant expression of the type of the template-parameter.
+    APValue Value;
+    ExprResult ArgResult = CheckConvertedConstantExpression(
+        Arg, ParamType, Value, CCEK_TemplateArg, Param);
+    if (ArgResult.isInvalid())
+      return ExprError();
+
+    // For a value-dependent argument, CheckConvertedConstantExpression is
+    // permitted (and expected) to be unable to determine a value.
+    if (ArgResult.get()->isValueDependent()) {
+      Converted = TemplateArgument(ArgResult.get());
+      return ArgResult;
+    }
+
+    // Convert the APValue to a TemplateArgument.
+    switch (Value.getKind()) {
+    case APValue::None:
+      assert(ParamType->isNullPtrType());
+      Converted = TemplateArgument(CanonParamType, /*isNullPtr*/true);
+      break;
+    case APValue::Indeterminate:
+      llvm_unreachable("result of constant evaluation should be initialized");
+      break;
+    case APValue::Int:
+      assert(ParamType->isIntegralOrEnumerationType());
+      Converted = TemplateArgument(Context, Value.getInt(), CanonParamType);
+      break;
+    case APValue::MemberPointer: {
+      assert(ParamType->isMemberPointerType());
+
+      // FIXME: We need TemplateArgument representation and mangling for these.
+      if (!Value.getMemberPointerPath().empty()) {
+        Diag(Arg->getBeginLoc(),
+             diag::err_template_arg_member_ptr_base_derived_not_supported)
+            << Value.getMemberPointerDecl() << ParamType
+            << Arg->getSourceRange();
+        return ExprError();
+      }
+
+      auto *VD = const_cast<ValueDecl*>(Value.getMemberPointerDecl());
+      Converted = VD ? TemplateArgument(VD, CanonParamType)
+                     : TemplateArgument(CanonParamType, /*isNullPtr*/true);
+      break;
+    }
+    case APValue::LValue: {
+      //   For a non-type template-parameter of pointer or reference type,
+      //   the value of the constant expression shall not refer to
+      assert(ParamType->isPointerType() || ParamType->isReferenceType() ||
+             ParamType->isNullPtrType());
+      // -- a temporary object
+      // -- a string literal
+      // -- the result of a typeid expression, or
+      // -- a predefined __func__ variable
+      APValue::LValueBase Base = Value.getLValueBase();
+      auto *VD = const_cast<ValueDecl *>(Base.dyn_cast<const ValueDecl *>());
+      if (Base && (!VD || isa<LifetimeExtendedTemporaryDecl>(VD))) {
+        Diag(Arg->getBeginLoc(), diag::err_template_arg_not_decl_ref)
+            << Arg->getSourceRange();
+        return ExprError();
+      }
+      // -- a subobject
+      // FIXME: Until C++20
+      if (Value.hasLValuePath() && Value.getLValuePath().size() == 1 &&
+          VD && VD->getType()->isArrayType() &&
+          Value.getLValuePath()[0].getAsArrayIndex() == 0 &&
+          !Value.isLValueOnePastTheEnd() && ParamType->isPointerType()) {
+        // Per defect report (no number yet):
+        //   ... other than a pointer to the first element of a complete array
+        //       object.
+      } else if (!Value.hasLValuePath() || Value.getLValuePath().size() ||
+                 Value.isLValueOnePastTheEnd()) {
+        Diag(StartLoc, diag::err_non_type_template_arg_subobject)
+          << Value.getAsString(Context, ParamType);
+        return ExprError();
+      }
+      assert((VD || !ParamType->isReferenceType()) &&
+             "null reference should not be a constant expression");
+      assert((!VD || !ParamType->isNullPtrType()) &&
+             "non-null value of type nullptr_t?");
+      Converted = VD ? TemplateArgument(VD, CanonParamType)
+                     : TemplateArgument(CanonParamType, /*isNullPtr*/true);
+      break;
+    }
+    case APValue::Struct:
+    case APValue::Union:
+      // Get or create the corresponding template parameter object.
+      Converted = TemplateArgument(
+          Context.getTemplateParamObjectDecl(CanonParamType, Value),
+          CanonParamType);
+      break;
+    case APValue::AddrLabelDiff:
+      return Diag(StartLoc, diag::err_non_type_template_arg_addr_label_diff);
+    case APValue::FixedPoint:
+    case APValue::Float:
+    case APValue::ComplexInt:
+    case APValue::ComplexFloat:
+    case APValue::Vector:
+    case APValue::Array:
+      return Diag(StartLoc, diag::err_non_type_template_arg_unsupported)
+             << ParamType;
+    }
+
+    return ArgResult.get();
+  }
+
+  // C++ [temp.arg.nontype]p5:
+  //   The following conversions are performed on each expression used
+  //   as a non-type template-argument. If a non-type
+  //   template-argument cannot be converted to the type of the
+  //   corresponding template-parameter then the program is
+  //   ill-formed.
+  if (ParamType->isIntegralOrEnumerationType()) {
+    // C++11:
+    //   -- for a non-type template-parameter of integral or
+    //      enumeration type, conversions permitted in a converted
+    //      constant expression are applied.
+    //
+    // C++98:
+    //   -- for a non-type template-parameter of integral or
+    //      enumeration type, integral promotions (4.5) and integral
+    //      conversions (4.7) are applied.
+
+    if (getLangOpts().CPlusPlus11) {
+      // C++ [temp.arg.nontype]p1:
+      //   A template-argument for a non-type, non-template template-parameter
+      //   shall be one of:
+      //
+      //     -- for a non-type template-parameter of integral or enumeration
+      //        type, a converted constant expression of the type of the
+      //        template-parameter; or
+      llvm::APSInt Value;
+      ExprResult ArgResult =
+        CheckConvertedConstantExpression(Arg, ParamType, Value,
+                                         CCEK_TemplateArg);
+      if (ArgResult.isInvalid())
+        return ExprError();
+
+      // We can't check arbitrary value-dependent arguments.
+      if (ArgResult.get()->isValueDependent()) {
+        Converted = TemplateArgument(ArgResult.get());
+        return ArgResult;
+      }
+
+      // Widen the argument value to sizeof(parameter type). This is almost
+      // always a no-op, except when the parameter type is bool. In
+      // that case, this may extend the argument from 1 bit to 8 bits.
+      QualType IntegerType = ParamType;
+      if (const EnumType *Enum = IntegerType->getAs<EnumType>())
+        IntegerType = Enum->getDecl()->getIntegerType();
+      Value = Value.extOrTrunc(IntegerType->isBitIntType()
+                                   ? Context.getIntWidth(IntegerType)
+                                   : Context.getTypeSize(IntegerType));
+
+      Converted = TemplateArgument(Context, Value,
+                                   Context.getCanonicalType(ParamType));
+      return ArgResult;
+    }
+
+    ExprResult ArgResult = DefaultLvalueConversion(Arg);
+    if (ArgResult.isInvalid())
+      return ExprError();
+    Arg = ArgResult.get();
+
+    QualType ArgType = Arg->getType();
+
+    // C++ [temp.arg.nontype]p1:
+    //   A template-argument for a non-type, non-template
+    //   template-parameter shall be one of:
+    //
+    //     -- an integral constant-expression of integral or enumeration
+    //        type; or
+    //     -- the name of a non-type template-parameter; or
+    llvm::APSInt Value;
+    if (!ArgType->isIntegralOrEnumerationType()) {
+      Diag(Arg->getBeginLoc(), diag::err_template_arg_not_integral_or_enumeral)
+          << ArgType << Arg->getSourceRange();
+      Diag(Param->getLocation(), diag::note_template_param_here);
+      return ExprError();
+    } else if (!Arg->isValueDependent()) {
+      class TmplArgICEDiagnoser : public VerifyICEDiagnoser {
+        QualType T;
+
+      public:
+        TmplArgICEDiagnoser(QualType T) : T(T) { }
+
+        SemaDiagnosticBuilder diagnoseNotICE(Sema &S,
+                                             SourceLocation Loc) override {
+          return S.Diag(Loc, diag::err_template_arg_not_ice) << T;
+        }
+      } Diagnoser(ArgType);
+
+      Arg = VerifyIntegerConstantExpression(Arg, &Value, Diagnoser).get();
+      if (!Arg)
+        return ExprError();
+    }
+
+    // From here on out, all we care about is the unqualified form
+    // of the argument type.
+    ArgType = ArgType.getUnqualifiedType();
+
+    // Try to convert the argument to the parameter's type.
+    if (Context.hasSameType(ParamType, ArgType)) {
+      // Okay: no conversion necessary
+    } else if (ParamType->isBooleanType()) {
+      // This is an integral-to-boolean conversion.
+      Arg = ImpCastExprToType(Arg, ParamType, CK_IntegralToBoolean).get();
+    } else if (IsIntegralPromotion(Arg, ArgType, ParamType) ||
+               !ParamType->isEnumeralType()) {
+      // This is an integral promotion or conversion.
+      Arg = ImpCastExprToType(Arg, ParamType, CK_IntegralCast).get();
+    } else {
+      // We can't perform this conversion.
+      Diag(Arg->getBeginLoc(), diag::err_template_arg_not_convertible)
+          << Arg->getType() << ParamType << Arg->getSourceRange();
+      Diag(Param->getLocation(), diag::note_template_param_here);
+      return ExprError();
+    }
+
+    // Add the value of this argument to the list of converted
+    // arguments. We use the bitwidth and signedness of the template
+    // parameter.
+    if (Arg->isValueDependent()) {
+      // The argument is value-dependent. Create a new
+      // TemplateArgument with the converted expression.
+      Converted = TemplateArgument(Arg);
+      return Arg;
+    }
+
+    QualType IntegerType = Context.getCanonicalType(ParamType);
+    if (const EnumType *Enum = IntegerType->getAs<EnumType>())
+      IntegerType = Context.getCanonicalType(Enum->getDecl()->getIntegerType());
+
+    if (ParamType->isBooleanType()) {
+      // Value must be zero or one.
+      Value = Value != 0;
+      unsigned AllowedBits = Context.getTypeSize(IntegerType);
+      if (Value.getBitWidth() != AllowedBits)
+        Value = Value.extOrTrunc(AllowedBits);
+      Value.setIsSigned(IntegerType->isSignedIntegerOrEnumerationType());
+    } else {
+      llvm::APSInt OldValue = Value;
+
+      // Coerce the template argument's value to the value it will have
+      // based on the template parameter's type.
+      unsigned AllowedBits = IntegerType->isBitIntType()
+                                 ? Context.getIntWidth(IntegerType)
+                                 : Context.getTypeSize(IntegerType);
+      if (Value.getBitWidth() != AllowedBits)
+        Value = Value.extOrTrunc(AllowedBits);
+      Value.setIsSigned(IntegerType->isSignedIntegerOrEnumerationType());
+
+      // Complain if an unsigned parameter received a negative value.
+      if (IntegerType->isUnsignedIntegerOrEnumerationType() &&
+          (OldValue.isSigned() && OldValue.isNegative())) {
+        Diag(Arg->getBeginLoc(), diag::warn_template_arg_negative)
+            << toString(OldValue, 10) << toString(Value, 10) << Param->getType()
+            << Arg->getSourceRange();
+        Diag(Param->getLocation(), diag::note_template_param_here);
+      }
+
+      // Complain if we overflowed the template parameter's type.
+      unsigned RequiredBits;
+      if (IntegerType->isUnsignedIntegerOrEnumerationType())
+        RequiredBits = OldValue.getActiveBits();
+      else if (OldValue.isUnsigned())
+        RequiredBits = OldValue.getActiveBits() + 1;
+      else
+        RequiredBits = OldValue.getMinSignedBits();
+      if (RequiredBits > AllowedBits) {
+        Diag(Arg->getBeginLoc(), diag::warn_template_arg_too_large)
+            << toString(OldValue, 10) << toString(Value, 10) << Param->getType()
+            << Arg->getSourceRange();
+        Diag(Param->getLocation(), diag::note_template_param_here);
+      }
+    }
+
+    Converted = TemplateArgument(Context, Value,
+                                 ParamType->isEnumeralType()
+                                   ? Context.getCanonicalType(ParamType)
+                                   : IntegerType);
+    return Arg;
+  }
+
+  QualType ArgType = Arg->getType();
+  DeclAccessPair FoundResult; // temporary for ResolveOverloadedFunction
+
+  // Handle pointer-to-function, reference-to-function, and
+  // pointer-to-member-function all in (roughly) the same way.
+  if (// -- For a non-type template-parameter of type pointer to
+      //    function, only the function-to-pointer conversion (4.3) is
+      //    applied. If the template-argument represents a set of
+      //    overloaded functions (or a pointer to such), the matching
+      //    function is selected from the set (13.4).
+      (ParamType->isPointerType() &&
+       ParamType->castAs<PointerType>()->getPointeeType()->isFunctionType()) ||
+      // -- For a non-type template-parameter of type reference to
+      //    function, no conversions apply. If the template-argument
+      //    represents a set of overloaded functions, the matching
+      //    function is selected from the set (13.4).
+      (ParamType->isReferenceType() &&
+       ParamType->castAs<ReferenceType>()->getPointeeType()->isFunctionType()) ||
+      // -- For a non-type template-parameter of type pointer to
+      //    member function, no conversions apply. If the
+      //    template-argument represents a set of overloaded member
+      //    functions, the matching member function is selected from
+      //    the set (13.4).
+      (ParamType->isMemberPointerType() &&
+       ParamType->castAs<MemberPointerType>()->getPointeeType()
+         ->isFunctionType())) {
+
+    if (Arg->getType() == Context.OverloadTy) {
+      if (FunctionDecl *Fn = ResolveAddressOfOverloadedFunction(Arg, ParamType,
+                                                                true,
+                                                                FoundResult)) {
+        if (DiagnoseUseOfDecl(Fn, Arg->getBeginLoc()))
+          return ExprError();
+
+        Arg = FixOverloadedFunctionReference(Arg, FoundResult, Fn);
+        ArgType = Arg->getType();
+      } else
+        return ExprError();
+    }
+
+    if (!ParamType->isMemberPointerType()) {
+      if (CheckTemplateArgumentAddressOfObjectOrFunction(*this, Param,
+                                                         ParamType,
+                                                         Arg, Converted))
+        return ExprError();
+      return Arg;
+    }
+
+    if (CheckTemplateArgumentPointerToMember(*this, Param, ParamType, Arg,
+                                             Converted))
+      return ExprError();
+    return Arg;
+  }
+
+  if (ParamType->isPointerType()) {
+    //   -- for a non-type template-parameter of type pointer to
+    //      object, qualification conversions (4.4) and the
+    //      array-to-pointer conversion (4.2) are applied.
+    // C++0x also allows a value of std::nullptr_t.
+    assert(ParamType->getPointeeType()->isIncompleteOrObjectType() &&
+           "Only object pointers allowed here");
+
+    if (CheckTemplateArgumentAddressOfObjectOrFunction(*this, Param,
+                                                       ParamType,
+                                                       Arg, Converted))
+      return ExprError();
+    return Arg;
+  }
+
+  if (const ReferenceType *ParamRefType = ParamType->getAs<ReferenceType>()) {
+    //   -- For a non-type template-parameter of type reference to
+    //      object, no conversions apply. The type referred to by the
+    //      reference may be more cv-qualified than the (otherwise
+    //      identical) type of the template-argument. The
+    //      template-parameter is bound directly to the
+    //      template-argument, which must be an lvalue.
+    assert(ParamRefType->getPointeeType()->isIncompleteOrObjectType() &&
+           "Only object references allowed here");
+
+    if (Arg->getType() == Context.OverloadTy) {
+      if (FunctionDecl *Fn = ResolveAddressOfOverloadedFunction(Arg,
+                                                 ParamRefType->getPointeeType(),
+                                                                true,
+                                                                FoundResult)) {
+        if (DiagnoseUseOfDecl(Fn, Arg->getBeginLoc()))
+          return ExprError();
+
+        Arg = FixOverloadedFunctionReference(Arg, FoundResult, Fn);
+        ArgType = Arg->getType();
+      } else
+        return ExprError();
+    }
+
+    if (CheckTemplateArgumentAddressOfObjectOrFunction(*this, Param,
+                                                       ParamType,
+                                                       Arg, Converted))
+      return ExprError();
+    return Arg;
+  }
+
+  // Deal with parameters of type std::nullptr_t.
+  if (ParamType->isNullPtrType()) {
+    if (Arg->isTypeDependent() || Arg->isValueDependent()) {
+      Converted = TemplateArgument(Arg);
+      return Arg;
+    }
+
+    switch (isNullPointerValueTemplateArgument(*this, Param, ParamType, Arg)) {
+    case NPV_NotNullPointer:
+      Diag(Arg->getExprLoc(), diag::err_template_arg_not_convertible)
+        << Arg->getType() << ParamType;
+      Diag(Param->getLocation(), diag::note_template_param_here);
+      return ExprError();
+
+    case NPV_Error:
+      return ExprError();
+
+    case NPV_NullPointer:
+      Diag(Arg->getExprLoc(), diag::warn_cxx98_compat_template_arg_null);
+      Converted = TemplateArgument(Context.getCanonicalType(ParamType),
+                                   /*isNullPtr*/true);
+      return Arg;
+    }
+  }
+
+  //     -- For a non-type template-parameter of type pointer to data
+  //        member, qualification conversions (4.4) are applied.
+  assert(ParamType->isMemberPointerType() && "Only pointers to members remain");
+
+  if (CheckTemplateArgumentPointerToMember(*this, Param, ParamType, Arg,
+                                           Converted))
+    return ExprError();
+  return Arg;
+}
+
+static void DiagnoseTemplateParameterListArityMismatch(
+    Sema &S, TemplateParameterList *New, TemplateParameterList *Old,
+    Sema::TemplateParameterListEqualKind Kind, SourceLocation TemplateArgLoc);
+
+/// Check a template argument against its corresponding
+/// template template parameter.
+///
+/// This routine implements the semantics of C++ [temp.arg.template].
+/// It returns true if an error occurred, and false otherwise.
+bool Sema::CheckTemplateTemplateArgument(TemplateTemplateParmDecl *Param,
+                                         TemplateParameterList *Params,
+                                         TemplateArgumentLoc &Arg) {
+  TemplateName Name = Arg.getArgument().getAsTemplateOrTemplatePattern();
+  TemplateDecl *Template = Name.getAsTemplateDecl();
+  if (!Template) {
+    // Any dependent template name is fine.
+    assert(Name.isDependent() && "Non-dependent template isn't a declaration?");
+    return false;
+  }
+
+  if (Template->isInvalidDecl())
+    return true;
+
+  // C++0x [temp.arg.template]p1:
+  //   A template-argument for a template template-parameter shall be
+  //   the name of a class template or an alias template, expressed as an
+  //   id-expression. When the template-argument names a class template, only
+  //   primary class templates are considered when matching the
+  //   template template argument with the corresponding parameter;
+  //   partial specializations are not considered even if their
+  //   parameter lists match that of the template template parameter.
+  //
+  // Note that we also allow template template parameters here, which
+  // will happen when we are dealing with, e.g., class template
+  // partial specializations.
+  if (!isa<ClassTemplateDecl>(Template) &&
+      !isa<TemplateTemplateParmDecl>(Template) &&
+      !isa<TypeAliasTemplateDecl>(Template) &&
+      !isa<BuiltinTemplateDecl>(Template)) {
+    assert(isa<FunctionTemplateDecl>(Template) &&
+           "Only function templates are possible here");
+    Diag(Arg.getLocation(), diag::err_template_arg_not_valid_template);
+    Diag(Template->getLocation(), diag::note_template_arg_refers_here_func)
+      << Template;
+  }
+
+  // C++1z [temp.arg.template]p3: (DR 150)
+  //   A template-argument matches a template template-parameter P when P
+  //   is at least as specialized as the template-argument A.
+  // FIXME: We should enable RelaxedTemplateTemplateArgs by default as it is a
+  //  defect report resolution from C++17 and shouldn't be introduced by
+  //  concepts.
+  if (getLangOpts().RelaxedTemplateTemplateArgs) {
+    // Quick check for the common case:
+    //   If P contains a parameter pack, then A [...] matches P if each of A's
+    //   template parameters matches the corresponding template parameter in
+    //   the template-parameter-list of P.
+    if (TemplateParameterListsAreEqual(
+            Template->getTemplateParameters(), Params, false,
+            TPL_TemplateTemplateArgumentMatch, Arg.getLocation()) &&
+        // If the argument has no associated constraints, then the parameter is
+        // definitely at least as specialized as the argument.
+        // Otherwise - we need a more thorough check.
+        !Template->hasAssociatedConstraints())
+      return false;
+
+    if (isTemplateTemplateParameterAtLeastAsSpecializedAs(Params, Template,
+                                                          Arg.getLocation())) {
+      // C++2a[temp.func.order]p2
+      //   [...] If both deductions succeed, the partial ordering selects the
+      //   more constrained template as described by the rules in
+      //   [temp.constr.order].
+      SmallVector<const Expr *, 3> ParamsAC, TemplateAC;
+      Params->getAssociatedConstraints(ParamsAC);
+      // C++2a[temp.arg.template]p3
+      //   [...] In this comparison, if P is unconstrained, the constraints on A
+      //   are not considered.
+      if (ParamsAC.empty())
+        return false;
+      Template->getAssociatedConstraints(TemplateAC);
+      bool IsParamAtLeastAsConstrained;
+      if (IsAtLeastAsConstrained(Param, ParamsAC, Template, TemplateAC,
+                                 IsParamAtLeastAsConstrained))
+        return true;
+      if (!IsParamAtLeastAsConstrained) {
+        Diag(Arg.getLocation(),
+             diag::err_template_template_parameter_not_at_least_as_constrained)
+            << Template << Param << Arg.getSourceRange();
+        Diag(Param->getLocation(), diag::note_entity_declared_at) << Param;
+        Diag(Template->getLocation(), diag::note_entity_declared_at)
+            << Template;
+        MaybeEmitAmbiguousAtomicConstraintsDiagnostic(Param, ParamsAC, Template,
+                                                      TemplateAC);
+        return true;
+      }
+      return false;
+    }
+    // FIXME: Produce better diagnostics for deduction failures.
+  }
+
+  return !TemplateParameterListsAreEqual(Template->getTemplateParameters(),
+                                         Params,
+                                         true,
+                                         TPL_TemplateTemplateArgumentMatch,
+                                         Arg.getLocation());
+}
+
+/// Given a non-type template argument that refers to a
+/// declaration and the type of its corresponding non-type template
+/// parameter, produce an expression that properly refers to that
+/// declaration.
+ExprResult
+Sema::BuildExpressionFromDeclTemplateArgument(const TemplateArgument &Arg,
+                                              QualType ParamType,
+                                              SourceLocation Loc) {
+  // C++ [temp.param]p8:
+  //
+  //   A non-type template-parameter of type "array of T" or
+  //   "function returning T" is adjusted to be of type "pointer to
+  //   T" or "pointer to function returning T", respectively.
+  if (ParamType->isArrayType())
+    ParamType = Context.getArrayDecayedType(ParamType);
+  else if (ParamType->isFunctionType())
+    ParamType = Context.getPointerType(ParamType);
+
+  // For a NULL non-type template argument, return nullptr casted to the
+  // parameter's type.
+  if (Arg.getKind() == TemplateArgument::NullPtr) {
+    return ImpCastExprToType(
+             new (Context) CXXNullPtrLiteralExpr(Context.NullPtrTy, Loc),
+                             ParamType,
+                             ParamType->getAs<MemberPointerType>()
+                               ? CK_NullToMemberPointer
+                               : CK_NullToPointer);
+  }
+  assert(Arg.getKind() == TemplateArgument::Declaration &&
+         "Only declaration template arguments permitted here");
+
+  ValueDecl *VD = Arg.getAsDecl();
+
+  CXXScopeSpec SS;
+  if (ParamType->isMemberPointerType()) {
+    // If this is a pointer to member, we need to use a qualified name to
+    // form a suitable pointer-to-member constant.
+    assert(VD->getDeclContext()->isRecord() &&
+           (isa<CXXMethodDecl>(VD) || isa<FieldDecl>(VD) ||
+            isa<IndirectFieldDecl>(VD)));
+    QualType ClassType
+      = Context.getTypeDeclType(cast<RecordDecl>(VD->getDeclContext()));
+    NestedNameSpecifier *Qualifier
+      = NestedNameSpecifier::Create(Context, nullptr, false,
+                                    ClassType.getTypePtr());
+    SS.MakeTrivial(Context, Qualifier, Loc);
+  }
+
+  ExprResult RefExpr = BuildDeclarationNameExpr(
+      SS, DeclarationNameInfo(VD->getDeclName(), Loc), VD);
+  if (RefExpr.isInvalid())
+    return ExprError();
+
+  // For a pointer, the argument declaration is the pointee. Take its address.
+  QualType ElemT(RefExpr.get()->getType()->getArrayElementTypeNoTypeQual(), 0);
+  if (ParamType->isPointerType() && !ElemT.isNull() &&
+      Context.hasSimilarType(ElemT, ParamType->getPointeeType())) {
+    // Decay an array argument if we want a pointer to its first element.
+    RefExpr = DefaultFunctionArrayConversion(RefExpr.get());
+    if (RefExpr.isInvalid())
+      return ExprError();
+  } else if (ParamType->isPointerType() || ParamType->isMemberPointerType()) {
+    // For any other pointer, take the address (or form a pointer-to-member).
+    RefExpr = CreateBuiltinUnaryOp(Loc, UO_AddrOf, RefExpr.get());
+    if (RefExpr.isInvalid())
+      return ExprError();
+  } else if (ParamType->isRecordType()) {
+    assert(isa<TemplateParamObjectDecl>(VD) &&
+           "arg for class template param not a template parameter object");
+    // No conversions apply in this case.
+    return RefExpr;
+  } else {
+    assert(ParamType->isReferenceType() &&
+           "unexpected type for decl template argument");
+  }
+
+  // At this point we should have the right value category.
+  assert(ParamType->isReferenceType() == RefExpr.get()->isLValue() &&
+         "value kind mismatch for non-type template argument");
+
+  // The type of the template parameter can differ from the type of the
+  // argument in various ways; convert it now if necessary.
+  QualType DestExprType = ParamType.getNonLValueExprType(Context);
+  if (!Context.hasSameType(RefExpr.get()->getType(), DestExprType)) {
+    CastKind CK;
+    QualType Ignored;
+    if (Context.hasSimilarType(RefExpr.get()->getType(), DestExprType) ||
+        IsFunctionConversion(RefExpr.get()->getType(), DestExprType, Ignored)) {
+      CK = CK_NoOp;
+    } else if (ParamType->isVoidPointerType() &&
+               RefExpr.get()->getType()->isPointerType()) {
+      CK = CK_BitCast;
+    } else {
+      // FIXME: Pointers to members can need conversion derived-to-base or
+      // base-to-derived conversions. We currently don't retain enough
+      // information to convert properly (we need to track a cast path or
+      // subobject number in the template argument).
+      llvm_unreachable(
+          "unexpected conversion required for non-type template argument");
+    }
+    RefExpr = ImpCastExprToType(RefExpr.get(), DestExprType, CK,
+                                RefExpr.get()->getValueKind());
+  }
+
+  return RefExpr;
+}
+
+/// Construct a new expression that refers to the given
+/// integral template argument with the given source-location
+/// information.
+///
+/// This routine takes care of the mapping from an integral template
+/// argument (which may have any integral type) to the appropriate
+/// literal value.
+ExprResult
+Sema::BuildExpressionFromIntegralTemplateArgument(const TemplateArgument &Arg,
+                                                  SourceLocation Loc) {
+  assert(Arg.getKind() == TemplateArgument::Integral &&
+         "Operation is only valid for integral template arguments");
+  QualType OrigT = Arg.getIntegralType();
+
+  // If this is an enum type that we're instantiating, we need to use an integer
+  // type the same size as the enumerator.  We don't want to build an
+  // IntegerLiteral with enum type.  The integer type of an enum type can be of
+  // any integral type with C++11 enum classes, make sure we create the right
+  // type of literal for it.
+  QualType T = OrigT;
+  if (const EnumType *ET = OrigT->getAs<EnumType>())
+    T = ET->getDecl()->getIntegerType();
+
+  Expr *E;
+  if (T->isAnyCharacterType()) {
+    CharacterLiteral::CharacterKind Kind;
+    if (T->isWideCharType())
+      Kind = CharacterLiteral::Wide;
+    else if (T->isChar8Type() && getLangOpts().Char8)
+      Kind = CharacterLiteral::UTF8;
+    else if (T->isChar16Type())
+      Kind = CharacterLiteral::UTF16;
+    else if (T->isChar32Type())
+      Kind = CharacterLiteral::UTF32;
+    else
+      Kind = CharacterLiteral::Ascii;
+
+    E = new (Context) CharacterLiteral(Arg.getAsIntegral().getZExtValue(),
+                                       Kind, T, Loc);
+  } else if (T->isBooleanType()) {
+    E = new (Context) CXXBoolLiteralExpr(Arg.getAsIntegral().getBoolValue(),
+                                         T, Loc);
+  } else if (T->isNullPtrType()) {
+    E = new (Context) CXXNullPtrLiteralExpr(Context.NullPtrTy, Loc);
+  } else {
+    E = IntegerLiteral::Create(Context, Arg.getAsIntegral(), T, Loc);
+  }
+
+  if (OrigT->isEnumeralType()) {
+    // FIXME: This is a hack. We need a better way to handle substituted
+    // non-type template parameters.
+    E = CStyleCastExpr::Create(Context, OrigT, VK_PRValue, CK_IntegralCast, E,
+                               nullptr, CurFPFeatureOverrides(),
+                               Context.getTrivialTypeSourceInfo(OrigT, Loc),
+                               Loc, Loc);
+  }
+
+  return E;
+}
+
+/// Match two template parameters within template parameter lists.
+static bool MatchTemplateParameterKind(Sema &S, NamedDecl *New, NamedDecl *Old,
+                                       bool Complain,
+                                     Sema::TemplateParameterListEqualKind Kind,
+                                       SourceLocation TemplateArgLoc) {
+  // Check the actual kind (type, non-type, template).
+  if (Old->getKind() != New->getKind()) {
+    if (Complain) {
+      unsigned NextDiag = diag::err_template_param_different_kind;
+      if (TemplateArgLoc.isValid()) {
+        S.Diag(TemplateArgLoc, diag::err_template_arg_template_params_mismatch);
+        NextDiag = diag::note_template_param_different_kind;
+      }
+      S.Diag(New->getLocation(), NextDiag)
+        << (Kind != Sema::TPL_TemplateMatch);
+      S.Diag(Old->getLocation(), diag::note_template_prev_declaration)
+        << (Kind != Sema::TPL_TemplateMatch);
+    }
+
+    return false;
+  }
+
+  // Check that both are parameter packs or neither are parameter packs.
+  // However, if we are matching a template template argument to a
+  // template template parameter, the template template parameter can have
+  // a parameter pack where the template template argument does not.
+  if (Old->isTemplateParameterPack() != New->isTemplateParameterPack() &&
+      !(Kind == Sema::TPL_TemplateTemplateArgumentMatch &&
+        Old->isTemplateParameterPack())) {
+    if (Complain) {
+      unsigned NextDiag = diag::err_template_parameter_pack_non_pack;
+      if (TemplateArgLoc.isValid()) {
+        S.Diag(TemplateArgLoc,
+             diag::err_template_arg_template_params_mismatch);
+        NextDiag = diag::note_template_parameter_pack_non_pack;
+      }
+
+      unsigned ParamKind = isa<TemplateTypeParmDecl>(New)? 0
+                      : isa<NonTypeTemplateParmDecl>(New)? 1
+                      : 2;
+      S.Diag(New->getLocation(), NextDiag)
+        << ParamKind << New->isParameterPack();
+      S.Diag(Old->getLocation(), diag::note_template_parameter_pack_here)
+        << ParamKind << Old->isParameterPack();
+    }
+
+    return false;
+  }
+
+  // For non-type template parameters, check the type of the parameter.
+  if (NonTypeTemplateParmDecl *OldNTTP
+                                    = dyn_cast<NonTypeTemplateParmDecl>(Old)) {
+    NonTypeTemplateParmDecl *NewNTTP = cast<NonTypeTemplateParmDecl>(New);
+
+    // If we are matching a template template argument to a template
+    // template parameter and one of the non-type template parameter types
+    // is dependent, then we must wait until template instantiation time
+    // to actually compare the arguments.
+    if (Kind != Sema::TPL_TemplateTemplateArgumentMatch ||
+        (!OldNTTP->getType()->isDependentType() &&
+         !NewNTTP->getType()->isDependentType()))
+      if (!S.Context.hasSameType(OldNTTP->getType(), NewNTTP->getType())) {
+        if (Complain) {
+          unsigned NextDiag = diag::err_template_nontype_parm_different_type;
+          if (TemplateArgLoc.isValid()) {
+            S.Diag(TemplateArgLoc,
+                   diag::err_template_arg_template_params_mismatch);
+            NextDiag = diag::note_template_nontype_parm_different_type;
+          }
+          S.Diag(NewNTTP->getLocation(), NextDiag)
+            << NewNTTP->getType()
+            << (Kind != Sema::TPL_TemplateMatch);
+          S.Diag(OldNTTP->getLocation(),
+                 diag::note_template_nontype_parm_prev_declaration)
+            << OldNTTP->getType();
+        }
+
+        return false;
+      }
+  }
+  // For template template parameters, check the template parameter types.
+  // The template parameter lists of template template
+  // parameters must agree.
+  else if (TemplateTemplateParmDecl *OldTTP
+                                    = dyn_cast<TemplateTemplateParmDecl>(Old)) {
+    TemplateTemplateParmDecl *NewTTP = cast<TemplateTemplateParmDecl>(New);
+    if (!S.TemplateParameterListsAreEqual(NewTTP->getTemplateParameters(),
+                                          OldTTP->getTemplateParameters(),
+                                          Complain,
+                                        (Kind == Sema::TPL_TemplateMatch
+                                           ? Sema::TPL_TemplateTemplateParmMatch
+                                           : Kind),
+                                          TemplateArgLoc))
+      return false;
+  } else if (Kind != Sema::TPL_TemplateTemplateArgumentMatch) {
+    const Expr *NewC = nullptr, *OldC = nullptr;
+    if (const auto *TC = cast<TemplateTypeParmDecl>(New)->getTypeConstraint())
+      NewC = TC->getImmediatelyDeclaredConstraint();
+    if (const auto *TC = cast<TemplateTypeParmDecl>(Old)->getTypeConstraint())
+      OldC = TC->getImmediatelyDeclaredConstraint();
+
+    auto Diagnose = [&] {
+      S.Diag(NewC ? NewC->getBeginLoc() : New->getBeginLoc(),
+           diag::err_template_different_type_constraint);
+      S.Diag(OldC ? OldC->getBeginLoc() : Old->getBeginLoc(),
+           diag::note_template_prev_declaration) << /*declaration*/0;
+    };
+
+    if (!NewC != !OldC) {
+      if (Complain)
+        Diagnose();
+      return false;
+    }
+
+    if (NewC) {
+      llvm::FoldingSetNodeID OldCID, NewCID;
+      OldC->Profile(OldCID, S.Context, /*Canonical=*/true);
+      NewC->Profile(NewCID, S.Context, /*Canonical=*/true);
+      if (OldCID != NewCID) {
+        if (Complain)
+          Diagnose();
+        return false;
+      }
+    }
+  }
+
+  return true;
+}
+
+/// Diagnose a known arity mismatch when comparing template argument
+/// lists.
+static
+void DiagnoseTemplateParameterListArityMismatch(Sema &S,
+                                                TemplateParameterList *New,
+                                                TemplateParameterList *Old,
+                                      Sema::TemplateParameterListEqualKind Kind,
+                                                SourceLocation TemplateArgLoc) {
+  unsigned NextDiag = diag::err_template_param_list_different_arity;
+  if (TemplateArgLoc.isValid()) {
+    S.Diag(TemplateArgLoc, diag::err_template_arg_template_params_mismatch);
+    NextDiag = diag::note_template_param_list_different_arity;
+  }
+  S.Diag(New->getTemplateLoc(), NextDiag)
+    << (New->size() > Old->size())
+    << (Kind != Sema::TPL_TemplateMatch)
+    << SourceRange(New->getTemplateLoc(), New->getRAngleLoc());
+  S.Diag(Old->getTemplateLoc(), diag::note_template_prev_declaration)
+    << (Kind != Sema::TPL_TemplateMatch)
+    << SourceRange(Old->getTemplateLoc(), Old->getRAngleLoc());
+}
+
+/// Determine whether the given template parameter lists are
+/// equivalent.
+///
+/// \param New  The new template parameter list, typically written in the
+/// source code as part of a new template declaration.
+///
+/// \param Old  The old template parameter list, typically found via
+/// name lookup of the template declared with this template parameter
+/// list.
+///
+/// \param Complain  If true, this routine will produce a diagnostic if
+/// the template parameter lists are not equivalent.
+///
+/// \param Kind describes how we are to match the template parameter lists.
+///
+/// \param TemplateArgLoc If this source location is valid, then we
+/// are actually checking the template parameter list of a template
+/// argument (New) against the template parameter list of its
+/// corresponding template template parameter (Old). We produce
+/// slightly different diagnostics in this scenario.
+///
+/// \returns True if the template parameter lists are equal, false
+/// otherwise.
+bool
+Sema::TemplateParameterListsAreEqual(TemplateParameterList *New,
+                                     TemplateParameterList *Old,
+                                     bool Complain,
+                                     TemplateParameterListEqualKind Kind,
+                                     SourceLocation TemplateArgLoc) {
+  if (Old->size() != New->size() && Kind != TPL_TemplateTemplateArgumentMatch) {
+    if (Complain)
+      DiagnoseTemplateParameterListArityMismatch(*this, New, Old, Kind,
+                                                 TemplateArgLoc);
+
+    return false;
+  }
+
+  // C++0x [temp.arg.template]p3:
+  //   A template-argument matches a template template-parameter (call it P)
+  //   when each of the template parameters in the template-parameter-list of
+  //   the template-argument's corresponding class template or alias template
+  //   (call it A) matches the corresponding template parameter in the
+  //   template-parameter-list of P. [...]
+  TemplateParameterList::iterator NewParm = New->begin();
+  TemplateParameterList::iterator NewParmEnd = New->end();
+  for (TemplateParameterList::iterator OldParm = Old->begin(),
+                                    OldParmEnd = Old->end();
+       OldParm != OldParmEnd; ++OldParm) {
+    if (Kind != TPL_TemplateTemplateArgumentMatch ||
+        !(*OldParm)->isTemplateParameterPack()) {
+      if (NewParm == NewParmEnd) {
+        if (Complain)
+          DiagnoseTemplateParameterListArityMismatch(*this, New, Old, Kind,
+                                                     TemplateArgLoc);
+
+        return false;
+      }
+
+      if (!MatchTemplateParameterKind(*this, *NewParm, *OldParm, Complain,
+                                      Kind, TemplateArgLoc))
+        return false;
+
+      ++NewParm;
+      continue;
+    }
+
+    // C++0x [temp.arg.template]p3:
+    //   [...] When P's template- parameter-list contains a template parameter
+    //   pack (14.5.3), the template parameter pack will match zero or more
+    //   template parameters or template parameter packs in the
+    //   template-parameter-list of A with the same type and form as the
+    //   template parameter pack in P (ignoring whether those template
+    //   parameters are template parameter packs).
+    for (; NewParm != NewParmEnd; ++NewParm) {
+      if (!MatchTemplateParameterKind(*this, *NewParm, *OldParm, Complain,
+                                      Kind, TemplateArgLoc))
+        return false;
+    }
+  }
+
+  // Make sure we exhausted all of the arguments.
+  if (NewParm != NewParmEnd) {
+    if (Complain)
+      DiagnoseTemplateParameterListArityMismatch(*this, New, Old, Kind,
+                                                 TemplateArgLoc);
+
+    return false;
+  }
+
+  if (Kind != TPL_TemplateTemplateArgumentMatch) {
+    const Expr *NewRC = New->getRequiresClause();
+    const Expr *OldRC = Old->getRequiresClause();
+
+    auto Diagnose = [&] {
+      Diag(NewRC ? NewRC->getBeginLoc() : New->getTemplateLoc(),
+           diag::err_template_different_requires_clause);
+      Diag(OldRC ? OldRC->getBeginLoc() : Old->getTemplateLoc(),
+           diag::note_template_prev_declaration) << /*declaration*/0;
+    };
+
+    if (!NewRC != !OldRC) {
+      if (Complain)
+        Diagnose();
+      return false;
+    }
+
+    if (NewRC) {
+      llvm::FoldingSetNodeID OldRCID, NewRCID;
+      OldRC->Profile(OldRCID, Context, /*Canonical=*/true);
+      NewRC->Profile(NewRCID, Context, /*Canonical=*/true);
+      if (OldRCID != NewRCID) {
+        if (Complain)
+          Diagnose();
+        return false;
+      }
+    }
+  }
+
+  return true;
+}
+
+/// Check whether a template can be declared within this scope.
+///
+/// If the template declaration is valid in this scope, returns
+/// false. Otherwise, issues a diagnostic and returns true.
+bool
+Sema::CheckTemplateDeclScope(Scope *S, TemplateParameterList *TemplateParams) {
+  if (!S)
+    return false;
+
+  // Find the nearest enclosing declaration scope.
+  while ((S->getFlags() & Scope::DeclScope) == 0 ||
+         (S->getFlags() & Scope::TemplateParamScope) != 0)
+    S = S->getParent();
+
+  // C++ [temp.pre]p6: [P2096]
+  //   A template, explicit specialization, or partial specialization shall not
+  //   have C linkage.
+  DeclContext *Ctx = S->getEntity();
+  if (Ctx && Ctx->isExternCContext()) {
+    Diag(TemplateParams->getTemplateLoc(), diag::err_template_linkage)
+        << TemplateParams->getSourceRange();
+    if (const LinkageSpecDecl *LSD = Ctx->getExternCContext())
+      Diag(LSD->getExternLoc(), diag::note_extern_c_begins_here);
+    return true;
+  }
+  Ctx = Ctx ? Ctx->getRedeclContext() : nullptr;
+
+  // C++ [temp]p2:
+  //   A template-declaration can appear only as a namespace scope or
+  //   class scope declaration.
+  // C++ [temp.expl.spec]p3:
+  //   An explicit specialization may be declared in any scope in which the
+  //   corresponding primary template may be defined.
+  // C++ [temp.class.spec]p6: [P2096]
+  //   A partial specialization may be declared in any scope in which the
+  //   corresponding primary template may be defined.
+  if (Ctx) {
+    if (Ctx->isFileContext())
+      return false;
+    if (CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(Ctx)) {
+      // C++ [temp.mem]p2:
+      //   A local class shall not have member templates.
+      if (RD->isLocalClass())
+        return Diag(TemplateParams->getTemplateLoc(),
+                    diag::err_template_inside_local_class)
+          << TemplateParams->getSourceRange();
+      else
+        return false;
+    }
+  }
+
+  return Diag(TemplateParams->getTemplateLoc(),
+              diag::err_template_outside_namespace_or_class_scope)
+    << TemplateParams->getSourceRange();
+}
+
+/// Determine what kind of template specialization the given declaration
+/// is.
+static TemplateSpecializationKind getTemplateSpecializationKind(Decl *D) {
+  if (!D)
+    return TSK_Undeclared;
+
+  if (CXXRecordDecl *Record = dyn_cast<CXXRecordDecl>(D))
+    return Record->getTemplateSpecializationKind();
+  if (FunctionDecl *Function = dyn_cast<FunctionDecl>(D))
+    return Function->getTemplateSpecializationKind();
+  if (VarDecl *Var = dyn_cast<VarDecl>(D))
+    return Var->getTemplateSpecializationKind();
+
+  return TSK_Undeclared;
+}
+
+/// Check whether a specialization is well-formed in the current
+/// context.
+///
+/// This routine determines whether a template specialization can be declared
+/// in the current context (C++ [temp.expl.spec]p2).
+///
+/// \param S the semantic analysis object for which this check is being
+/// performed.
+///
+/// \param Specialized the entity being specialized or instantiated, which
+/// may be a kind of template (class template, function template, etc.) or
+/// a member of a class template (member function, static data member,
+/// member class).
+///
+/// \param PrevDecl the previous declaration of this entity, if any.
+///
+/// \param Loc the location of the explicit specialization or instantiation of
+/// this entity.
+///
+/// \param IsPartialSpecialization whether this is a partial specialization of
+/// a class template.
+///
+/// \returns true if there was an error that we cannot recover from, false
+/// otherwise.
+static bool CheckTemplateSpecializationScope(Sema &S,
+                                             NamedDecl *Specialized,
+                                             NamedDecl *PrevDecl,
+                                             SourceLocation Loc,
+                                             bool IsPartialSpecialization) {
+  // Keep these "kind" numbers in sync with the %select statements in the
+  // various diagnostics emitted by this routine.
+  int EntityKind = 0;
+  if (isa<ClassTemplateDecl>(Specialized))
+    EntityKind = IsPartialSpecialization? 1 : 0;
+  else if (isa<VarTemplateDecl>(Specialized))
+    EntityKind = IsPartialSpecialization ? 3 : 2;
+  else if (isa<FunctionTemplateDecl>(Specialized))
+    EntityKind = 4;
+  else if (isa<CXXMethodDecl>(Specialized))
+    EntityKind = 5;
+  else if (isa<VarDecl>(Specialized))
+    EntityKind = 6;
+  else if (isa<RecordDecl>(Specialized))
+    EntityKind = 7;
+  else if (isa<EnumDecl>(Specialized) && S.getLangOpts().CPlusPlus11)
+    EntityKind = 8;
+  else {
+    S.Diag(Loc, diag::err_template_spec_unknown_kind)
+      << S.getLangOpts().CPlusPlus11;
+    S.Diag(Specialized->getLocation(), diag::note_specialized_entity);
+    return true;
+  }
+
+  // C++ [temp.expl.spec]p2:
+  //   An explicit specialization may be declared in any scope in which
+  //   the corresponding primary template may be defined.
+  if (S.CurContext->getRedeclContext()->isFunctionOrMethod()) {
+    S.Diag(Loc, diag::err_template_spec_decl_function_scope)
+      << Specialized;
+    return true;
+  }
+
+  // C++ [temp.class.spec]p6:
+  //   A class template partial specialization may be declared in any
+  //   scope in which the primary template may be defined.
+  DeclContext *SpecializedContext =
+      Specialized->getDeclContext()->getRedeclContext();
+  DeclContext *DC = S.CurContext->getRedeclContext();
+
+  // Make sure that this redeclaration (or definition) occurs in the same
+  // scope or an enclosing namespace.
+  if (!(DC->isFileContext() ? DC->Encloses(SpecializedContext)
+                            : DC->Equals(SpecializedContext))) {
+    if (isa<TranslationUnitDecl>(SpecializedContext))
+      S.Diag(Loc, diag::err_template_spec_redecl_global_scope)
+        << EntityKind << Specialized;
+    else {
+      auto *ND = cast<NamedDecl>(SpecializedContext);
+      int Diag = diag::err_template_spec_redecl_out_of_scope;
+      if (S.getLangOpts().MicrosoftExt && !DC->isRecord())
+        Diag = diag::ext_ms_template_spec_redecl_out_of_scope;
+      S.Diag(Loc, Diag) << EntityKind << Specialized
+                        << ND << isa<CXXRecordDecl>(ND);
+    }
+
+    S.Diag(Specialized->getLocation(), diag::note_specialized_entity);
+
+    // Don't allow specializing in the wrong class during error recovery.
+    // Otherwise, things can go horribly wrong.
+    if (DC->isRecord())
+      return true;
+  }
+
+  return false;
+}
+
+static SourceRange findTemplateParameterInType(unsigned Depth, Expr *E) {
+  if (!E->isTypeDependent())
+    return SourceLocation();
+  DependencyChecker Checker(Depth, /*IgnoreNonTypeDependent*/true);
+  Checker.TraverseStmt(E);
+  if (Checker.MatchLoc.isInvalid())
+    return E->getSourceRange();
+  return Checker.MatchLoc;
+}
+
+static SourceRange findTemplateParameter(unsigned Depth, TypeLoc TL) {
+  if (!TL.getType()->isDependentType())
+    return SourceLocation();
+  DependencyChecker Checker(Depth, /*IgnoreNonTypeDependent*/true);
+  Checker.TraverseTypeLoc(TL);
+  if (Checker.MatchLoc.isInvalid())
+    return TL.getSourceRange();
+  return Checker.MatchLoc;
+}
+
+/// Subroutine of Sema::CheckTemplatePartialSpecializationArgs
+/// that checks non-type template partial specialization arguments.
+static bool CheckNonTypeTemplatePartialSpecializationArgs(
+    Sema &S, SourceLocation TemplateNameLoc, NonTypeTemplateParmDecl *Param,
+    const TemplateArgument *Args, unsigned NumArgs, bool IsDefaultArgument) {
+  for (unsigned I = 0; I != NumArgs; ++I) {
+    if (Args[I].getKind() == TemplateArgument::Pack) {
+      if (CheckNonTypeTemplatePartialSpecializationArgs(
+              S, TemplateNameLoc, Param, Args[I].pack_begin(),
+              Args[I].pack_size(), IsDefaultArgument))
+        return true;
+
+      continue;
+    }
+
+    if (Args[I].getKind() != TemplateArgument::Expression)
+      continue;
+
+    Expr *ArgExpr = Args[I].getAsExpr();
+
+    // We can have a pack expansion of any of the bullets below.
+    if (PackExpansionExpr *Expansion = dyn_cast<PackExpansionExpr>(ArgExpr))
+      ArgExpr = Expansion->getPattern();
+
+    // Strip off any implicit casts we added as part of type checking.
+    while (ImplicitCastExpr *ICE = dyn_cast<ImplicitCastExpr>(ArgExpr))
+      ArgExpr = ICE->getSubExpr();
+
+    // C++ [temp.class.spec]p8:
+    //   A non-type argument is non-specialized if it is the name of a
+    //   non-type parameter. All other non-type arguments are
+    //   specialized.
+    //
+    // Below, we check the two conditions that only apply to
+    // specialized non-type arguments, so skip any non-specialized
+    // arguments.
+    if (DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(ArgExpr))
+      if (isa<NonTypeTemplateParmDecl>(DRE->getDecl()))
+        continue;
+
+    // C++ [temp.class.spec]p9:
+    //   Within the argument list of a class template partial
+    //   specialization, the following restrictions apply:
+    //     -- A partially specialized non-type argument expression
+    //        shall not involve a template parameter of the partial
+    //        specialization except when the argument expression is a
+    //        simple identifier.
+    //     -- The type of a template parameter corresponding to a
+    //        specialized non-type argument shall not be dependent on a
+    //        parameter of the specialization.
+    // DR1315 removes the first bullet, leaving an incoherent set of rules.
+    // We implement a compromise between the original rules and DR1315:
+    //     --  A specialized non-type template argument shall not be
+    //         type-dependent and the corresponding template parameter
+    //         shall have a non-dependent type.
+    SourceRange ParamUseRange =
+        findTemplateParameterInType(Param->getDepth(), ArgExpr);
+    if (ParamUseRange.isValid()) {
+      if (IsDefaultArgument) {
+        S.Diag(TemplateNameLoc,
+               diag::err_dependent_non_type_arg_in_partial_spec);
+        S.Diag(ParamUseRange.getBegin(),
+               diag::note_dependent_non_type_default_arg_in_partial_spec)
+          << ParamUseRange;
+      } else {
+        S.Diag(ParamUseRange.getBegin(),
+               diag::err_dependent_non_type_arg_in_partial_spec)
+          << ParamUseRange;
+      }
+      return true;
+    }
+
+    ParamUseRange = findTemplateParameter(
+        Param->getDepth(), Param->getTypeSourceInfo()->getTypeLoc());
+    if (ParamUseRange.isValid()) {
+      S.Diag(IsDefaultArgument ? TemplateNameLoc : ArgExpr->getBeginLoc(),
+             diag::err_dependent_typed_non_type_arg_in_partial_spec)
+          << Param->getType();
+      S.Diag(Param->getLocation(), diag::note_template_param_here)
+        << (IsDefaultArgument ? ParamUseRange : SourceRange())
+        << ParamUseRange;
+      return true;
+    }
+  }
+
+  return false;
+}
+
+/// Check the non-type template arguments of a class template
+/// partial specialization according to C++ [temp.class.spec]p9.
+///
+/// \param TemplateNameLoc the location of the template name.
+/// \param PrimaryTemplate the template parameters of the primary class
+///        template.
+/// \param NumExplicit the number of explicitly-specified template arguments.
+/// \param TemplateArgs the template arguments of the class template
+///        partial specialization.
+///
+/// \returns \c true if there was an error, \c false otherwise.
+bool Sema::CheckTemplatePartialSpecializationArgs(
+    SourceLocation TemplateNameLoc, TemplateDecl *PrimaryTemplate,
+    unsigned NumExplicit, ArrayRef<TemplateArgument> TemplateArgs) {
+  // We have to be conservative when checking a template in a dependent
+  // context.
+  if (PrimaryTemplate->getDeclContext()->isDependentContext())
+    return false;
+
+  TemplateParameterList *TemplateParams =
+      PrimaryTemplate->getTemplateParameters();
+  for (unsigned I = 0, N = TemplateParams->size(); I != N; ++I) {
+    NonTypeTemplateParmDecl *Param
+      = dyn_cast<NonTypeTemplateParmDecl>(TemplateParams->getParam(I));
+    if (!Param)
+      continue;
+
+    if (CheckNonTypeTemplatePartialSpecializationArgs(*this, TemplateNameLoc,
+                                                      Param, &TemplateArgs[I],
+                                                      1, I >= NumExplicit))
+      return true;
+  }
+
+  return false;
+}
+
+DeclResult Sema::ActOnClassTemplateSpecialization(
+    Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
+    SourceLocation ModulePrivateLoc, CXXScopeSpec &SS,
+    TemplateIdAnnotation &TemplateId, const ParsedAttributesView &Attr,
+    MultiTemplateParamsArg TemplateParameterLists, SkipBodyInfo *SkipBody) {
+  assert(TUK != TUK_Reference && "References are not specializations");
+
+  // NOTE: KWLoc is the location of the tag keyword. This will instead
+  // store the location of the outermost template keyword in the declaration.
+  SourceLocation TemplateKWLoc = TemplateParameterLists.size() > 0
+    ? TemplateParameterLists[0]->getTemplateLoc() : KWLoc;
+  SourceLocation TemplateNameLoc = TemplateId.TemplateNameLoc;
+  SourceLocation LAngleLoc = TemplateId.LAngleLoc;
+  SourceLocation RAngleLoc = TemplateId.RAngleLoc;
+
+  // Find the class template we're specializing
+  TemplateName Name = TemplateId.Template.get();
+  ClassTemplateDecl *ClassTemplate
+    = dyn_cast_or_null<ClassTemplateDecl>(Name.getAsTemplateDecl());
+
+  if (!ClassTemplate) {
+    Diag(TemplateNameLoc, diag::err_not_class_template_specialization)
+      << (Name.getAsTemplateDecl() &&
+          isa<TemplateTemplateParmDecl>(Name.getAsTemplateDecl()));
+    return true;
+  }
+
+  bool isMemberSpecialization = false;
+  bool isPartialSpecialization = false;
+
+  // Check the validity of the template headers that introduce this
+  // template.
+  // FIXME: We probably shouldn't complain about these headers for
+  // friend declarations.
+  bool Invalid = false;
+  TemplateParameterList *TemplateParams =
+      MatchTemplateParametersToScopeSpecifier(
+          KWLoc, TemplateNameLoc, SS, &TemplateId,
+          TemplateParameterLists, TUK == TUK_Friend, isMemberSpecialization,
+          Invalid);
+  if (Invalid)
+    return true;
+
+  // Check that we can declare a template specialization here.
+  if (TemplateParams && CheckTemplateDeclScope(S, TemplateParams))
+    return true;
+
+  if (TemplateParams && TemplateParams->size() > 0) {
+    isPartialSpecialization = true;
+
+    if (TUK == TUK_Friend) {
+      Diag(KWLoc, diag::err_partial_specialization_friend)
+        << SourceRange(LAngleLoc, RAngleLoc);
+      return true;
+    }
+
+    // C++ [temp.class.spec]p10:
+    //   The template parameter list of a specialization shall not
+    //   contain default template argument values.
+    for (unsigned I = 0, N = TemplateParams->size(); I != N; ++I) {
+      Decl *Param = TemplateParams->getParam(I);
+      if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(Param)) {
+        if (TTP->hasDefaultArgument()) {
+          Diag(TTP->getDefaultArgumentLoc(),
+               diag::err_default_arg_in_partial_spec);
+          TTP->removeDefaultArgument();
+        }
+      } else if (NonTypeTemplateParmDecl *NTTP
+                   = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
+        if (Expr *DefArg = NTTP->getDefaultArgument()) {
+          Diag(NTTP->getDefaultArgumentLoc(),
+               diag::err_default_arg_in_partial_spec)
+            << DefArg->getSourceRange();
+          NTTP->removeDefaultArgument();
+        }
+      } else {
+        TemplateTemplateParmDecl *TTP = cast<TemplateTemplateParmDecl>(Param);
+        if (TTP->hasDefaultArgument()) {
+          Diag(TTP->getDefaultArgument().getLocation(),
+               diag::err_default_arg_in_partial_spec)
+            << TTP->getDefaultArgument().getSourceRange();
+          TTP->removeDefaultArgument();
+        }
+      }
+    }
+  } else if (TemplateParams) {
+    if (TUK == TUK_Friend)
+      Diag(KWLoc, diag::err_template_spec_friend)
+        << FixItHint::CreateRemoval(
+                                SourceRange(TemplateParams->getTemplateLoc(),
+                                            TemplateParams->getRAngleLoc()))
+        << SourceRange(LAngleLoc, RAngleLoc);
+  } else {
+    assert(TUK == TUK_Friend && "should have a 'template<>' for this decl");
+  }
+
+  // Check that the specialization uses the same tag kind as the
+  // original template.
+  TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForTypeSpec(TagSpec);
+  assert(Kind != TTK_Enum && "Invalid enum tag in class template spec!");
+  if (!isAcceptableTagRedeclaration(ClassTemplate->getTemplatedDecl(),
+                                    Kind, TUK == TUK_Definition, KWLoc,
+                                    ClassTemplate->getIdentifier())) {
+    Diag(KWLoc, diag::err_use_with_wrong_tag)
+      << ClassTemplate
+      << FixItHint::CreateReplacement(KWLoc,
+                            ClassTemplate->getTemplatedDecl()->getKindName());
+    Diag(ClassTemplate->getTemplatedDecl()->getLocation(),
+         diag::note_previous_use);
+    Kind = ClassTemplate->getTemplatedDecl()->getTagKind();
+  }
+
+  // Translate the parser's template argument list in our AST format.
+  TemplateArgumentListInfo TemplateArgs =
+      makeTemplateArgumentListInfo(*this, TemplateId);
+
+  // Check for unexpanded parameter packs in any of the template arguments.
+  for (unsigned I = 0, N = TemplateArgs.size(); I != N; ++I)
+    if (DiagnoseUnexpandedParameterPack(TemplateArgs[I],
+                                        UPPC_PartialSpecialization))
+      return true;
+
+  // Check that the template argument list is well-formed for this
+  // template.
+  SmallVector<TemplateArgument, 4> Converted;
+  if (CheckTemplateArgumentList(ClassTemplate, TemplateNameLoc,
+                                TemplateArgs, false, Converted,
+                                /*UpdateArgsWithConversions=*/true))
+    return true;
+
+  // Find the class template (partial) specialization declaration that
+  // corresponds to these arguments.
+  if (isPartialSpecialization) {
+    if (CheckTemplatePartialSpecializationArgs(TemplateNameLoc, ClassTemplate,
+                                               TemplateArgs.size(), Converted))
+      return true;
+
+    // FIXME: Move this to CheckTemplatePartialSpecializationArgs so we
+    // also do it during instantiation.
+    if (!Name.isDependent() &&
+        !TemplateSpecializationType::anyDependentTemplateArguments(TemplateArgs,
+                                                                   Converted)) {
+      Diag(TemplateNameLoc, diag::err_partial_spec_fully_specialized)
+        << ClassTemplate->getDeclName();
+      isPartialSpecialization = false;
+    }
+  }
+
+  void *InsertPos = nullptr;
+  ClassTemplateSpecializationDecl *PrevDecl = nullptr;
+
+  if (isPartialSpecialization)
+    PrevDecl = ClassTemplate->findPartialSpecialization(Converted,
+                                                        TemplateParams,
+                                                        InsertPos);
+  else
+    PrevDecl = ClassTemplate->findSpecialization(Converted, InsertPos);
+
+  ClassTemplateSpecializationDecl *Specialization = nullptr;
+
+  // Check whether we can declare a class template specialization in
+  // the current scope.
+  if (TUK != TUK_Friend &&
+      CheckTemplateSpecializationScope(*this, ClassTemplate, PrevDecl,
+                                       TemplateNameLoc,
+                                       isPartialSpecialization))
+    return true;
+
+  // The canonical type
+  QualType CanonType;
+  if (isPartialSpecialization) {
+    // Build the canonical type that describes the converted template
+    // arguments of the class template partial specialization.
+    TemplateName CanonTemplate = Context.getCanonicalTemplateName(Name);
+    CanonType = Context.getTemplateSpecializationType(CanonTemplate,
+                                                      Converted);
+
+    if (Context.hasSameType(CanonType,
+                        ClassTemplate->getInjectedClassNameSpecialization()) &&
+        (!Context.getLangOpts().CPlusPlus20 ||
+         !TemplateParams->hasAssociatedConstraints())) {
+      // C++ [temp.class.spec]p9b3:
+      //
+      //   -- The argument list of the specialization shall not be identical
+      //      to the implicit argument list of the primary template.
+      //
+      // This rule has since been removed, because it's redundant given DR1495,
+      // but we keep it because it produces better diagnostics and recovery.
+      Diag(TemplateNameLoc, diag::err_partial_spec_args_match_primary_template)
+        << /*class template*/0 << (TUK == TUK_Definition)
+        << FixItHint::CreateRemoval(SourceRange(LAngleLoc, RAngleLoc));
+      return CheckClassTemplate(S, TagSpec, TUK, KWLoc, SS,
+                                ClassTemplate->getIdentifier(),
+                                TemplateNameLoc,
+                                Attr,
+                                TemplateParams,
+                                AS_none, /*ModulePrivateLoc=*/SourceLocation(),
+                                /*FriendLoc*/SourceLocation(),
+                                TemplateParameterLists.size() - 1,
+                                TemplateParameterLists.data());
+    }
+
+    // Create a new class template partial specialization declaration node.
+    ClassTemplatePartialSpecializationDecl *PrevPartial
+      = cast_or_null<ClassTemplatePartialSpecializationDecl>(PrevDecl);
+    ClassTemplatePartialSpecializationDecl *Partial
+      = ClassTemplatePartialSpecializationDecl::Create(Context, Kind,
+                                             ClassTemplate->getDeclContext(),
+                                                       KWLoc, TemplateNameLoc,
+                                                       TemplateParams,
+                                                       ClassTemplate,
+                                                       Converted,
+                                                       TemplateArgs,
+                                                       CanonType,
+                                                       PrevPartial);
+    SetNestedNameSpecifier(*this, Partial, SS);
+    if (TemplateParameterLists.size() > 1 && SS.isSet()) {
+      Partial->setTemplateParameterListsInfo(
+          Context, TemplateParameterLists.drop_back(1));
+    }
+
+    if (!PrevPartial)
+      ClassTemplate->AddPartialSpecialization(Partial, InsertPos);
+    Specialization = Partial;
+
+    // If we are providing an explicit specialization of a member class
+    // template specialization, make a note of that.
+    if (PrevPartial && PrevPartial->getInstantiatedFromMember())
+      PrevPartial->setMemberSpecialization();
+
+    CheckTemplatePartialSpecialization(Partial);
+  } else {
+    // Create a new class template specialization declaration node for
+    // this explicit specialization or friend declaration.
+    Specialization
+      = ClassTemplateSpecializationDecl::Create(Context, Kind,
+                                             ClassTemplate->getDeclContext(),
+                                                KWLoc, TemplateNameLoc,
+                                                ClassTemplate,
+                                                Converted,
+                                                PrevDecl);
+    SetNestedNameSpecifier(*this, Specialization, SS);
+    if (TemplateParameterLists.size() > 0) {
+      Specialization->setTemplateParameterListsInfo(Context,
+                                                    TemplateParameterLists);
+    }
+
+    if (!PrevDecl)
+      ClassTemplate->AddSpecialization(Specialization, InsertPos);
+
+    if (CurContext->isDependentContext()) {
+      TemplateName CanonTemplate = Context.getCanonicalTemplateName(Name);
+      CanonType = Context.getTemplateSpecializationType(
+          CanonTemplate, Converted);
+    } else {
+      CanonType = Context.getTypeDeclType(Specialization);
+    }
+  }
+
+  // C++ [temp.expl.spec]p6:
+  //   If a template, a member template or the member of a class template is
+  //   explicitly specialized then that specialization shall be declared
+  //   before the first use of that specialization that would cause an implicit
+  //   instantiation to take place, in every translation unit in which such a
+  //   use occurs; no diagnostic is required.
+  if (PrevDecl && PrevDecl->getPointOfInstantiation().isValid()) {
+    bool Okay = false;
+    for (Decl *Prev = PrevDecl; Prev; Prev = Prev->getPreviousDecl()) {
+      // Is there any previous explicit specialization declaration?
+      if (getTemplateSpecializationKind(Prev) == TSK_ExplicitSpecialization) {
+        Okay = true;
+        break;
+      }
+    }
+
+    if (!Okay) {
+      SourceRange Range(TemplateNameLoc, RAngleLoc);
+      Diag(TemplateNameLoc, diag::err_specialization_after_instantiation)
+        << Context.getTypeDeclType(Specialization) << Range;
+
+      Diag(PrevDecl->getPointOfInstantiation(),
+           diag::note_instantiation_required_here)
+        << (PrevDecl->getTemplateSpecializationKind()
+                                                != TSK_ImplicitInstantiation);
+      return true;
+    }
+  }
+
+  // If this is not a friend, note that this is an explicit specialization.
+  if (TUK != TUK_Friend)
+    Specialization->setSpecializationKind(TSK_ExplicitSpecialization);
+
+  // Check that this isn't a redefinition of this specialization.
+  if (TUK == TUK_Definition) {
+    RecordDecl *Def = Specialization->getDefinition();
+    NamedDecl *Hidden = nullptr;
+    if (Def && SkipBody && !hasVisibleDefinition(Def, &Hidden)) {
+      SkipBody->ShouldSkip = true;
+      SkipBody->Previous = Def;
+      makeMergedDefinitionVisible(Hidden);
+    } else if (Def) {
+      SourceRange Range(TemplateNameLoc, RAngleLoc);
+      Diag(TemplateNameLoc, diag::err_redefinition) << Specialization << Range;
+      Diag(Def->getLocation(), diag::note_previous_definition);
+      Specialization->setInvalidDecl();
+      return true;
+    }
+  }
+
+  ProcessDeclAttributeList(S, Specialization, Attr);
+
+  // Add alignment attributes if necessary; these attributes are checked when
+  // the ASTContext lays out the structure.
+  if (TUK == TUK_Definition && (!SkipBody || !SkipBody->ShouldSkip)) {
+    AddAlignmentAttributesForRecord(Specialization);
+    AddMsStructLayoutForRecord(Specialization);
+  }
+
+  if (ModulePrivateLoc.isValid())
+    Diag(Specialization->getLocation(), diag::err_module_private_specialization)
+      << (isPartialSpecialization? 1 : 0)
+      << FixItHint::CreateRemoval(ModulePrivateLoc);
+
+  // Build the fully-sugared type for this class template
+  // specialization as the user wrote in the specialization
+  // itself. This means that we'll pretty-print the type retrieved
+  // from the specialization's declaration the way that the user
+  // actually wrote the specialization, rather than formatting the
+  // name based on the "canonical" representation used to store the
+  // template arguments in the specialization.
+  TypeSourceInfo *WrittenTy
+    = Context.getTemplateSpecializationTypeInfo(Name, TemplateNameLoc,
+                                                TemplateArgs, CanonType);
+  if (TUK != TUK_Friend) {
+    Specialization->setTypeAsWritten(WrittenTy);
+    Specialization->setTemplateKeywordLoc(TemplateKWLoc);
+  }
+
+  // C++ [temp.expl.spec]p9:
+  //   A template explicit specialization is in the scope of the
+  //   namespace in which the template was defined.
+  //
+  // We actually implement this paragraph where we set the semantic
+  // context (in the creation of the ClassTemplateSpecializationDecl),
+  // but we also maintain the lexical context where the actual
+  // definition occurs.
+  Specialization->setLexicalDeclContext(CurContext);
+
+  // We may be starting the definition of this specialization.
+  if (TUK == TUK_Definition && (!SkipBody || !SkipBody->ShouldSkip))
+    Specialization->startDefinition();
+
+  if (TUK == TUK_Friend) {
+    FriendDecl *Friend = FriendDecl::Create(Context, CurContext,
+                                            TemplateNameLoc,
+                                            WrittenTy,
+                                            /*FIXME:*/KWLoc);
+    Friend->setAccess(AS_public);
+    CurContext->addDecl(Friend);
+  } else {
+    // Add the specialization into its lexical context, so that it can
+    // be seen when iterating through the list of declarations in that
+    // context. However, specializations are not found by name lookup.
+    CurContext->addDecl(Specialization);
+  }
+
+  if (SkipBody && SkipBody->ShouldSkip)
+    return SkipBody->Previous;
+
+  return Specialization;
+}
+
+Decl *Sema::ActOnTemplateDeclarator(Scope *S,
+                              MultiTemplateParamsArg TemplateParameterLists,
+                                    Declarator &D) {
+  Decl *NewDecl = HandleDeclarator(S, D, TemplateParameterLists);
+  ActOnDocumentableDecl(NewDecl);
+  return NewDecl;
+}
+
+Decl *Sema::ActOnConceptDefinition(Scope *S,
+                              MultiTemplateParamsArg TemplateParameterLists,
+                                   IdentifierInfo *Name, SourceLocation NameLoc,
+                                   Expr *ConstraintExpr) {
+  DeclContext *DC = CurContext;
+
+  if (!DC->getRedeclContext()->isFileContext()) {
+    Diag(NameLoc,
+      diag::err_concept_decls_may_only_appear_in_global_namespace_scope);
+    return nullptr;
+  }
+
+  if (TemplateParameterLists.size() > 1) {
+    Diag(NameLoc, diag::err_concept_extra_headers);
+    return nullptr;
+  }
+
+  if (TemplateParameterLists.front()->size() == 0) {
+    Diag(NameLoc, diag::err_concept_no_parameters);
+    return nullptr;
+  }
+
+  if (DiagnoseUnexpandedParameterPack(ConstraintExpr))
+    return nullptr;
+
+  ConceptDecl *NewDecl = ConceptDecl::Create(Context, DC, NameLoc, Name,
+                                             TemplateParameterLists.front(),
+                                             ConstraintExpr);
+
+  if (NewDecl->hasAssociatedConstraints()) {
+    // C++2a [temp.concept]p4:
+    // A concept shall not have associated constraints.
+    Diag(NameLoc, diag::err_concept_no_associated_constraints);
+    NewDecl->setInvalidDecl();
+  }
+
+  // Check for conflicting previous declaration.
+  DeclarationNameInfo NameInfo(NewDecl->getDeclName(), NameLoc);
+  LookupResult Previous(*this, NameInfo, LookupOrdinaryName,
+                        ForVisibleRedeclaration);
+  LookupName(Previous, S);
+
+  FilterLookupForScope(Previous, DC, S, /*ConsiderLinkage=*/false,
+                       /*AllowInlineNamespace*/false);
+  if (!Previous.empty()) {
+    auto *Old = Previous.getRepresentativeDecl();
+    Diag(NameLoc, isa<ConceptDecl>(Old) ? diag::err_redefinition :
+         diag::err_redefinition_different_kind) << NewDecl->getDeclName();
+    Diag(Old->getLocation(), diag::note_previous_definition);
+  }
+
+  ActOnDocumentableDecl(NewDecl);
+  PushOnScopeChains(NewDecl, S);
+  return NewDecl;
+}
+
+/// \brief Strips various properties off an implicit instantiation
+/// that has just been explicitly specialized.
+static void StripImplicitInstantiation(NamedDecl *D) {
+  D->dropAttr<DLLImportAttr>();
+  D->dropAttr<DLLExportAttr>();
+
+  if (FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
+    FD->setInlineSpecified(false);
+}
+
+/// Compute the diagnostic location for an explicit instantiation
+//  declaration or definition.
+static SourceLocation DiagLocForExplicitInstantiation(
+    NamedDecl* D, SourceLocation PointOfInstantiation) {
+  // Explicit instantiations following a specialization have no effect and
+  // hence no PointOfInstantiation. In that case, walk decl backwards
+  // until a valid name loc is found.
+  SourceLocation PrevDiagLoc = PointOfInstantiation;
+  for (Decl *Prev = D; Prev && !PrevDiagLoc.isValid();
+       Prev = Prev->getPreviousDecl()) {
+    PrevDiagLoc = Prev->getLocation();
+  }
+  assert(PrevDiagLoc.isValid() &&
+         "Explicit instantiation without point of instantiation?");
+  return PrevDiagLoc;
+}
+
+/// Diagnose cases where we have an explicit template specialization
+/// before/after an explicit template instantiation, producing diagnostics
+/// for those cases where they are required and determining whether the
+/// new specialization/instantiation will have any effect.
+///
+/// \param NewLoc the location of the new explicit specialization or
+/// instantiation.
+///
+/// \param NewTSK the kind of the new explicit specialization or instantiation.
+///
+/// \param PrevDecl the previous declaration of the entity.
+///
+/// \param PrevTSK the kind of the old explicit specialization or instantiatin.
+///
+/// \param PrevPointOfInstantiation if valid, indicates where the previous
+/// declaration was instantiated (either implicitly or explicitly).
+///
+/// \param HasNoEffect will be set to true to indicate that the new
+/// specialization or instantiation has no effect and should be ignored.
+///
+/// \returns true if there was an error that should prevent the introduction of
+/// the new declaration into the AST, false otherwise.
+bool
+Sema::CheckSpecializationInstantiationRedecl(SourceLocation NewLoc,
+                                             TemplateSpecializationKind NewTSK,
+                                             NamedDecl *PrevDecl,
+                                             TemplateSpecializationKind PrevTSK,
+                                        SourceLocation PrevPointOfInstantiation,
+                                             bool &HasNoEffect) {
+  HasNoEffect = false;
+
+  switch (NewTSK) {
+  case TSK_Undeclared:
+  case TSK_ImplicitInstantiation:
+    assert(
+        (PrevTSK == TSK_Undeclared || PrevTSK == TSK_ImplicitInstantiation) &&
+        "previous declaration must be implicit!");
+    return false;
+
+  case TSK_ExplicitSpecialization:
+    switch (PrevTSK) {
+    case TSK_Undeclared:
+    case TSK_ExplicitSpecialization:
+      // Okay, we're just specializing something that is either already
+      // explicitly specialized or has merely been mentioned without any
+      // instantiation.
+      return false;
+
+    case TSK_ImplicitInstantiation:
+      if (PrevPointOfInstantiation.isInvalid()) {
+        // The declaration itself has not actually been instantiated, so it is
+        // still okay to specialize it.
+        StripImplicitInstantiation(PrevDecl);
+        return false;
+      }
+      // Fall through
+      LLVM_FALLTHROUGH;
+
+    case TSK_ExplicitInstantiationDeclaration:
+    case TSK_ExplicitInstantiationDefinition:
+      assert((PrevTSK == TSK_ImplicitInstantiation ||
+              PrevPointOfInstantiation.isValid()) &&
+             "Explicit instantiation without point of instantiation?");
+
+      // C++ [temp.expl.spec]p6:
+      //   If a template, a member template or the member of a class template
+      //   is explicitly specialized then that specialization shall be declared
+      //   before the first use of that specialization that would cause an
+      //   implicit instantiation to take place, in every translation unit in
+      //   which such a use occurs; no diagnostic is required.
+      for (Decl *Prev = PrevDecl; Prev; Prev = Prev->getPreviousDecl()) {
+        // Is there any previous explicit specialization declaration?
+        if (getTemplateSpecializationKind(Prev) == TSK_ExplicitSpecialization)
+          return false;
+      }
+
+      Diag(NewLoc, diag::err_specialization_after_instantiation)
+        << PrevDecl;
+      Diag(PrevPointOfInstantiation, diag::note_instantiation_required_here)
+        << (PrevTSK != TSK_ImplicitInstantiation);
+
+      return true;
+    }
+    llvm_unreachable("The switch over PrevTSK must be exhaustive.");
+
+  case TSK_ExplicitInstantiationDeclaration:
+    switch (PrevTSK) {
+    case TSK_ExplicitInstantiationDeclaration:
+      // This explicit instantiation declaration is redundant (that's okay).
+      HasNoEffect = true;
+      return false;
+
+    case TSK_Undeclared:
+    case TSK_ImplicitInstantiation:
+      // We're explicitly instantiating something that may have already been
+      // implicitly instantiated; that's fine.
+      return false;
+
+    case TSK_ExplicitSpecialization:
+      // C++0x [temp.explicit]p4:
+      //   For a given set of template parameters, if an explicit instantiation
+      //   of a template appears after a declaration of an explicit
+      //   specialization for that template, the explicit instantiation has no
+      //   effect.
+      HasNoEffect = true;
+      return false;
+
+    case TSK_ExplicitInstantiationDefinition:
+      // C++0x [temp.explicit]p10:
+      //   If an entity is the subject of both an explicit instantiation
+      //   declaration and an explicit instantiation definition in the same
+      //   translation unit, the definition shall follow the declaration.
+      Diag(NewLoc,
+           diag::err_explicit_instantiation_declaration_after_definition);
+
+      // Explicit instantiations following a specialization have no effect and
+      // hence no PrevPointOfInstantiation. In that case, walk decl backwards
+      // until a valid name loc is found.
+      Diag(DiagLocForExplicitInstantiation(PrevDecl, PrevPointOfInstantiation),
+           diag::note_explicit_instantiation_definition_here);
+      HasNoEffect = true;
+      return false;
+    }
+    llvm_unreachable("Unexpected TemplateSpecializationKind!");
+
+  case TSK_ExplicitInstantiationDefinition:
+    switch (PrevTSK) {
+    case TSK_Undeclared:
+    case TSK_ImplicitInstantiation:
+      // We're explicitly instantiating something that may have already been
+      // implicitly instantiated; that's fine.
+      return false;
+
+    case TSK_ExplicitSpecialization:
+      // C++ DR 259, C++0x [temp.explicit]p4:
+      //   For a given set of template parameters, if an explicit
+      //   instantiation of a template appears after a declaration of
+      //   an explicit specialization for that template, the explicit
+      //   instantiation has no effect.
+      Diag(NewLoc, diag::warn_explicit_instantiation_after_specialization)
+        << PrevDecl;
+      Diag(PrevDecl->getLocation(),
+           diag::note_previous_template_specialization);
+      HasNoEffect = true;
+      return false;
+
+    case TSK_ExplicitInstantiationDeclaration:
+      // We're explicitly instantiating a definition for something for which we
+      // were previously asked to suppress instantiations. That's fine.
+
+      // C++0x [temp.explicit]p4:
+      //   For a given set of template parameters, if an explicit instantiation
+      //   of a template appears after a declaration of an explicit
+      //   specialization for that template, the explicit instantiation has no
+      //   effect.
+      for (Decl *Prev = PrevDecl; Prev; Prev = Prev->getPreviousDecl()) {
+        // Is there any previous explicit specialization declaration?
+        if (getTemplateSpecializationKind(Prev) == TSK_ExplicitSpecialization) {
+          HasNoEffect = true;
+          break;
+        }
+      }
+
+      return false;
+
+    case TSK_ExplicitInstantiationDefinition:
+      // C++0x [temp.spec]p5:
+      //   For a given template and a given set of template-arguments,
+      //     - an explicit instantiation definition shall appear at most once
+      //       in a program,
+
+      // MSVCCompat: MSVC silently ignores duplicate explicit instantiations.
+      Diag(NewLoc, (getLangOpts().MSVCCompat)
+                       ? diag::ext_explicit_instantiation_duplicate
+                       : diag::err_explicit_instantiation_duplicate)
+          << PrevDecl;
+      Diag(DiagLocForExplicitInstantiation(PrevDecl, PrevPointOfInstantiation),
+           diag::note_previous_explicit_instantiation);
+      HasNoEffect = true;
+      return false;
+    }
+  }
+
+  llvm_unreachable("Missing specialization/instantiation case?");
+}
+
+/// Perform semantic analysis for the given dependent function
+/// template specialization.
+///
+/// The only possible way to get a dependent function template specialization
+/// is with a friend declaration, like so:
+///
+/// \code
+///   template \<class T> void foo(T);
+///   template \<class T> class A {
+///     friend void foo<>(T);
+///   };
+/// \endcode
+///
+/// There really isn't any useful analysis we can do here, so we
+/// just store the information.
+bool
+Sema::CheckDependentFunctionTemplateSpecialization(FunctionDecl *FD,
+                   const TemplateArgumentListInfo &ExplicitTemplateArgs,
+                                                   LookupResult &Previous) {
+  // Remove anything from Previous that isn't a function template in
+  // the correct context.
+  DeclContext *FDLookupContext = FD->getDeclContext()->getRedeclContext();
+  LookupResult::Filter F = Previous.makeFilter();
+  enum DiscardReason { NotAFunctionTemplate, NotAMemberOfEnclosing };
+  SmallVector<std::pair<DiscardReason, Decl *>, 8> DiscardedCandidates;
+  while (F.hasNext()) {
+    NamedDecl *D = F.next()->getUnderlyingDecl();
+    if (!isa<FunctionTemplateDecl>(D)) {
+      F.erase();
+      DiscardedCandidates.push_back(std::make_pair(NotAFunctionTemplate, D));
+      continue;
+    }
+
+    if (!FDLookupContext->InEnclosingNamespaceSetOf(
+            D->getDeclContext()->getRedeclContext())) {
+      F.erase();
+      DiscardedCandidates.push_back(std::make_pair(NotAMemberOfEnclosing, D));
+      continue;
+    }
+  }
+  F.done();
+
+  if (Previous.empty()) {
+    Diag(FD->getLocation(),
+         diag::err_dependent_function_template_spec_no_match);
+    for (auto &P : DiscardedCandidates)
+      Diag(P.second->getLocation(),
+           diag::note_dependent_function_template_spec_discard_reason)
+          << P.first;
+    return true;
+  }
+
+  FD->setDependentTemplateSpecialization(Context, Previous.asUnresolvedSet(),
+                                         ExplicitTemplateArgs);
+  return false;
+}
+
+/// Perform semantic analysis for the given function template
+/// specialization.
+///
+/// This routine performs all of the semantic analysis required for an
+/// explicit function template specialization. On successful completion,
+/// the function declaration \p FD will become a function template
+/// specialization.
+///
+/// \param FD the function declaration, which will be updated to become a
+/// function template specialization.
+///
+/// \param ExplicitTemplateArgs the explicitly-provided template arguments,
+/// if any. Note that this may be valid info even when 0 arguments are
+/// explicitly provided as in, e.g., \c void sort<>(char*, char*);
+/// as it anyway contains info on the angle brackets locations.
+///
+/// \param Previous the set of declarations that may be specialized by
+/// this function specialization.
+///
+/// \param QualifiedFriend whether this is a lookup for a qualified friend
+/// declaration with no explicit template argument list that might be
+/// befriending a function template specialization.
+bool Sema::CheckFunctionTemplateSpecialization(
+    FunctionDecl *FD, TemplateArgumentListInfo *ExplicitTemplateArgs,
+    LookupResult &Previous, bool QualifiedFriend) {
+  // The set of function template specializations that could match this
+  // explicit function template specialization.
+  UnresolvedSet<8> Candidates;
+  TemplateSpecCandidateSet FailedCandidates(FD->getLocation(),
+                                            /*ForTakingAddress=*/false);
+
+  llvm::SmallDenseMap<FunctionDecl *, TemplateArgumentListInfo, 8>
+      ConvertedTemplateArgs;
+
+  DeclContext *FDLookupContext = FD->getDeclContext()->getRedeclContext();
+  for (LookupResult::iterator I = Previous.begin(), E = Previous.end();
+         I != E; ++I) {
+    NamedDecl *Ovl = (*I)->getUnderlyingDecl();
+    if (FunctionTemplateDecl *FunTmpl = dyn_cast<FunctionTemplateDecl>(Ovl)) {
+      // Only consider templates found within the same semantic lookup scope as
+      // FD.
+      if (!FDLookupContext->InEnclosingNamespaceSetOf(
+                                Ovl->getDeclContext()->getRedeclContext()))
+        continue;
+
+      // When matching a constexpr member function template specialization
+      // against the primary template, we don't yet know whether the
+      // specialization has an implicit 'const' (because we don't know whether
+      // it will be a static member function until we know which template it
+      // specializes), so adjust it now assuming it specializes this template.
+      QualType FT = FD->getType();
+      if (FD->isConstexpr()) {
+        CXXMethodDecl *OldMD =
+          dyn_cast<CXXMethodDecl>(FunTmpl->getTemplatedDecl());
+        if (OldMD && OldMD->isConst()) {
+          const FunctionProtoType *FPT = FT->castAs<FunctionProtoType>();
+          FunctionProtoType::ExtProtoInfo EPI = FPT->getExtProtoInfo();
+          EPI.TypeQuals.addConst();
+          FT = Context.getFunctionType(FPT->getReturnType(),
+                                       FPT->getParamTypes(), EPI);
+        }
+      }
+
+      TemplateArgumentListInfo Args;
+      if (ExplicitTemplateArgs)
+        Args = *ExplicitTemplateArgs;
+
+      // C++ [temp.expl.spec]p11:
+      //   A trailing template-argument can be left unspecified in the
+      //   template-id naming an explicit function template specialization
+      //   provided it can be deduced from the function argument type.
+      // Perform template argument deduction to determine whether we may be
+      // specializing this template.
+      // FIXME: It is somewhat wasteful to build
+      TemplateDeductionInfo Info(FailedCandidates.getLocation());
+      FunctionDecl *Specialization = nullptr;
+      if (TemplateDeductionResult TDK = DeduceTemplateArguments(
+              cast<FunctionTemplateDecl>(FunTmpl->getFirstDecl()),
+              ExplicitTemplateArgs ? &Args : nullptr, FT, Specialization,
+              Info)) {
+        // Template argument deduction failed; record why it failed, so
+        // that we can provide nifty diagnostics.
+        FailedCandidates.addCandidate().set(
+            I.getPair(), FunTmpl->getTemplatedDecl(),
+            MakeDeductionFailureInfo(Context, TDK, Info));
+        (void)TDK;
+        continue;
+      }
+
+      // Target attributes are part of the cuda function signature, so
+      // the deduced template's cuda target must match that of the
+      // specialization.  Given that C++ template deduction does not
+      // take target attributes into account, we reject candidates
+      // here that have a different target.
+      if (LangOpts.CUDA &&
+          IdentifyCUDATarget(Specialization,
+                             /* IgnoreImplicitHDAttr = */ true) !=
+              IdentifyCUDATarget(FD, /* IgnoreImplicitHDAttr = */ true)) {
+        FailedCandidates.addCandidate().set(
+            I.getPair(), FunTmpl->getTemplatedDecl(),
+            MakeDeductionFailureInfo(Context, TDK_CUDATargetMismatch, Info));
+        continue;
+      }
+
+      // Record this candidate.
+      if (ExplicitTemplateArgs)
+        ConvertedTemplateArgs[Specialization] = std::move(Args);
+      Candidates.addDecl(Specialization, I.getAccess());
+    }
+  }
+
+  // For a qualified friend declaration (with no explicit marker to indicate
+  // that a template specialization was intended), note all (template and
+  // non-template) candidates.
+  if (QualifiedFriend && Candidates.empty()) {
+    Diag(FD->getLocation(), diag::err_qualified_friend_no_match)
+        << FD->getDeclName() << FDLookupContext;
+    // FIXME: We should form a single candidate list and diagnose all
+    // candidates at once, to get proper sorting and limiting.
+    for (auto *OldND : Previous) {
+      if (auto *OldFD = dyn_cast<FunctionDecl>(OldND->getUnderlyingDecl()))
+        NoteOverloadCandidate(OldND, OldFD, CRK_None, FD->getType(), false);
+    }
+    FailedCandidates.NoteCandidates(*this, FD->getLocation());
+    return true;
+  }
+
+  // Find the most specialized function template.
+  UnresolvedSetIterator Result = getMostSpecialized(
+      Candidates.begin(), Candidates.end(), FailedCandidates, FD->getLocation(),
+      PDiag(diag::err_function_template_spec_no_match) << FD->getDeclName(),
+      PDiag(diag::err_function_template_spec_ambiguous)
+          << FD->getDeclName() << (ExplicitTemplateArgs != nullptr),
+      PDiag(diag::note_function_template_spec_matched));
+
+  if (Result == Candidates.end())
+    return true;
+
+  // Ignore access information;  it doesn't figure into redeclaration checking.
+  FunctionDecl *Specialization = cast<FunctionDecl>(*Result);
+
+  FunctionTemplateSpecializationInfo *SpecInfo
+    = Specialization->getTemplateSpecializationInfo();
+  assert(SpecInfo && "Function template specialization info missing?");
+
+  // Note: do not overwrite location info if previous template
+  // specialization kind was explicit.
+  TemplateSpecializationKind TSK = SpecInfo->getTemplateSpecializationKind();
+  if (TSK == TSK_Undeclared || TSK == TSK_ImplicitInstantiation) {
+    Specialization->setLocation(FD->getLocation());
+    Specialization->setLexicalDeclContext(FD->getLexicalDeclContext());
+    // C++11 [dcl.constexpr]p1: An explicit specialization of a constexpr
+    // function can differ from the template declaration with respect to
+    // the constexpr specifier.
+    // FIXME: We need an update record for this AST mutation.
+    // FIXME: What if there are multiple such prior declarations (for instance,
+    // from different modules)?
+    Specialization->setConstexprKind(FD->getConstexprKind());
+  }
+
+  // FIXME: Check if the prior specialization has a point of instantiation.
+  // If so, we have run afoul of .
+
+  // If this is a friend declaration, then we're not really declaring
+  // an explicit specialization.
+  bool isFriend = (FD->getFriendObjectKind() != Decl::FOK_None);
+
+  // Check the scope of this explicit specialization.
+  if (!isFriend &&
+      CheckTemplateSpecializationScope(*this,
+                                       Specialization->getPrimaryTemplate(),
+                                       Specialization, FD->getLocation(),
+                                       false))
+    return true;
+
+  // C++ [temp.expl.spec]p6:
+  //   If a template, a member template or the member of a class template is
+  //   explicitly specialized then that specialization shall be declared
+  //   before the first use of that specialization that would cause an implicit
+  //   instantiation to take place, in every translation unit in which such a
+  //   use occurs; no diagnostic is required.
+  bool HasNoEffect = false;
+  if (!isFriend &&
+      CheckSpecializationInstantiationRedecl(FD->getLocation(),
+                                             TSK_ExplicitSpecialization,
+                                             Specialization,
+                                   SpecInfo->getTemplateSpecializationKind(),
+                                         SpecInfo->getPointOfInstantiation(),
+                                             HasNoEffect))
+    return true;
+
+  // Mark the prior declaration as an explicit specialization, so that later
+  // clients know that this is an explicit specialization.
+  if (!isFriend) {
+    // Since explicit specializations do not inherit '=delete' from their
+    // primary function template - check if the 'specialization' that was
+    // implicitly generated (during template argument deduction for partial
+    // ordering) from the most specialized of all the function templates that
+    // 'FD' could have been specializing, has a 'deleted' definition.  If so,
+    // first check that it was implicitly generated during template argument
+    // deduction by making sure it wasn't referenced, and then reset the deleted
+    // flag to not-deleted, so that we can inherit that information from 'FD'.
+    if (Specialization->isDeleted() && !SpecInfo->isExplicitSpecialization() &&
+        !Specialization->getCanonicalDecl()->isReferenced()) {
+      // FIXME: This assert will not hold in the presence of modules.
+      assert(
+          Specialization->getCanonicalDecl() == Specialization &&
+          "This must be the only existing declaration of this specialization");
+      // FIXME: We need an update record for this AST mutation.
+      Specialization->setDeletedAsWritten(false);
+    }
+    // FIXME: We need an update record for this AST mutation.
+    SpecInfo->setTemplateSpecializationKind(TSK_ExplicitSpecialization);
+    MarkUnusedFileScopedDecl(Specialization);
+  }
+
+  // Turn the given function declaration into a function template
+  // specialization, with the template arguments from the previous
+  // specialization.
+  // Take copies of (semantic and syntactic) template argument lists.
+  const TemplateArgumentList* TemplArgs = new (Context)
+    TemplateArgumentList(Specialization->getTemplateSpecializationArgs());
+  FD->setFunctionTemplateSpecialization(
+      Specialization->getPrimaryTemplate(), TemplArgs, /*InsertPos=*/nullptr,
+      SpecInfo->getTemplateSpecializationKind(),
+      ExplicitTemplateArgs ? &ConvertedTemplateArgs[Specialization] : nullptr);
+
+  // A function template specialization inherits the target attributes
+  // of its template.  (We require the attributes explicitly in the
+  // code to match, but a template may have implicit attributes by
+  // virtue e.g. of being constexpr, and it passes these implicit
+  // attributes on to its specializations.)
+  if (LangOpts.CUDA)
+    inheritCUDATargetAttrs(FD, *Specialization->getPrimaryTemplate());
+
+  // The "previous declaration" for this function template specialization is
+  // the prior function template specialization.
+  Previous.clear();
+  Previous.addDecl(Specialization);
+  return false;
+}
+
+/// Perform semantic analysis for the given non-template member
+/// specialization.
+///
+/// This routine performs all of the semantic analysis required for an
+/// explicit member function specialization. On successful completion,
+/// the function declaration \p FD will become a member function
+/// specialization.
+///
+/// \param Member the member declaration, which will be updated to become a
+/// specialization.
+///
+/// \param Previous the set of declarations, one of which may be specialized
+/// by this function specialization;  the set will be modified to contain the
+/// redeclared member.
+bool
+Sema::CheckMemberSpecialization(NamedDecl *Member, LookupResult &Previous) {
+  assert(!isa<TemplateDecl>(Member) && "Only for non-template members");
+
+  // Try to find the member we are instantiating.
+  NamedDecl *FoundInstantiation = nullptr;
+  NamedDecl *Instantiation = nullptr;
+  NamedDecl *InstantiatedFrom = nullptr;
+  MemberSpecializationInfo *MSInfo = nullptr;
+
+  if (Previous.empty()) {
+    // Nowhere to look anyway.
+  } else if (FunctionDecl *Function = dyn_cast<FunctionDecl>(Member)) {
+    for (LookupResult::iterator I = Previous.begin(), E = Previous.end();
+           I != E; ++I) {
+      NamedDecl *D = (*I)->getUnderlyingDecl();
+      if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(D)) {
+        QualType Adjusted = Function->getType();
+        if (!hasExplicitCallingConv(Adjusted))
+          Adjusted = adjustCCAndNoReturn(Adjusted, Method->getType());
+        // This doesn't handle deduced return types, but both function
+        // declarations should be undeduced at this point.
+        if (Context.hasSameType(Adjusted, Method->getType())) {
+          FoundInstantiation = *I;
+          Instantiation = Method;
+          InstantiatedFrom = Method->getInstantiatedFromMemberFunction();
+          MSInfo = Method->getMemberSpecializationInfo();
+          break;
+        }
+      }
+    }
+  } else if (isa<VarDecl>(Member)) {
+    VarDecl *PrevVar;
+    if (Previous.isSingleResult() &&
+        (PrevVar = dyn_cast<VarDecl>(Previous.getFoundDecl())))
+      if (PrevVar->isStaticDataMember()) {
+        FoundInstantiation = Previous.getRepresentativeDecl();
+        Instantiation = PrevVar;
+        InstantiatedFrom = PrevVar->getInstantiatedFromStaticDataMember();
+        MSInfo = PrevVar->getMemberSpecializationInfo();
+      }
+  } else if (isa<RecordDecl>(Member)) {
+    CXXRecordDecl *PrevRecord;
+    if (Previous.isSingleResult() &&
+        (PrevRecord = dyn_cast<CXXRecordDecl>(Previous.getFoundDecl()))) {
+      FoundInstantiation = Previous.getRepresentativeDecl();
+      Instantiation = PrevRecord;
+      InstantiatedFrom = PrevRecord->getInstantiatedFromMemberClass();
+      MSInfo = PrevRecord->getMemberSpecializationInfo();
+    }
+  } else if (isa<EnumDecl>(Member)) {
+    EnumDecl *PrevEnum;
+    if (Previous.isSingleResult() &&
+        (PrevEnum = dyn_cast<EnumDecl>(Previous.getFoundDecl()))) {
+      FoundInstantiation = Previous.getRepresentativeDecl();
+      Instantiation = PrevEnum;
+      InstantiatedFrom = PrevEnum->getInstantiatedFromMemberEnum();
+      MSInfo = PrevEnum->getMemberSpecializationInfo();
+    }
+  }
+
+  if (!Instantiation) {
+    // There is no previous declaration that matches. Since member
+    // specializations are always out-of-line, the caller will complain about
+    // this mismatch later.
+    return false;
+  }
+
+  // A member specialization in a friend declaration isn't really declaring
+  // an explicit specialization, just identifying a specific (possibly implicit)
+  // specialization. Don't change the template specialization kind.
+  //
+  // FIXME: Is this really valid? Other compilers reject.
+  if (Member->getFriendObjectKind() != Decl::FOK_None) {
+    // Preserve instantiation information.
+    if (InstantiatedFrom && isa<CXXMethodDecl>(Member)) {
+      cast<CXXMethodDecl>(Member)->setInstantiationOfMemberFunction(
+                                      cast<CXXMethodDecl>(InstantiatedFrom),
+        cast<CXXMethodDecl>(Instantiation)->getTemplateSpecializationKind());
+    } else if (InstantiatedFrom && isa<CXXRecordDecl>(Member)) {
+      cast<CXXRecordDecl>(Member)->setInstantiationOfMemberClass(
+                                      cast<CXXRecordDecl>(InstantiatedFrom),
+        cast<CXXRecordDecl>(Instantiation)->getTemplateSpecializationKind());
+    }
+
+    Previous.clear();
+    Previous.addDecl(FoundInstantiation);
+    return false;
+  }
+
+  // Make sure that this is a specialization of a member.
+  if (!InstantiatedFrom) {
+    Diag(Member->getLocation(), diag::err_spec_member_not_instantiated)
+      << Member;
+    Diag(Instantiation->getLocation(), diag::note_specialized_decl);
+    return true;
+  }
+
+  // C++ [temp.expl.spec]p6:
+  //   If a template, a member template or the member of a class template is
+  //   explicitly specialized then that specialization shall be declared
+  //   before the first use of that specialization that would cause an implicit
+  //   instantiation to take place, in every translation unit in which such a
+  //   use occurs; no diagnostic is required.
+  assert(MSInfo && "Member specialization info missing?");
+
+  bool HasNoEffect = false;
+  if (CheckSpecializationInstantiationRedecl(Member->getLocation(),
+                                             TSK_ExplicitSpecialization,
+                                             Instantiation,
+                                     MSInfo->getTemplateSpecializationKind(),
+                                           MSInfo->getPointOfInstantiation(),
+                                             HasNoEffect))
+    return true;
+
+  // Check the scope of this explicit specialization.
+  if (CheckTemplateSpecializationScope(*this,
+                                       InstantiatedFrom,
+                                       Instantiation, Member->getLocation(),
+                                       false))
+    return true;
+
+  // Note that this member specialization is an "instantiation of" the
+  // corresponding member of the original template.
+  if (auto *MemberFunction = dyn_cast<FunctionDecl>(Member)) {
+    FunctionDecl *InstantiationFunction = cast<FunctionDecl>(Instantiation);
+    if (InstantiationFunction->getTemplateSpecializationKind() ==
+          TSK_ImplicitInstantiation) {
+      // Explicit specializations of member functions of class templates do not
+      // inherit '=delete' from the member function they are specializing.
+      if (InstantiationFunction->isDeleted()) {
+        // FIXME: This assert will not hold in the presence of modules.
+        assert(InstantiationFunction->getCanonicalDecl() ==
+               InstantiationFunction);
+        // FIXME: We need an update record for this AST mutation.
+        InstantiationFunction->setDeletedAsWritten(false);
+      }
+    }
+
+    MemberFunction->setInstantiationOfMemberFunction(
+        cast<CXXMethodDecl>(InstantiatedFrom), TSK_ExplicitSpecialization);
+  } else if (auto *MemberVar = dyn_cast<VarDecl>(Member)) {
+    MemberVar->setInstantiationOfStaticDataMember(
+        cast<VarDecl>(InstantiatedFrom), TSK_ExplicitSpecialization);
+  } else if (auto *MemberClass = dyn_cast<CXXRecordDecl>(Member)) {
+    MemberClass->setInstantiationOfMemberClass(
+        cast<CXXRecordDecl>(InstantiatedFrom), TSK_ExplicitSpecialization);
+  } else if (auto *MemberEnum = dyn_cast<EnumDecl>(Member)) {
+    MemberEnum->setInstantiationOfMemberEnum(
+        cast<EnumDecl>(InstantiatedFrom), TSK_ExplicitSpecialization);
+  } else {
+    llvm_unreachable("unknown member specialization kind");
+  }
+
+  // Save the caller the trouble of having to figure out which declaration
+  // this specialization matches.
+  Previous.clear();
+  Previous.addDecl(FoundInstantiation);
+  return false;
+}
+
+/// Complete the explicit specialization of a member of a class template by
+/// updating the instantiated member to be marked as an explicit specialization.
+///
+/// \param OrigD The member declaration instantiated from the template.
+/// \param Loc The location of the explicit specialization of the member.
+template<typename DeclT>
+static void completeMemberSpecializationImpl(Sema &S, DeclT *OrigD,
+                                             SourceLocation Loc) {
+  if (OrigD->getTemplateSpecializationKind() != TSK_ImplicitInstantiation)
+    return;
+
+  // FIXME: Inform AST mutation listeners of this AST mutation.
+  // FIXME: If there are multiple in-class declarations of the member (from
+  // multiple modules, or a declaration and later definition of a member type),
+  // should we update all of them?
+  OrigD->setTemplateSpecializationKind(TSK_ExplicitSpecialization);
+  OrigD->setLocation(Loc);
+}
+
+void Sema::CompleteMemberSpecialization(NamedDecl *Member,
+                                        LookupResult &Previous) {
+  NamedDecl *Instantiation = cast<NamedDecl>(Member->getCanonicalDecl());
+  if (Instantiation == Member)
+    return;
+
+  if (auto *Function = dyn_cast<CXXMethodDecl>(Instantiation))
+    completeMemberSpecializationImpl(*this, Function, Member->getLocation());
+  else if (auto *Var = dyn_cast<VarDecl>(Instantiation))
+    completeMemberSpecializationImpl(*this, Var, Member->getLocation());
+  else if (auto *Record = dyn_cast<CXXRecordDecl>(Instantiation))
+    completeMemberSpecializationImpl(*this, Record, Member->getLocation());
+  else if (auto *Enum = dyn_cast<EnumDecl>(Instantiation))
+    completeMemberSpecializationImpl(*this, Enum, Member->getLocation());
+  else
+    llvm_unreachable("unknown member specialization kind");
+}
+
+/// Check the scope of an explicit instantiation.
+///
+/// \returns true if a serious error occurs, false otherwise.
+static bool CheckExplicitInstantiationScope(Sema &S, NamedDecl *D,
+                                            SourceLocation InstLoc,
+                                            bool WasQualifiedName) {
+  DeclContext *OrigContext= D->getDeclContext()->getEnclosingNamespaceContext();
+  DeclContext *CurContext = S.CurContext->getRedeclContext();
+
+  if (CurContext->isRecord()) {
+    S.Diag(InstLoc, diag::err_explicit_instantiation_in_class)
+      << D;
+    return true;
+  }
+
+  // C++11 [temp.explicit]p3:
+  //   An explicit instantiation shall appear in an enclosing namespace of its
+  //   template. If the name declared in the explicit instantiation is an
+  //   unqualified name, the explicit instantiation shall appear in the
+  //   namespace where its template is declared or, if that namespace is inline
+  //   (7.3.1), any namespace from its enclosing namespace set.
+  //
+  // This is DR275, which we do not retroactively apply to C++98/03.
+  if (WasQualifiedName) {
+    if (CurContext->Encloses(OrigContext))
+      return false;
+  } else {
+    if (CurContext->InEnclosingNamespaceSetOf(OrigContext))
+      return false;
+  }
+
+  if (NamespaceDecl *NS = dyn_cast<NamespaceDecl>(OrigContext)) {
+    if (WasQualifiedName)
+      S.Diag(InstLoc,
+             S.getLangOpts().CPlusPlus11?
+               diag::err_explicit_instantiation_out_of_scope :
+               diag::warn_explicit_instantiation_out_of_scope_0x)
+        << D << NS;
+    else
+      S.Diag(InstLoc,
+             S.getLangOpts().CPlusPlus11?
+               diag::err_explicit_instantiation_unqualified_wrong_namespace :
+               diag::warn_explicit_instantiation_unqualified_wrong_namespace_0x)
+        << D << NS;
+  } else
+    S.Diag(InstLoc,
+           S.getLangOpts().CPlusPlus11?
+             diag::err_explicit_instantiation_must_be_global :
+             diag::warn_explicit_instantiation_must_be_global_0x)
+      << D;
+  S.Diag(D->getLocation(), diag::note_explicit_instantiation_here);
+  return false;
+}
+
+/// Common checks for whether an explicit instantiation of \p D is valid.
+static bool CheckExplicitInstantiation(Sema &S, NamedDecl *D,
+                                       SourceLocation InstLoc,
+                                       bool WasQualifiedName,
+                                       TemplateSpecializationKind TSK) {
+  // C++ [temp.explicit]p13:
+  //   An explicit instantiation declaration shall not name a specialization of
+  //   a template with internal linkage.
+  if (TSK == TSK_ExplicitInstantiationDeclaration &&
+      D->getFormalLinkage() == InternalLinkage) {
+    S.Diag(InstLoc, diag::err_explicit_instantiation_internal_linkage) << D;
+    return true;
+  }
+
+  // C++11 [temp.explicit]p3: [DR 275]
+  //   An explicit instantiation shall appear in an enclosing namespace of its
+  //   template.
+  if (CheckExplicitInstantiationScope(S, D, InstLoc, WasQualifiedName))
+    return true;
+
+  return false;
+}
+
+/// Determine whether the given scope specifier has a template-id in it.
+static bool ScopeSpecifierHasTemplateId(const CXXScopeSpec &SS) {
+  if (!SS.isSet())
+    return false;
+
+  // C++11 [temp.explicit]p3:
+  //   If the explicit instantiation is for a member function, a member class
+  //   or a static data member of a class template specialization, the name of
+  //   the class template specialization in the qualified-id for the member
+  //   name shall be a simple-template-id.
+  //
+  // C++98 has the same restriction, just worded differently.
+  for (NestedNameSpecifier *NNS = SS.getScopeRep(); NNS;
+       NNS = NNS->getPrefix())
+    if (const Type *T = NNS->getAsType())
+      if (isa<TemplateSpecializationType>(T))
+        return true;
+
+  return false;
+}
+
+/// Make a dllexport or dllimport attr on a class template specialization take
+/// effect.
+static void dllExportImportClassTemplateSpecialization(
+    Sema &S, ClassTemplateSpecializationDecl *Def) {
+  auto *A = cast_or_null<InheritableAttr>(getDLLAttr(Def));
+  assert(A && "dllExportImportClassTemplateSpecialization called "
+              "on Def without dllexport or dllimport");
+
+  // We reject explicit instantiations in class scope, so there should
+  // never be any delayed exported classes to worry about.
+  assert(S.DelayedDllExportClasses.empty() &&
+         "delayed exports present at explicit instantiation");
+  S.checkClassLevelDLLAttribute(Def);
+
+  // Propagate attribute to base class templates.
+  for (auto &B : Def->bases()) {
+    if (auto *BT = dyn_cast_or_null<ClassTemplateSpecializationDecl>(
+            B.getType()->getAsCXXRecordDecl()))
+      S.propagateDLLAttrToBaseClassTemplate(Def, A, BT, B.getBeginLoc());
+  }
+
+  S.referenceDLLExportedClassMethods();
+}
+
+// Explicit instantiation of a class template specialization
+DeclResult Sema::ActOnExplicitInstantiation(
+    Scope *S, SourceLocation ExternLoc, SourceLocation TemplateLoc,
+    unsigned TagSpec, SourceLocation KWLoc, const CXXScopeSpec &SS,
+    TemplateTy TemplateD, SourceLocation TemplateNameLoc,
+    SourceLocation LAngleLoc, ASTTemplateArgsPtr TemplateArgsIn,
+    SourceLocation RAngleLoc, const ParsedAttributesView &Attr) {
+  // Find the class template we're specializing
+  TemplateName Name = TemplateD.get();
+  TemplateDecl *TD = Name.getAsTemplateDecl();
+  // Check that the specialization uses the same tag kind as the
+  // original template.
+  TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForTypeSpec(TagSpec);
+  assert(Kind != TTK_Enum &&
+         "Invalid enum tag in class template explicit instantiation!");
+
+  ClassTemplateDecl *ClassTemplate = dyn_cast<ClassTemplateDecl>(TD);
+
+  if (!ClassTemplate) {
+    NonTagKind NTK = getNonTagTypeDeclKind(TD, Kind);
+    Diag(TemplateNameLoc, diag::err_tag_reference_non_tag) << TD << NTK << Kind;
+    Diag(TD->getLocation(), diag::note_previous_use);
+    return true;
+  }
+
+  if (!isAcceptableTagRedeclaration(ClassTemplate->getTemplatedDecl(),
+                                    Kind, /*isDefinition*/false, KWLoc,
+                                    ClassTemplate->getIdentifier())) {
+    Diag(KWLoc, diag::err_use_with_wrong_tag)
+      << ClassTemplate
+      << FixItHint::CreateReplacement(KWLoc,
+                            ClassTemplate->getTemplatedDecl()->getKindName());
+    Diag(ClassTemplate->getTemplatedDecl()->getLocation(),
+         diag::note_previous_use);
+    Kind = ClassTemplate->getTemplatedDecl()->getTagKind();
+  }
+
+  // C++0x [temp.explicit]p2:
+  //   There are two forms of explicit instantiation: an explicit instantiation
+  //   definition and an explicit instantiation declaration. An explicit
+  //   instantiation declaration begins with the extern keyword. [...]
+  TemplateSpecializationKind TSK = ExternLoc.isInvalid()
+                                       ? TSK_ExplicitInstantiationDefinition
+                                       : TSK_ExplicitInstantiationDeclaration;
+
+  if (TSK == TSK_ExplicitInstantiationDeclaration &&
+      !Context.getTargetInfo().getTriple().isWindowsGNUEnvironment()) {
+    // Check for dllexport class template instantiation declarations,
+    // except for MinGW mode.
+    for (const ParsedAttr &AL : Attr) {
+      if (AL.getKind() == ParsedAttr::AT_DLLExport) {
+        Diag(ExternLoc,
+             diag::warn_attribute_dllexport_explicit_instantiation_decl);
+        Diag(AL.getLoc(), diag::note_attribute);
+        break;
+      }
+    }
+
+    if (auto *A = ClassTemplate->getTemplatedDecl()->getAttr<DLLExportAttr>()) {
+      Diag(ExternLoc,
+           diag::warn_attribute_dllexport_explicit_instantiation_decl);
+      Diag(A->getLocation(), diag::note_attribute);
+    }
+  }
+
+  // In MSVC mode, dllimported explicit instantiation definitions are treated as
+  // instantiation declarations for most purposes.
+  bool DLLImportExplicitInstantiationDef = false;
+  if (TSK == TSK_ExplicitInstantiationDefinition &&
+      Context.getTargetInfo().getCXXABI().isMicrosoft()) {
+    // Check for dllimport class template instantiation definitions.
+    bool DLLImport =
+        ClassTemplate->getTemplatedDecl()->getAttr<DLLImportAttr>();
+    for (const ParsedAttr &AL : Attr) {
+      if (AL.getKind() == ParsedAttr::AT_DLLImport)
+        DLLImport = true;
+      if (AL.getKind() == ParsedAttr::AT_DLLExport) {
+        // dllexport trumps dllimport here.
+        DLLImport = false;
+        break;
+      }
+    }
+    if (DLLImport) {
+      TSK = TSK_ExplicitInstantiationDeclaration;
+      DLLImportExplicitInstantiationDef = true;
+    }
+  }
+
+  // Translate the parser's template argument list in our AST format.
+  TemplateArgumentListInfo TemplateArgs(LAngleLoc, RAngleLoc);
+  translateTemplateArguments(TemplateArgsIn, TemplateArgs);
+
+  // Check that the template argument list is well-formed for this
+  // template.
+  SmallVector<TemplateArgument, 4> Converted;
+  if (CheckTemplateArgumentList(ClassTemplate, TemplateNameLoc,
+                                TemplateArgs, false, Converted,
+                                /*UpdateArgsWithConversions=*/true))
+    return true;
+
+  // Find the class template specialization declaration that
+  // corresponds to these arguments.
+  void *InsertPos = nullptr;
+  ClassTemplateSpecializationDecl *PrevDecl
+    = ClassTemplate->findSpecialization(Converted, InsertPos);
+
+  TemplateSpecializationKind PrevDecl_TSK
+    = PrevDecl ? PrevDecl->getTemplateSpecializationKind() : TSK_Undeclared;
+
+  if (TSK == TSK_ExplicitInstantiationDefinition && PrevDecl != nullptr &&
+      Context.getTargetInfo().getTriple().isWindowsGNUEnvironment()) {
+    // Check for dllexport class template instantiation definitions in MinGW
+    // mode, if a previous declaration of the instantiation was seen.
+    for (const ParsedAttr &AL : Attr) {
+      if (AL.getKind() == ParsedAttr::AT_DLLExport) {
+        Diag(AL.getLoc(),
+             diag::warn_attribute_dllexport_explicit_instantiation_def);
+        break;
+      }
+    }
+  }
+
+  if (CheckExplicitInstantiation(*this, ClassTemplate, TemplateNameLoc,
+                                 SS.isSet(), TSK))
+    return true;
+
+  ClassTemplateSpecializationDecl *Specialization = nullptr;
+
+  bool HasNoEffect = false;
+  if (PrevDecl) {
+    if (CheckSpecializationInstantiationRedecl(TemplateNameLoc, TSK,
+                                               PrevDecl, PrevDecl_TSK,
+                                            PrevDecl->getPointOfInstantiation(),
+                                               HasNoEffect))
+      return PrevDecl;
+
+    // Even though HasNoEffect == true means that this explicit instantiation
+    // has no effect on semantics, we go on to put its syntax in the AST.
+
+    if (PrevDecl_TSK == TSK_ImplicitInstantiation ||
+        PrevDecl_TSK == TSK_Undeclared) {
+      // Since the only prior class template specialization with these
+      // arguments was referenced but not declared, reuse that
+      // declaration node as our own, updating the source location
+      // for the template name to reflect our new declaration.
+      // (Other source locations will be updated later.)
+      Specialization = PrevDecl;
+      Specialization->setLocation(TemplateNameLoc);
+      PrevDecl = nullptr;
+    }
+
+    if (PrevDecl_TSK == TSK_ExplicitInstantiationDeclaration &&
+        DLLImportExplicitInstantiationDef) {
+      // The new specialization might add a dllimport attribute.
+      HasNoEffect = false;
+    }
+  }
+
+  if (!Specialization) {
+    // Create a new class template specialization declaration node for
+    // this explicit specialization.
+    Specialization
+      = ClassTemplateSpecializationDecl::Create(Context, Kind,
+                                             ClassTemplate->getDeclContext(),
+                                                KWLoc, TemplateNameLoc,
+                                                ClassTemplate,
+                                                Converted,
+                                                PrevDecl);
+    SetNestedNameSpecifier(*this, Specialization, SS);
+
+    if (!HasNoEffect && !PrevDecl) {
+      // Insert the new specialization.
+      ClassTemplate->AddSpecialization(Specialization, InsertPos);
+    }
+  }
+
+  // Build the fully-sugared type for this explicit instantiation as
+  // the user wrote in the explicit instantiation itself. This means
+  // that we'll pretty-print the type retrieved from the
+  // specialization's declaration the way that the user actually wrote
+  // the explicit instantiation, rather than formatting the name based
+  // on the "canonical" representation used to store the template
+  // arguments in the specialization.
+  TypeSourceInfo *WrittenTy
+    = Context.getTemplateSpecializationTypeInfo(Name, TemplateNameLoc,
+                                                TemplateArgs,
+                                  Context.getTypeDeclType(Specialization));
+  Specialization->setTypeAsWritten(WrittenTy);
+
+  // Set source locations for keywords.
+  Specialization->setExternLoc(ExternLoc);
+  Specialization->setTemplateKeywordLoc(TemplateLoc);
+  Specialization->setBraceRange(SourceRange());
+
+  bool PreviouslyDLLExported = Specialization->hasAttr<DLLExportAttr>();
+  ProcessDeclAttributeList(S, Specialization, Attr);
+
+  // Add the explicit instantiation into its lexical context. However,
+  // since explicit instantiations are never found by name lookup, we
+  // just put it into the declaration context directly.
+  Specialization->setLexicalDeclContext(CurContext);
+  CurContext->addDecl(Specialization);
+
+  // Syntax is now OK, so return if it has no other effect on semantics.
+  if (HasNoEffect) {
+    // Set the template specialization kind.
+    Specialization->setTemplateSpecializationKind(TSK);
+    return Specialization;
+  }
+
+  // C++ [temp.explicit]p3:
+  //   A definition of a class template or class member template
+  //   shall be in scope at the point of the explicit instantiation of
+  //   the class template or class member template.
+  //
+  // This check comes when we actually try to perform the
+  // instantiation.
+  ClassTemplateSpecializationDecl *Def
+    = cast_or_null<ClassTemplateSpecializationDecl>(
+                                              Specialization->getDefinition());
+  if (!Def)
+    InstantiateClassTemplateSpecialization(TemplateNameLoc, Specialization, TSK);
+  else if (TSK == TSK_ExplicitInstantiationDefinition) {
+    MarkVTableUsed(TemplateNameLoc, Specialization, true);
+    Specialization->setPointOfInstantiation(Def->getPointOfInstantiation());
+  }
+
+  // Instantiate the members of this class template specialization.
+  Def = cast_or_null<ClassTemplateSpecializationDecl>(
+                                       Specialization->getDefinition());
+  if (Def) {
+    TemplateSpecializationKind Old_TSK = Def->getTemplateSpecializationKind();
+    // Fix a TSK_ExplicitInstantiationDeclaration followed by a
+    // TSK_ExplicitInstantiationDefinition
+    if (Old_TSK == TSK_ExplicitInstantiationDeclaration &&
+        (TSK == TSK_ExplicitInstantiationDefinition ||
+         DLLImportExplicitInstantiationDef)) {
+      // FIXME: Need to notify the ASTMutationListener that we did this.
+      Def->setTemplateSpecializationKind(TSK);
+
+      if (!getDLLAttr(Def) && getDLLAttr(Specialization) &&
+          (Context.getTargetInfo().shouldDLLImportComdatSymbols() &&
+           !Context.getTargetInfo().getTriple().isPS4CPU())) {
+        // An explicit instantiation definition can add a dll attribute to a
+        // template with a previous instantiation declaration. MinGW doesn't
+        // allow this.
+        auto *A = cast<InheritableAttr>(
+            getDLLAttr(Specialization)->clone(getASTContext()));
+        A->setInherited(true);
+        Def->addAttr(A);
+        dllExportImportClassTemplateSpecialization(*this, Def);
+      }
+    }
+
+    // Fix a TSK_ImplicitInstantiation followed by a
+    // TSK_ExplicitInstantiationDefinition
+    bool NewlyDLLExported =
+        !PreviouslyDLLExported && Specialization->hasAttr<DLLExportAttr>();
+    if (Old_TSK == TSK_ImplicitInstantiation && NewlyDLLExported &&
+        (Context.getTargetInfo().shouldDLLImportComdatSymbols() &&
+         !Context.getTargetInfo().getTriple().isPS4CPU())) {
+      // An explicit instantiation definition can add a dll attribute to a
+      // template with a previous implicit instantiation. MinGW doesn't allow
+      // this. We limit clang to only adding dllexport, to avoid potentially
+      // strange codegen behavior. For example, if we extend this conditional
+      // to dllimport, and we have a source file calling a method on an
+      // implicitly instantiated template class instance and then declaring a
+      // dllimport explicit instantiation definition for the same template
+      // class, the codegen for the method call will not respect the dllimport,
+      // while it will with cl. The Def will already have the DLL attribute,
+      // since the Def and Specialization will be the same in the case of
+      // Old_TSK == TSK_ImplicitInstantiation, and we already added the
+      // attribute to the Specialization; we just need to make it take effect.
+      assert(Def == Specialization &&
+             "Def and Specialization should match for implicit instantiation");
+      dllExportImportClassTemplateSpecialization(*this, Def);
+    }
+
+    // In MinGW mode, export the template instantiation if the declaration
+    // was marked dllexport.
+    if (PrevDecl_TSK == TSK_ExplicitInstantiationDeclaration &&
+        Context.getTargetInfo().getTriple().isWindowsGNUEnvironment() &&
+        PrevDecl->hasAttr<DLLExportAttr>()) {
+      dllExportImportClassTemplateSpecialization(*this, Def);
+    }
+
+    if (Def->hasAttr<MSInheritanceAttr>()) {
+      Specialization->addAttr(Def->getAttr<MSInheritanceAttr>());
+      Consumer.AssignInheritanceModel(Specialization);
+    }
+
+    // Set the template specialization kind. Make sure it is set before
+    // instantiating the members which will trigger ASTConsumer callbacks.
+    Specialization->setTemplateSpecializationKind(TSK);
+    InstantiateClassTemplateSpecializationMembers(TemplateNameLoc, Def, TSK);
+  } else {
+
+    // Set the template specialization kind.
+    Specialization->setTemplateSpecializationKind(TSK);
+  }
+
+  return Specialization;
+}
+
+// Explicit instantiation of a member class of a class template.
+DeclResult
+Sema::ActOnExplicitInstantiation(Scope *S, SourceLocation ExternLoc,
+                                 SourceLocation TemplateLoc, unsigned TagSpec,
+                                 SourceLocation KWLoc, CXXScopeSpec &SS,
+                                 IdentifierInfo *Name, SourceLocation NameLoc,
+                                 const ParsedAttributesView &Attr) {
+
+  bool Owned = false;
+  bool IsDependent = false;
+  Decl *TagD = ActOnTag(S, TagSpec, Sema::TUK_Reference,
+                        KWLoc, SS, Name, NameLoc, Attr, AS_none,
+                        /*ModulePrivateLoc=*/SourceLocation(),
+                        MultiTemplateParamsArg(), Owned, IsDependent,
+                        SourceLocation(), false, TypeResult(),
+                        /*IsTypeSpecifier*/false,
+                        /*IsTemplateParamOrArg*/false);
+  assert(!IsDependent && "explicit instantiation of dependent name not yet handled");
+
+  if (!TagD)
+    return true;
+
+  TagDecl *Tag = cast<TagDecl>(TagD);
+  assert(!Tag->isEnum() && "shouldn't see enumerations here");
+
+  if (Tag->isInvalidDecl())
+    return true;
+
+  CXXRecordDecl *Record = cast<CXXRecordDecl>(Tag);
+  CXXRecordDecl *Pattern = Record->getInstantiatedFromMemberClass();
+  if (!Pattern) {
+    Diag(TemplateLoc, diag::err_explicit_instantiation_nontemplate_type)
+      << Context.getTypeDeclType(Record);
+    Diag(Record->getLocation(), diag::note_nontemplate_decl_here);
+    return true;
+  }
+
+  // C++0x [temp.explicit]p2:
+  //   If the explicit instantiation is for a class or member class, the
+  //   elaborated-type-specifier in the declaration shall include a
+  //   simple-template-id.
+  //
+  // C++98 has the same restriction, just worded differently.
+  if (!ScopeSpecifierHasTemplateId(SS))
+    Diag(TemplateLoc, diag::ext_explicit_instantiation_without_qualified_id)
+      << Record << SS.getRange();
+
+  // C++0x [temp.explicit]p2:
+  //   There are two forms of explicit instantiation: an explicit instantiation
+  //   definition and an explicit instantiation declaration. An explicit
+  //   instantiation declaration begins with the extern keyword. [...]
+  TemplateSpecializationKind TSK
+    = ExternLoc.isInvalid()? TSK_ExplicitInstantiationDefinition
+                           : TSK_ExplicitInstantiationDeclaration;
+
+  CheckExplicitInstantiation(*this, Record, NameLoc, true, TSK);
+
+  // Verify that it is okay to explicitly instantiate here.
+  CXXRecordDecl *PrevDecl
+    = cast_or_null<CXXRecordDecl>(Record->getPreviousDecl());
+  if (!PrevDecl && Record->getDefinition())
+    PrevDecl = Record;
+  if (PrevDecl) {
+    MemberSpecializationInfo *MSInfo = PrevDecl->getMemberSpecializationInfo();
+    bool HasNoEffect = false;
+    assert(MSInfo && "No member specialization information?");
+    if (CheckSpecializationInstantiationRedecl(TemplateLoc, TSK,
+                                               PrevDecl,
+                                        MSInfo->getTemplateSpecializationKind(),
+                                             MSInfo->getPointOfInstantiation(),
+                                               HasNoEffect))
+      return true;
+    if (HasNoEffect)
+      return TagD;
+  }
+
+  CXXRecordDecl *RecordDef
+    = cast_or_null<CXXRecordDecl>(Record->getDefinition());
+  if (!RecordDef) {
+    // C++ [temp.explicit]p3:
+    //   A definition of a member class of a class template shall be in scope
+    //   at the point of an explicit instantiation of the member class.
+    CXXRecordDecl *Def
+      = cast_or_null<CXXRecordDecl>(Pattern->getDefinition());
+    if (!Def) {
+      Diag(TemplateLoc, diag::err_explicit_instantiation_undefined_member)
+        << 0 << Record->getDeclName() << Record->getDeclContext();
+      Diag(Pattern->getLocation(), diag::note_forward_declaration)
+        << Pattern;
+      return true;
+    } else {
+      if (InstantiateClass(NameLoc, Record, Def,
+                           getTemplateInstantiationArgs(Record),
+                           TSK))
+        return true;
+
+      RecordDef = cast_or_null<CXXRecordDecl>(Record->getDefinition());
+      if (!RecordDef)
+        return true;
+    }
+  }
+
+  // Instantiate all of the members of the class.
+  InstantiateClassMembers(NameLoc, RecordDef,
+                          getTemplateInstantiationArgs(Record), TSK);
+
+  if (TSK == TSK_ExplicitInstantiationDefinition)
+    MarkVTableUsed(NameLoc, RecordDef, true);
+
+  // FIXME: We don't have any representation for explicit instantiations of
+  // member classes. Such a representation is not needed for compilation, but it
+  // should be available for clients that want to see all of the declarations in
+  // the source code.
+  return TagD;
+}
+
+DeclResult Sema::ActOnExplicitInstantiation(Scope *S,
+                                            SourceLocation ExternLoc,
+                                            SourceLocation TemplateLoc,
+                                            Declarator &D) {
+  // Explicit instantiations always require a name.
+  // TODO: check if/when DNInfo should replace Name.
+  DeclarationNameInfo NameInfo = GetNameForDeclarator(D);
+  DeclarationName Name = NameInfo.getName();
+  if (!Name) {
+    if (!D.isInvalidType())
+      Diag(D.getDeclSpec().getBeginLoc(),
+           diag::err_explicit_instantiation_requires_name)
+          << D.getDeclSpec().getSourceRange() << D.getSourceRange();
+
+    return true;
+  }
+
+  // The scope passed in may not be a decl scope.  Zip up the scope tree until
+  // we find one that is.
+  while ((S->getFlags() & Scope::DeclScope) == 0 ||
+         (S->getFlags() & Scope::TemplateParamScope) != 0)
+    S = S->getParent();
+
+  // Determine the type of the declaration.
+  TypeSourceInfo *T = GetTypeForDeclarator(D, S);
+  QualType R = T->getType();
+  if (R.isNull())
+    return true;
+
+  // C++ [dcl.stc]p1:
+  //   A storage-class-specifier shall not be specified in [...] an explicit
+  //   instantiation (14.7.2) directive.
+  if (D.getDeclSpec().getStorageClassSpec() == DeclSpec::SCS_typedef) {
+    Diag(D.getIdentifierLoc(), diag::err_explicit_instantiation_of_typedef)
+      << Name;
+    return true;
+  } else if (D.getDeclSpec().getStorageClassSpec()
+                                                != DeclSpec::SCS_unspecified) {
+    // Complain about then remove the storage class specifier.
+    Diag(D.getIdentifierLoc(), diag::err_explicit_instantiation_storage_class)
+      << FixItHint::CreateRemoval(D.getDeclSpec().getStorageClassSpecLoc());
+
+    D.getMutableDeclSpec().ClearStorageClassSpecs();
+  }
+
+  // C++0x [temp.explicit]p1:
+  //   [...] An explicit instantiation of a function template shall not use the
+  //   inline or constexpr specifiers.
+  // Presumably, this also applies to member functions of class templates as
+  // well.
+  if (D.getDeclSpec().isInlineSpecified())
+    Diag(D.getDeclSpec().getInlineSpecLoc(),
+         getLangOpts().CPlusPlus11 ?
+           diag::err_explicit_instantiation_inline :
+           diag::warn_explicit_instantiation_inline_0x)
+      << FixItHint::CreateRemoval(D.getDeclSpec().getInlineSpecLoc());
+  if (D.getDeclSpec().hasConstexprSpecifier() && R->isFunctionType())
+    // FIXME: Add a fix-it to remove the 'constexpr' and add a 'const' if one is
+    // not already specified.
+    Diag(D.getDeclSpec().getConstexprSpecLoc(),
+         diag::err_explicit_instantiation_constexpr);
+
+  // A deduction guide is not on the list of entities that can be explicitly
+  // instantiated.
+  if (Name.getNameKind() == DeclarationName::CXXDeductionGuideName) {
+    Diag(D.getDeclSpec().getBeginLoc(), diag::err_deduction_guide_specialized)
+        << /*explicit instantiation*/ 0;
+    return true;
+  }
+
+  // C++0x [temp.explicit]p2:
+  //   There are two forms of explicit instantiation: an explicit instantiation
+  //   definition and an explicit instantiation declaration. An explicit
+  //   instantiation declaration begins with the extern keyword. [...]
+  TemplateSpecializationKind TSK
+    = ExternLoc.isInvalid()? TSK_ExplicitInstantiationDefinition
+                           : TSK_ExplicitInstantiationDeclaration;
+
+  LookupResult Previous(*this, NameInfo, LookupOrdinaryName);
+  LookupParsedName(Previous, S, &D.getCXXScopeSpec());
+
+  if (!R->isFunctionType()) {
+    // C++ [temp.explicit]p1:
+    //   A [...] static data member of a class template can be explicitly
+    //   instantiated from the member definition associated with its class
+    //   template.
+    // C++1y [temp.explicit]p1:
+    //   A [...] variable [...] template specialization can be explicitly
+    //   instantiated from its template.
+    if (Previous.isAmbiguous())
+      return true;
+
+    VarDecl *Prev = Previous.getAsSingle<VarDecl>();
+    VarTemplateDecl *PrevTemplate = Previous.getAsSingle<VarTemplateDecl>();
+
+    if (!PrevTemplate) {
+      if (!Prev || !Prev->isStaticDataMember()) {
+        // We expect to see a static data member here.
+        Diag(D.getIdentifierLoc(), diag::err_explicit_instantiation_not_known)
+            << Name;
+        for (LookupResult::iterator P = Previous.begin(), PEnd = Previous.end();
+             P != PEnd; ++P)
+          Diag((*P)->getLocation(), diag::note_explicit_instantiation_here);
+        return true;
+      }
+
+      if (!Prev->getInstantiatedFromStaticDataMember()) {
+        // FIXME: Check for explicit specialization?
+        Diag(D.getIdentifierLoc(),
+             diag::err_explicit_instantiation_data_member_not_instantiated)
+            << Prev;
+        Diag(Prev->getLocation(), diag::note_explicit_instantiation_here);
+        // FIXME: Can we provide a note showing where this was declared?
+        return true;
+      }
+    } else {
+      // Explicitly instantiate a variable template.
+
+      // C++1y [dcl.spec.auto]p6:
+      //   ... A program that uses auto or decltype(auto) in a context not
+      //   explicitly allowed in this section is ill-formed.
+      //
+      // This includes auto-typed variable template instantiations.
+      if (R->isUndeducedType()) {
+        Diag(T->getTypeLoc().getBeginLoc(),
+             diag::err_auto_not_allowed_var_inst);
+        return true;
+      }
+
+      if (D.getName().getKind() != UnqualifiedIdKind::IK_TemplateId) {
+        // C++1y [temp.explicit]p3:
+        //   If the explicit instantiation is for a variable, the unqualified-id
+        //   in the declaration shall be a template-id.
+        Diag(D.getIdentifierLoc(),
+             diag::err_explicit_instantiation_without_template_id)
+          << PrevTemplate;
+        Diag(PrevTemplate->getLocation(),
+             diag::note_explicit_instantiation_here);
+        return true;
+      }
+
+      // Translate the parser's template argument list into our AST format.
+      TemplateArgumentListInfo TemplateArgs =
+          makeTemplateArgumentListInfo(*this, *D.getName().TemplateId);
+
+      DeclResult Res = CheckVarTemplateId(PrevTemplate, TemplateLoc,
+                                          D.getIdentifierLoc(), TemplateArgs);
+      if (Res.isInvalid())
+        return true;
+
+      if (!Res.isUsable()) {
+        // We somehow specified dependent template arguments in an explicit
+        // instantiation. This should probably only happen during error
+        // recovery.
+        Diag(D.getIdentifierLoc(), diag::err_explicit_instantiation_dependent);
+        return true;
+      }
+
+      // Ignore access control bits, we don't need them for redeclaration
+      // checking.
+      Prev = cast<VarDecl>(Res.get());
+    }
+
+    // C++0x [temp.explicit]p2:
+    //   If the explicit instantiation is for a member function, a member class
+    //   or a static data member of a class template specialization, the name of
+    //   the class template specialization in the qualified-id for the member
+    //   name shall be a simple-template-id.
+    //
+    // C++98 has the same restriction, just worded differently.
+    //
+    // This does not apply to variable template specializations, where the
+    // template-id is in the unqualified-id instead.
+    if (!ScopeSpecifierHasTemplateId(D.getCXXScopeSpec()) && !PrevTemplate)
+      Diag(D.getIdentifierLoc(),
+           diag::ext_explicit_instantiation_without_qualified_id)
+        << Prev << D.getCXXScopeSpec().getRange();
+
+    CheckExplicitInstantiation(*this, Prev, D.getIdentifierLoc(), true, TSK);
+
+    // Verify that it is okay to explicitly instantiate here.
+    TemplateSpecializationKind PrevTSK = Prev->getTemplateSpecializationKind();
+    SourceLocation POI = Prev->getPointOfInstantiation();
+    bool HasNoEffect = false;
+    if (CheckSpecializationInstantiationRedecl(D.getIdentifierLoc(), TSK, Prev,
+                                               PrevTSK, POI, HasNoEffect))
+      return true;
+
+    if (!HasNoEffect) {
+      // Instantiate static data member or variable template.
+      Prev->setTemplateSpecializationKind(TSK, D.getIdentifierLoc());
+      // Merge attributes.
+      ProcessDeclAttributeList(S, Prev, D.getDeclSpec().getAttributes());
+      if (TSK == TSK_ExplicitInstantiationDefinition)
+        InstantiateVariableDefinition(D.getIdentifierLoc(), Prev);
+    }
+
+    // Check the new variable specialization against the parsed input.
+    if (PrevTemplate && Prev && !Context.hasSameType(Prev->getType(), R)) {
+      Diag(T->getTypeLoc().getBeginLoc(),
+           diag::err_invalid_var_template_spec_type)
+          << 0 << PrevTemplate << R << Prev->getType();
+      Diag(PrevTemplate->getLocation(), diag::note_template_declared_here)
+          << 2 << PrevTemplate->getDeclName();
+      return true;
+    }
+
+    // FIXME: Create an ExplicitInstantiation node?
+    return (Decl*) nullptr;
+  }
+
+  // If the declarator is a template-id, translate the parser's template
+  // argument list into our AST format.
+  bool HasExplicitTemplateArgs = false;
+  TemplateArgumentListInfo TemplateArgs;
+  if (D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId) {
+    TemplateArgs = makeTemplateArgumentListInfo(*this, *D.getName().TemplateId);
+    HasExplicitTemplateArgs = true;
+  }
+
+  // C++ [temp.explicit]p1:
+  //   A [...] function [...] can be explicitly instantiated from its template.
+  //   A member function [...] of a class template can be explicitly
+  //  instantiated from the member definition associated with its class
+  //  template.
+  UnresolvedSet<8> TemplateMatches;
+  FunctionDecl *NonTemplateMatch = nullptr;
+  TemplateSpecCandidateSet FailedCandidates(D.getIdentifierLoc());
+  for (LookupResult::iterator P = Previous.begin(), PEnd = Previous.end();
+       P != PEnd; ++P) {
+    NamedDecl *Prev = *P;
+    if (!HasExplicitTemplateArgs) {
+      if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Prev)) {
+        QualType Adjusted = adjustCCAndNoReturn(R, Method->getType(),
+                                                /*AdjustExceptionSpec*/true);
+        if (Context.hasSameUnqualifiedType(Method->getType(), Adjusted)) {
+          if (Method->getPrimaryTemplate()) {
+            TemplateMatches.addDecl(Method, P.getAccess());
+          } else {
+            // FIXME: Can this assert ever happen?  Needs a test.
+            assert(!NonTemplateMatch && "Multiple NonTemplateMatches");
+            NonTemplateMatch = Method;
+          }
+        }
+      }
+    }
+
+    FunctionTemplateDecl *FunTmpl = dyn_cast<FunctionTemplateDecl>(Prev);
+    if (!FunTmpl)
+      continue;
+
+    TemplateDeductionInfo Info(FailedCandidates.getLocation());
+    FunctionDecl *Specialization = nullptr;
+    if (TemplateDeductionResult TDK
+          = DeduceTemplateArguments(FunTmpl,
+                               (HasExplicitTemplateArgs ? &TemplateArgs
+                                                        : nullptr),
+                                    R, Specialization, Info)) {
+      // Keep track of almost-matches.
+      FailedCandidates.addCandidate()
+          .set(P.getPair(), FunTmpl->getTemplatedDecl(),
+               MakeDeductionFailureInfo(Context, TDK, Info));
+      (void)TDK;
+      continue;
+    }
+
+    // Target attributes are part of the cuda function signature, so
+    // the cuda target of the instantiated function must match that of its
+    // template.  Given that C++ template deduction does not take
+    // target attributes into account, we reject candidates here that
+    // have a different target.
+    if (LangOpts.CUDA &&
+        IdentifyCUDATarget(Specialization,
+                           /* IgnoreImplicitHDAttr = */ true) !=
+            IdentifyCUDATarget(D.getDeclSpec().getAttributes())) {
+      FailedCandidates.addCandidate().set(
+          P.getPair(), FunTmpl->getTemplatedDecl(),
+          MakeDeductionFailureInfo(Context, TDK_CUDATargetMismatch, Info));
+      continue;
+    }
+
+    TemplateMatches.addDecl(Specialization, P.getAccess());
+  }
+
+  FunctionDecl *Specialization = NonTemplateMatch;
+  if (!Specialization) {
+    // Find the most specialized function template specialization.
+    UnresolvedSetIterator Result = getMostSpecialized(
+        TemplateMatches.begin(), TemplateMatches.end(), FailedCandidates,
+        D.getIdentifierLoc(),
+        PDiag(diag::err_explicit_instantiation_not_known) << Name,
+        PDiag(diag::err_explicit_instantiation_ambiguous) << Name,
+        PDiag(diag::note_explicit_instantiation_candidate));
+
+    if (Result == TemplateMatches.end())
+      return true;
+
+    // Ignore access control bits, we don't need them for redeclaration checking.
+    Specialization = cast<FunctionDecl>(*Result);
+  }
+
+  // C++11 [except.spec]p4
+  // In an explicit instantiation an exception-specification may be specified,
+  // but is not required.
+  // If an exception-specification is specified in an explicit instantiation
+  // directive, it shall be compatible with the exception-specifications of
+  // other declarations of that function.
+  if (auto *FPT = R->getAs<FunctionProtoType>())
+    if (FPT->hasExceptionSpec()) {
+      unsigned DiagID =
+          diag::err_mismatched_exception_spec_explicit_instantiation;
+      if (getLangOpts().MicrosoftExt)
+        DiagID = diag::ext_mismatched_exception_spec_explicit_instantiation;
+      bool Result = CheckEquivalentExceptionSpec(
+          PDiag(DiagID) << Specialization->getType(),
+          PDiag(diag::note_explicit_instantiation_here),
+          Specialization->getType()->getAs<FunctionProtoType>(),
+          Specialization->getLocation(), FPT, D.getBeginLoc());
+      // In Microsoft mode, mismatching exception specifications just cause a
+      // warning.
+      if (!getLangOpts().MicrosoftExt && Result)
+        return true;
+    }
+
+  if (Specialization->getTemplateSpecializationKind() == TSK_Undeclared) {
+    Diag(D.getIdentifierLoc(),
+         diag::err_explicit_instantiation_member_function_not_instantiated)
+      << Specialization
+      << (Specialization->getTemplateSpecializationKind() ==
+          TSK_ExplicitSpecialization);
+    Diag(Specialization->getLocation(), diag::note_explicit_instantiation_here);
+    return true;
+  }
+
+  FunctionDecl *PrevDecl = Specialization->getPreviousDecl();
+  if (!PrevDecl && Specialization->isThisDeclarationADefinition())
+    PrevDecl = Specialization;
+
+  if (PrevDecl) {
+    bool HasNoEffect = false;
+    if (CheckSpecializationInstantiationRedecl(D.getIdentifierLoc(), TSK,
+                                               PrevDecl,
+                                     PrevDecl->getTemplateSpecializationKind(),
+                                          PrevDecl->getPointOfInstantiation(),
+                                               HasNoEffect))
+      return true;
+
+    // FIXME: We may still want to build some representation of this
+    // explicit specialization.
+    if (HasNoEffect)
+      return (Decl*) nullptr;
+  }
+
+  // HACK: libc++ has a bug where it attempts to explicitly instantiate the
+  // functions
+  //     valarray<size_t>::valarray(size_t) and
+  //     valarray<size_t>::~valarray()
+  // that it declared to have internal linkage with the internal_linkage
+  // attribute. Ignore the explicit instantiation declaration in this case.
+  if (Specialization->hasAttr<InternalLinkageAttr>() &&
+      TSK == TSK_ExplicitInstantiationDeclaration) {
+    if (auto *RD = dyn_cast<CXXRecordDecl>(Specialization->getDeclContext()))
+      if (RD->getIdentifier() && RD->getIdentifier()->isStr("valarray") &&
+          RD->isInStdNamespace())
+        return (Decl*) nullptr;
+  }
+
+  ProcessDeclAttributeList(S, Specialization, D.getDeclSpec().getAttributes());
+
+  // In MSVC mode, dllimported explicit instantiation definitions are treated as
+  // instantiation declarations.
+  if (TSK == TSK_ExplicitInstantiationDefinition &&
+      Specialization->hasAttr<DLLImportAttr>() &&
+      Context.getTargetInfo().getCXXABI().isMicrosoft())
+    TSK = TSK_ExplicitInstantiationDeclaration;
+
+  Specialization->setTemplateSpecializationKind(TSK, D.getIdentifierLoc());
+
+  if (Specialization->isDefined()) {
+    // Let the ASTConsumer know that this function has been explicitly
+    // instantiated now, and its linkage might have changed.
+    Consumer.HandleTopLevelDecl(DeclGroupRef(Specialization));
+  } else if (TSK == TSK_ExplicitInstantiationDefinition)
+    InstantiateFunctionDefinition(D.getIdentifierLoc(), Specialization);
+
+  // C++0x [temp.explicit]p2:
+  //   If the explicit instantiation is for a member function, a member class
+  //   or a static data member of a class template specialization, the name of
+  //   the class template specialization in the qualified-id for the member
+  //   name shall be a simple-template-id.
+  //
+  // C++98 has the same restriction, just worded differently.
+  FunctionTemplateDecl *FunTmpl = Specialization->getPrimaryTemplate();
+  if (D.getName().getKind() != UnqualifiedIdKind::IK_TemplateId && !FunTmpl &&
+      D.getCXXScopeSpec().isSet() &&
+      !ScopeSpecifierHasTemplateId(D.getCXXScopeSpec()))
+    Diag(D.getIdentifierLoc(),
+         diag::ext_explicit_instantiation_without_qualified_id)
+    << Specialization << D.getCXXScopeSpec().getRange();
+
+  CheckExplicitInstantiation(
+      *this,
+      FunTmpl ? (NamedDecl *)FunTmpl
+              : Specialization->getInstantiatedFromMemberFunction(),
+      D.getIdentifierLoc(), D.getCXXScopeSpec().isSet(), TSK);
+
+  // FIXME: Create some kind of ExplicitInstantiationDecl here.
+  return (Decl*) nullptr;
+}
+
+TypeResult
+Sema::ActOnDependentTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
+                        const CXXScopeSpec &SS, IdentifierInfo *Name,
+                        SourceLocation TagLoc, SourceLocation NameLoc) {
+  // This has to hold, because SS is expected to be defined.
+  assert(Name && "Expected a name in a dependent tag");
+
+  NestedNameSpecifier *NNS = SS.getScopeRep();
+  if (!NNS)
+    return true;
+
+  TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForTypeSpec(TagSpec);
+
+  if (TUK == TUK_Declaration || TUK == TUK_Definition) {
+    Diag(NameLoc, diag::err_dependent_tag_decl)
+      << (TUK == TUK_Definition) << Kind << SS.getRange();
+    return true;
+  }
+
+  // Create the resulting type.
+  ElaboratedTypeKeyword Kwd = TypeWithKeyword::getKeywordForTagTypeKind(Kind);
+  QualType Result = Context.getDependentNameType(Kwd, NNS, Name);
+
+  // Create type-source location information for this type.
+  TypeLocBuilder TLB;
+  DependentNameTypeLoc TL = TLB.push<DependentNameTypeLoc>(Result);
+  TL.setElaboratedKeywordLoc(TagLoc);
+  TL.setQualifierLoc(SS.getWithLocInContext(Context));
+  TL.setNameLoc(NameLoc);
+  return CreateParsedType(Result, TLB.getTypeSourceInfo(Context, Result));
+}
+
+TypeResult
+Sema::ActOnTypenameType(Scope *S, SourceLocation TypenameLoc,
+                        const CXXScopeSpec &SS, const IdentifierInfo &II,
+                        SourceLocation IdLoc) {
+  if (SS.isInvalid())
+    return true;
+
+  if (TypenameLoc.isValid() && S && !S->getTemplateParamParent())
+    Diag(TypenameLoc,
+         getLangOpts().CPlusPlus11 ?
+           diag::warn_cxx98_compat_typename_outside_of_template :
+           diag::ext_typename_outside_of_template)
+      << FixItHint::CreateRemoval(TypenameLoc);
+
+  NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);
+  TypeSourceInfo *TSI = nullptr;
+  QualType T = CheckTypenameType(TypenameLoc.isValid()? ETK_Typename : ETK_None,
+                                 TypenameLoc, QualifierLoc, II, IdLoc, &TSI,
+                                 /*DeducedTSTContext=*/true);
+  if (T.isNull())
+    return true;
+  return CreateParsedType(T, TSI);
+}
+
+TypeResult
+Sema::ActOnTypenameType(Scope *S,
+                        SourceLocation TypenameLoc,
+                        const CXXScopeSpec &SS,
+                        SourceLocation TemplateKWLoc,
+                        TemplateTy TemplateIn,
+                        IdentifierInfo *TemplateII,
+                        SourceLocation TemplateIILoc,
+                        SourceLocation LAngleLoc,
+                        ASTTemplateArgsPtr TemplateArgsIn,
+                        SourceLocation RAngleLoc) {
+  if (TypenameLoc.isValid() && S && !S->getTemplateParamParent())
+    Diag(TypenameLoc,
+         getLangOpts().CPlusPlus11 ?
+           diag::warn_cxx98_compat_typename_outside_of_template :
+           diag::ext_typename_outside_of_template)
+      << FixItHint::CreateRemoval(TypenameLoc);
+
+  // Strangely, non-type results are not ignored by this lookup, so the
+  // program is ill-formed if it finds an injected-class-name.
+  if (TypenameLoc.isValid()) {
+    auto *LookupRD =
+        dyn_cast_or_null<CXXRecordDecl>(computeDeclContext(SS, false));
+    if (LookupRD && LookupRD->getIdentifier() == TemplateII) {
+      Diag(TemplateIILoc,
+           diag::ext_out_of_line_qualified_id_type_names_constructor)
+        << TemplateII << 0 /*injected-class-name used as template name*/
+        << (TemplateKWLoc.isValid() ? 1 : 0 /*'template'/'typename' keyword*/);
+    }
+  }
+
+  // Translate the parser's template argument list in our AST format.
+  TemplateArgumentListInfo TemplateArgs(LAngleLoc, RAngleLoc);
+  translateTemplateArguments(TemplateArgsIn, TemplateArgs);
+
+  TemplateName Template = TemplateIn.get();
+  if (DependentTemplateName *DTN = Template.getAsDependentTemplateName()) {
+    // Construct a dependent template specialization type.
+    assert(DTN && "dependent template has non-dependent name?");
+    assert(DTN->getQualifier() == SS.getScopeRep());
+    QualType T = Context.getDependentTemplateSpecializationType(ETK_Typename,
+                                                          DTN->getQualifier(),
+                                                          DTN->getIdentifier(),
+                                                                TemplateArgs);
+
+    // Create source-location information for this type.
+    TypeLocBuilder Builder;
+    DependentTemplateSpecializationTypeLoc SpecTL
+    = Builder.push<DependentTemplateSpecializationTypeLoc>(T);
+    SpecTL.setElaboratedKeywordLoc(TypenameLoc);
+    SpecTL.setQualifierLoc(SS.getWithLocInContext(Context));
+    SpecTL.setTemplateKeywordLoc(TemplateKWLoc);
+    SpecTL.setTemplateNameLoc(TemplateIILoc);
+    SpecTL.setLAngleLoc(LAngleLoc);
+    SpecTL.setRAngleLoc(RAngleLoc);
+    for (unsigned I = 0, N = TemplateArgs.size(); I != N; ++I)
+      SpecTL.setArgLocInfo(I, TemplateArgs[I].getLocInfo());
+    return CreateParsedType(T, Builder.getTypeSourceInfo(Context, T));
+  }
+
+  QualType T = CheckTemplateIdType(Template, TemplateIILoc, TemplateArgs);
+  if (T.isNull())
+    return true;
+
+  // Provide source-location information for the template specialization type.
+  TypeLocBuilder Builder;
+  TemplateSpecializationTypeLoc SpecTL
+    = Builder.push<TemplateSpecializationTypeLoc>(T);
+  SpecTL.setTemplateKeywordLoc(TemplateKWLoc);
+  SpecTL.setTemplateNameLoc(TemplateIILoc);
+  SpecTL.setLAngleLoc(LAngleLoc);
+  SpecTL.setRAngleLoc(RAngleLoc);
+  for (unsigned I = 0, N = TemplateArgs.size(); I != N; ++I)
+    SpecTL.setArgLocInfo(I, TemplateArgs[I].getLocInfo());
+
+  T = Context.getElaboratedType(ETK_Typename, SS.getScopeRep(), T);
+  ElaboratedTypeLoc TL = Builder.push<ElaboratedTypeLoc>(T);
+  TL.setElaboratedKeywordLoc(TypenameLoc);
+  TL.setQualifierLoc(SS.getWithLocInContext(Context));
+
+  TypeSourceInfo *TSI = Builder.getTypeSourceInfo(Context, T);
+  return CreateParsedType(T, TSI);
+}
+
+
+/// Determine whether this failed name lookup should be treated as being
+/// disabled by a usage of std::enable_if.
+static bool isEnableIf(NestedNameSpecifierLoc NNS, const IdentifierInfo &II,
+                       SourceRange &CondRange, Expr *&Cond) {
+  // We must be looking for a ::type...
+  if (!II.isStr("type"))
+    return false;
+
+  // ... within an explicitly-written template specialization...
+  if (!NNS || !NNS.getNestedNameSpecifier()->getAsType())
+    return false;
+  TypeLoc EnableIfTy = NNS.getTypeLoc();
+  TemplateSpecializationTypeLoc EnableIfTSTLoc =
+      EnableIfTy.getAs<TemplateSpecializationTypeLoc>();
+  if (!EnableIfTSTLoc || EnableIfTSTLoc.getNumArgs() == 0)
+    return false;
+  const TemplateSpecializationType *EnableIfTST = EnableIfTSTLoc.getTypePtr();
+
+  // ... which names a complete class template declaration...
+  const TemplateDecl *EnableIfDecl =
+    EnableIfTST->getTemplateName().getAsTemplateDecl();
+  if (!EnableIfDecl || EnableIfTST->isIncompleteType())
+    return false;
+
+  // ... called "enable_if".
+  const IdentifierInfo *EnableIfII =
+    EnableIfDecl->getDeclName().getAsIdentifierInfo();
+  if (!EnableIfII || !EnableIfII->isStr("enable_if"))
+    return false;
+
+  // Assume the first template argument is the condition.
+  CondRange = EnableIfTSTLoc.getArgLoc(0).getSourceRange();
+
+  // Dig out the condition.
+  Cond = nullptr;
+  if (EnableIfTSTLoc.getArgLoc(0).getArgument().getKind()
+        != TemplateArgument::Expression)
+    return true;
+
+  Cond = EnableIfTSTLoc.getArgLoc(0).getSourceExpression();
+
+  // Ignore Boolean literals; they add no value.
+  if (isa<CXXBoolLiteralExpr>(Cond->IgnoreParenCasts()))
+    Cond = nullptr;
+
+  return true;
+}
+
+QualType
+Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
+                        SourceLocation KeywordLoc,
+                        NestedNameSpecifierLoc QualifierLoc,
+                        const IdentifierInfo &II,
+                        SourceLocation IILoc,
+                        TypeSourceInfo **TSI,
+                        bool DeducedTSTContext) {
+  QualType T = CheckTypenameType(Keyword, KeywordLoc, QualifierLoc, II, IILoc,
+                                 DeducedTSTContext);
+  if (T.isNull())
+    return QualType();
+
+  *TSI = Context.CreateTypeSourceInfo(T);
+  if (isa<DependentNameType>(T)) {
+    DependentNameTypeLoc TL =
+        (*TSI)->getTypeLoc().castAs<DependentNameTypeLoc>();
+    TL.setElaboratedKeywordLoc(KeywordLoc);
+    TL.setQualifierLoc(QualifierLoc);
+    TL.setNameLoc(IILoc);
+  } else {
+    ElaboratedTypeLoc TL = (*TSI)->getTypeLoc().castAs<ElaboratedTypeLoc>();
+    TL.setElaboratedKeywordLoc(KeywordLoc);
+    TL.setQualifierLoc(QualifierLoc);
+    TL.getNamedTypeLoc().castAs<TypeSpecTypeLoc>().setNameLoc(IILoc);
+  }
+  return T;
+}
+
+/// Build the type that describes a C++ typename specifier,
+/// e.g., "typename T::type".
+QualType
+Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
+                        SourceLocation KeywordLoc,
+                        NestedNameSpecifierLoc QualifierLoc,
+                        const IdentifierInfo &II,
+                        SourceLocation IILoc, bool DeducedTSTContext) {
+  CXXScopeSpec SS;
+  SS.Adopt(QualifierLoc);
+
+  DeclContext *Ctx = nullptr;
+  if (QualifierLoc) {
+    Ctx = computeDeclContext(SS);
+    if (!Ctx) {
+      // If the nested-name-specifier is dependent and couldn't be
+      // resolved to a type, build a typename type.
+      assert(QualifierLoc.getNestedNameSpecifier()->isDependent());
+      return Context.getDependentNameType(Keyword,
+                                          QualifierLoc.getNestedNameSpecifier(),
+                                          &II);
+    }
+
+    // If the nested-name-specifier refers to the current instantiation,
+    // the "typename" keyword itself is superfluous. In C++03, the
+    // program is actually ill-formed. However, DR 382 (in C++0x CD1)
+    // allows such extraneous "typename" keywords, and we retroactively
+    // apply this DR to C++03 code with only a warning. In any case we continue.
+
+    if (RequireCompleteDeclContext(SS, Ctx))
+      return QualType();
+  }
+
+  DeclarationName Name(&II);
+  LookupResult Result(*this, Name, IILoc, LookupOrdinaryName);
+  if (Ctx)
+    LookupQualifiedName(Result, Ctx, SS);
+  else
+    LookupName(Result, CurScope);
+  unsigned DiagID = 0;
+  Decl *Referenced = nullptr;
+  switch (Result.getResultKind()) {
+  case LookupResult::NotFound: {
+    // If we're looking up 'type' within a template named 'enable_if', produce
+    // a more specific diagnostic.
+    SourceRange CondRange;
+    Expr *Cond = nullptr;
+    if (Ctx && isEnableIf(QualifierLoc, II, CondRange, Cond)) {
+      // If we have a condition, narrow it down to the specific failed
+      // condition.
+      if (Cond) {
+        Expr *FailedCond;
+        std::string FailedDescription;
+        std::tie(FailedCond, FailedDescription) =
+          findFailedBooleanCondition(Cond);
+
+        Diag(FailedCond->getExprLoc(),
+             diag::err_typename_nested_not_found_requirement)
+          << FailedDescription
+          << FailedCond->getSourceRange();
+        return QualType();
+      }
+
+      Diag(CondRange.getBegin(),
+           diag::err_typename_nested_not_found_enable_if)
+          << Ctx << CondRange;
+      return QualType();
+    }
+
+    DiagID = Ctx ? diag::err_typename_nested_not_found
+                 : diag::err_unknown_typename;
+    break;
+  }
+
+  case LookupResult::FoundUnresolvedValue: {
+    // We found a using declaration that is a value. Most likely, the using
+    // declaration itself is meant to have the 'typename' keyword.
+    SourceRange FullRange(KeywordLoc.isValid() ? KeywordLoc : SS.getBeginLoc(),
+                          IILoc);
+    Diag(IILoc, diag::err_typename_refers_to_using_value_decl)
+      << Name << Ctx << FullRange;
+    if (UnresolvedUsingValueDecl *Using
+          = dyn_cast<UnresolvedUsingValueDecl>(Result.getRepresentativeDecl())){
+      SourceLocation Loc = Using->getQualifierLoc().getBeginLoc();
+      Diag(Loc, diag::note_using_value_decl_missing_typename)
+        << FixItHint::CreateInsertion(Loc, "typename ");
+    }
+  }
+  // Fall through to create a dependent typename type, from which we can recover
+  // better.
+  LLVM_FALLTHROUGH;
+
+  case LookupResult::NotFoundInCurrentInstantiation:
+    // Okay, it's a member of an unknown instantiation.
+    return Context.getDependentNameType(Keyword,
+                                        QualifierLoc.getNestedNameSpecifier(),
+                                        &II);
+
+  case LookupResult::Found:
+    if (TypeDecl *Type = dyn_cast<TypeDecl>(Result.getFoundDecl())) {
+      // C++ [class.qual]p2:
+      //   In a lookup in which function names are not ignored and the
+      //   nested-name-specifier nominates a class C, if the name specified
+      //   after the nested-name-specifier, when looked up in C, is the
+      //   injected-class-name of C [...] then the name is instead considered
+      //   to name the constructor of class C.
+      //
+      // Unlike in an elaborated-type-specifier, function names are not ignored
+      // in typename-specifier lookup. However, they are ignored in all the
+      // contexts where we form a typename type with no keyword (that is, in
+      // mem-initializer-ids, base-specifiers, and elaborated-type-specifiers).
+      //
+      // FIXME: That's not strictly true: mem-initializer-id lookup does not
+      // ignore functions, but that appears to be an oversight.
+      auto *LookupRD = dyn_cast_or_null<CXXRecordDecl>(Ctx);
+      auto *FoundRD = dyn_cast<CXXRecordDecl>(Type);
+      if (Keyword == ETK_Typename && LookupRD && FoundRD &&
+          FoundRD->isInjectedClassName() &&
+          declaresSameEntity(LookupRD, cast<Decl>(FoundRD->getParent())))
+        Diag(IILoc, diag::ext_out_of_line_qualified_id_type_names_constructor)
+            << &II << 1 << 0 /*'typename' keyword used*/;
+
+      // We found a type. Build an ElaboratedType, since the
+      // typename-specifier was just sugar.
+      MarkAnyDeclReferenced(Type->getLocation(), Type, /*OdrUse=*/false);
+      return Context.getElaboratedType(Keyword,
+                                       QualifierLoc.getNestedNameSpecifier(),
+                                       Context.getTypeDeclType(Type));
+    }
+
+    // C++ [dcl.type.simple]p2:
+    //   A type-specifier of the form
+    //     typename[opt] nested-name-specifier[opt] template-name
+    //   is a placeholder for a deduced class type [...].
+    if (getLangOpts().CPlusPlus17) {
+      if (auto *TD = getAsTypeTemplateDecl(Result.getFoundDecl())) {
+        if (!DeducedTSTContext) {
+          QualType T(QualifierLoc
+                         ? QualifierLoc.getNestedNameSpecifier()->getAsType()
+                         : nullptr, 0);
+          if (!T.isNull())
+            Diag(IILoc, diag::err_dependent_deduced_tst)
+              << (int)getTemplateNameKindForDiagnostics(TemplateName(TD)) << T;
+          else
+            Diag(IILoc, diag::err_deduced_tst)
+              << (int)getTemplateNameKindForDiagnostics(TemplateName(TD));
+          Diag(TD->getLocation(), diag::note_template_decl_here);
+          return QualType();
+        }
+        return Context.getElaboratedType(
+            Keyword, QualifierLoc.getNestedNameSpecifier(),
+            Context.getDeducedTemplateSpecializationType(TemplateName(TD),
+                                                         QualType(), false));
+      }
+    }
+
+    DiagID = Ctx ? diag::err_typename_nested_not_type
+                 : diag::err_typename_not_type;
+    Referenced = Result.getFoundDecl();
+    break;
+
+  case LookupResult::FoundOverloaded:
+    DiagID = Ctx ? diag::err_typename_nested_not_type
+                 : diag::err_typename_not_type;
+    Referenced = *Result.begin();
+    break;
+
+  case LookupResult::Ambiguous:
+    return QualType();
+  }
+
+  // If we get here, it's because name lookup did not find a
+  // type. Emit an appropriate diagnostic and return an error.
+  SourceRange FullRange(KeywordLoc.isValid() ? KeywordLoc : SS.getBeginLoc(),
+                        IILoc);
+  if (Ctx)
+    Diag(IILoc, DiagID) << FullRange << Name << Ctx;
+  else
+    Diag(IILoc, DiagID) << FullRange << Name;
+  if (Referenced)
+    Diag(Referenced->getLocation(),
+         Ctx ? diag::note_typename_member_refers_here
+             : diag::note_typename_refers_here)
+      << Name;
+  return QualType();
+}
+
+namespace {
+  // See Sema::RebuildTypeInCurrentInstantiation
+  class CurrentInstantiationRebuilder
+    : public TreeTransform<CurrentInstantiationRebuilder> {
+    SourceLocation Loc;
+    DeclarationName Entity;
+
+  public:
+    typedef TreeTransform<CurrentInstantiationRebuilder> inherited;
+
+    CurrentInstantiationRebuilder(Sema &SemaRef,
+                                  SourceLocation Loc,
+                                  DeclarationName Entity)
+    : TreeTransform<CurrentInstantiationRebuilder>(SemaRef),
+      Loc(Loc), Entity(Entity) { }
+
+    /// Determine whether the given type \p T has already been
+    /// transformed.
+    ///
+    /// For the purposes of type reconstruction, a type has already been
+    /// transformed if it is NULL or if it is not dependent.
+    bool AlreadyTransformed(QualType T) {
+      return T.isNull() || !T->isInstantiationDependentType();
+    }
+
+    /// Returns the location of the entity whose type is being
+    /// rebuilt.
+    SourceLocation getBaseLocation() { return Loc; }
+
+    /// Returns the name of the entity whose type is being rebuilt.
+    DeclarationName getBaseEntity() { return Entity; }
+
+    /// Sets the "base" location and entity when that
+    /// information is known based on another transformation.
+    void setBase(SourceLocation Loc, DeclarationName Entity) {
+      this->Loc = Loc;
+      this->Entity = Entity;
+    }
+
+    ExprResult TransformLambdaExpr(LambdaExpr *E) {
+      // Lambdas never need to be transformed.
+      return E;
+    }
+  };
+} // end anonymous namespace
+
+/// Rebuilds a type within the context of the current instantiation.
+///
+/// The type \p T is part of the type of an out-of-line member definition of
+/// a class template (or class template partial specialization) that was parsed
+/// and constructed before we entered the scope of the class template (or
+/// partial specialization thereof). This routine will rebuild that type now
+/// that we have entered the declarator's scope, which may produce different
+/// canonical types, e.g.,
+///
+/// \code
+/// template<typename T>
+/// struct X {
+///   typedef T* pointer;
+///   pointer data();
+/// };
+///
+/// template<typename T>
+/// typename X<T>::pointer X<T>::data() { ... }
+/// \endcode
+///
+/// Here, the type "typename X<T>::pointer" will be created as a DependentNameType,
+/// since we do not know that we can look into X<T> when we parsed the type.
+/// This function will rebuild the type, performing the lookup of "pointer"
+/// in X<T> and returning an ElaboratedType whose canonical type is the same
+/// as the canonical type of T*, allowing the return types of the out-of-line
+/// definition and the declaration to match.
+TypeSourceInfo *Sema::RebuildTypeInCurrentInstantiation(TypeSourceInfo *T,
+                                                        SourceLocation Loc,
+                                                        DeclarationName Name) {
+  if (!T || !T->getType()->isInstantiationDependentType())
+    return T;
+
+  CurrentInstantiationRebuilder Rebuilder(*this, Loc, Name);
+  return Rebuilder.TransformType(T);
+}
+
+ExprResult Sema::RebuildExprInCurrentInstantiation(Expr *E) {
+  CurrentInstantiationRebuilder Rebuilder(*this, E->getExprLoc(),
+                                          DeclarationName());
+  return Rebuilder.TransformExpr(E);
+}
+
+bool Sema::RebuildNestedNameSpecifierInCurrentInstantiation(CXXScopeSpec &SS) {
+  if (SS.isInvalid())
+    return true;
+
+  NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);
+  CurrentInstantiationRebuilder Rebuilder(*this, SS.getRange().getBegin(),
+                                          DeclarationName());
+  NestedNameSpecifierLoc Rebuilt
+    = Rebuilder.TransformNestedNameSpecifierLoc(QualifierLoc);
+  if (!Rebuilt)
+    return true;
+
+  SS.Adopt(Rebuilt);
+  return false;
+}
+
+/// Rebuild the template parameters now that we know we're in a current
+/// instantiation.
+bool Sema::RebuildTemplateParamsInCurrentInstantiation(
+                                               TemplateParameterList *Params) {
+  for (unsigned I = 0, N = Params->size(); I != N; ++I) {
+    Decl *Param = Params->getParam(I);
+
+    // There is nothing to rebuild in a type parameter.
+    if (isa<TemplateTypeParmDecl>(Param))
+      continue;
+
+    // Rebuild the template parameter list of a template template parameter.
+    if (TemplateTemplateParmDecl *TTP
+        = dyn_cast<TemplateTemplateParmDecl>(Param)) {
+      if (RebuildTemplateParamsInCurrentInstantiation(
+            TTP->getTemplateParameters()))
+        return true;
+
+      continue;
+    }
+
+    // Rebuild the type of a non-type template parameter.
+    NonTypeTemplateParmDecl *NTTP = cast<NonTypeTemplateParmDecl>(Param);
+    TypeSourceInfo *NewTSI
+      = RebuildTypeInCurrentInstantiation(NTTP->getTypeSourceInfo(),
+                                          NTTP->getLocation(),
+                                          NTTP->getDeclName());
+    if (!NewTSI)
+      return true;
+
+    if (NewTSI->getType()->isUndeducedType()) {
+      // C++17 [temp.dep.expr]p3:
+      //   An id-expression is type-dependent if it contains
+      //    - an identifier associated by name lookup with a non-type
+      //      template-parameter declared with a type that contains a
+      //      placeholder type (7.1.7.4),
+      NewTSI = SubstAutoTypeSourceInfoDependent(NewTSI);
+    }
+
+    if (NewTSI != NTTP->getTypeSourceInfo()) {
+      NTTP->setTypeSourceInfo(NewTSI);
+      NTTP->setType(NewTSI->getType());
+    }
+  }
+
+  return false;
+}
+
+/// Produces a formatted string that describes the binding of
+/// template parameters to template arguments.
+std::string
+Sema::getTemplateArgumentBindingsText(const TemplateParameterList *Params,
+                                      const TemplateArgumentList &Args) {
+  return getTemplateArgumentBindingsText(Params, Args.data(), Args.size());
+}
+
+std::string
+Sema::getTemplateArgumentBindingsText(const TemplateParameterList *Params,
+                                      const TemplateArgument *Args,
+                                      unsigned NumArgs) {
+  SmallString<128> Str;
+  llvm::raw_svector_ostream Out(Str);
+
+  if (!Params || Params->size() == 0 || NumArgs == 0)
+    return std::string();
+
+  for (unsigned I = 0, N = Params->size(); I != N; ++I) {
+    if (I >= NumArgs)
+      break;
+
+    if (I == 0)
+      Out << "[with ";
+    else
+      Out << ", ";
+
+    if (const IdentifierInfo *Id = Params->getParam(I)->getIdentifier()) {
+      Out << Id->getName();
+    } else {
+      Out << '$' << I;
+    }
+
+    Out << " = ";
+    Args[I].print(getPrintingPolicy(), Out,
+                  TemplateParameterList::shouldIncludeTypeForArgument(
+                      getPrintingPolicy(), Params, I));
+  }
+
+  Out << ']';
+  return std::string(Out.str());
+}
+
+void Sema::MarkAsLateParsedTemplate(FunctionDecl *FD, Decl *FnD,
+                                    CachedTokens &Toks) {
+  if (!FD)
+    return;
+
+  auto LPT = std::make_unique<LateParsedTemplate>();
+
+  // Take tokens to avoid allocations
+  LPT->Toks.swap(Toks);
+  LPT->D = FnD;
+  LateParsedTemplateMap.insert(std::make_pair(FD, std::move(LPT)));
+
+  FD->setLateTemplateParsed(true);
+}
+
+void Sema::UnmarkAsLateParsedTemplate(FunctionDecl *FD) {
+  if (!FD)
+    return;
+  FD->setLateTemplateParsed(false);
+}
+
+bool Sema::IsInsideALocalClassWithinATemplateFunction() {
+  DeclContext *DC = CurContext;
+
+  while (DC) {
+    if (CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(CurContext)) {
+      const FunctionDecl *FD = RD->isLocalClass();
+      return (FD && FD->getTemplatedKind() != FunctionDecl::TK_NonTemplate);
+    } else if (DC->isTranslationUnit() || DC->isNamespace())
+      return false;
+
+    DC = DC->getParent();
+  }
+  return false;
+}
+
+namespace {
+/// Walk the path from which a declaration was instantiated, and check
+/// that every explicit specialization along that path is visible. This enforces
+/// C++ [temp.expl.spec]/6:
+///
+///   If a template, a member template or a member of a class template is
+///   explicitly specialized then that specialization shall be declared before
+///   the first use of that specialization that would cause an implicit
+///   instantiation to take place, in every translation unit in which such a
+///   use occurs; no diagnostic is required.
+///
+/// and also C++ [temp.class.spec]/1:
+///
+///   A partial specialization shall be declared before the first use of a
+///   class template specialization that would make use of the partial
+///   specialization as the result of an implicit or explicit instantiation
+///   in every translation unit in which such a use occurs; no diagnostic is
+///   required.
+class ExplicitSpecializationVisibilityChecker {
+  Sema &S;
+  SourceLocation Loc;
+  llvm::SmallVector<Module *, 8> Modules;
+
+public:
+  ExplicitSpecializationVisibilityChecker(Sema &S, SourceLocation Loc)
+      : S(S), Loc(Loc) {}
+
+  void check(NamedDecl *ND) {
+    if (auto *FD = dyn_cast<FunctionDecl>(ND))
+      return checkImpl(FD);
+    if (auto *RD = dyn_cast<CXXRecordDecl>(ND))
+      return checkImpl(RD);
+    if (auto *VD = dyn_cast<VarDecl>(ND))
+      return checkImpl(VD);
+    if (auto *ED = dyn_cast<EnumDecl>(ND))
+      return checkImpl(ED);
+  }
+
+private:
+  void diagnose(NamedDecl *D, bool IsPartialSpec) {
+    auto Kind = IsPartialSpec ? Sema::MissingImportKind::PartialSpecialization
+                              : Sema::MissingImportKind::ExplicitSpecialization;
+    const bool Recover = true;
+
+    // If we got a custom set of modules (because only a subset of the
+    // declarations are interesting), use them, otherwise let
+    // diagnoseMissingImport intelligently pick some.
+    if (Modules.empty())
+      S.diagnoseMissingImport(Loc, D, Kind, Recover);
+    else
+      S.diagnoseMissingImport(Loc, D, D->getLocation(), Modules, Kind, Recover);
+  }
+
+  // Check a specific declaration. There are three problematic cases:
+  //
+  //  1) The declaration is an explicit specialization of a template
+  //     specialization.
+  //  2) The declaration is an explicit specialization of a member of an
+  //     templated class.
+  //  3) The declaration is an instantiation of a template, and that template
+  //     is an explicit specialization of a member of a templated class.
+  //
+  // We don't need to go any deeper than that, as the instantiation of the
+  // surrounding class / etc is not triggered by whatever triggered this
+  // instantiation, and thus should be checked elsewhere.
+  template<typename SpecDecl>
+  void checkImpl(SpecDecl *Spec) {
+    bool IsHiddenExplicitSpecialization = false;
+    if (Spec->getTemplateSpecializationKind() == TSK_ExplicitSpecialization) {
+      IsHiddenExplicitSpecialization =
+          Spec->getMemberSpecializationInfo()
+              ? !S.hasVisibleMemberSpecialization(Spec, &Modules)
+              : !S.hasVisibleExplicitSpecialization(Spec, &Modules);
+    } else {
+      checkInstantiated(Spec);
+    }
+
+    if (IsHiddenExplicitSpecialization)
+      diagnose(Spec->getMostRecentDecl(), false);
+  }
+
+  void checkInstantiated(FunctionDecl *FD) {
+    if (auto *TD = FD->getPrimaryTemplate())
+      checkTemplate(TD);
+  }
+
+  void checkInstantiated(CXXRecordDecl *RD) {
+    auto *SD = dyn_cast<ClassTemplateSpecializationDecl>(RD);
+    if (!SD)
+      return;
+
+    auto From = SD->getSpecializedTemplateOrPartial();
+    if (auto *TD = From.dyn_cast<ClassTemplateDecl *>())
+      checkTemplate(TD);
+    else if (auto *TD =
+                 From.dyn_cast<ClassTemplatePartialSpecializationDecl *>()) {
+      if (!S.hasVisibleDeclaration(TD))
+        diagnose(TD, true);
+      checkTemplate(TD);
+    }
+  }
+
+  void checkInstantiated(VarDecl *RD) {
+    auto *SD = dyn_cast<VarTemplateSpecializationDecl>(RD);
+    if (!SD)
+      return;
+
+    auto From = SD->getSpecializedTemplateOrPartial();
+    if (auto *TD = From.dyn_cast<VarTemplateDecl *>())
+      checkTemplate(TD);
+    else if (auto *TD =
+                 From.dyn_cast<VarTemplatePartialSpecializationDecl *>()) {
+      if (!S.hasVisibleDeclaration(TD))
+        diagnose(TD, true);
+      checkTemplate(TD);
+    }
+  }
+
+  void checkInstantiated(EnumDecl *FD) {}
+
+  template<typename TemplDecl>
+  void checkTemplate(TemplDecl *TD) {
+    if (TD->isMemberSpecialization()) {
+      if (!S.hasVisibleMemberSpecialization(TD, &Modules))
+        diagnose(TD->getMostRecentDecl(), false);
+    }
+  }
+};
+} // end anonymous namespace
+
+void Sema::checkSpecializationVisibility(SourceLocation Loc, NamedDecl *Spec) {
+  if (!getLangOpts().Modules)
+    return;
+
+  ExplicitSpecializationVisibilityChecker(*this, Loc).check(Spec);
+}