path: root/contrib/tools/bison/NEWS

GNU Bison NEWS

* Noteworthy changes in release 3.5.4 (2020-04-05) [stable]

** WARNING: Future backward-incompatibilities!

  TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".

  Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
  that still depend on it will produce Yacc-like error messages (just
  "syntax error").  It was superseded by the "%error-verbose" directive in
  Bison 1.875 (2003-01-01).  Bison 2.6 (2012-07-19) clearly announced that
  support for YYERROR_VERBOSE would be removed.  Note that since Bison 3.0
  (2013-07-25), "%error-verbose" is deprecated in favor of "%define
  parse.error verbose".

** Bug fixes

  Fix portability issues of the package itself on old compilers.

  Fix api.token.raw support in Java.

* Noteworthy changes in release 3.5.3 (2020-03-08) [stable]

** Bug fixes

  Error messages could quote lines containing zero-width characters (such as
  \005) with incorrect styling.  Fixes for similar issues with unexpectedly
  short lines (e.g., the file was changed between parsing and diagnosing).

  Several unlikely crashes found by fuzzing have been fixed.

* Noteworthy changes in release 3.5.2 (2020-02-13) [stable]

** Bug fixes

  Portability issues and minor cosmetic issues.

  The lalr1.cc skeleton properly rejects unsupported values for parse.lac
  (as yacc.c does).

* Noteworthy changes in release 3.5.1 (2020-01-19) [stable]

** Bug fixes

  Portability fixes.

  Fix compiler warnings.

* Noteworthy changes in release 3.5 (2019-12-11) [stable]

** Backward incompatible changes

  Lone carriage-return characters (aka \r or ^M) in the grammar files are no
  longer treated as end-of-lines.  This changes the diagnostics, and in
  particular their locations.

  In C++, line numbers and columns are now represented as 'int' not
  'unsigned', so that integer overflow on positions is easily checkable via
  'gcc -fsanitize=undefined' and the like.  This affects the API for
  positions.  The default position and location classes now expose
  'counter_type' (int), used to define line and column numbers.

** Deprecated features

  The YYPRINT macro, which works only with yacc.c and only for tokens, was
  obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
  It is deprecated and its support will be removed eventually.

** New features

*** Lookahead correction in C++

  Contributed by Adrian Vogelsgesang.

  The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
  %define variable parse.lac.

*** Variable api.token.raw: Optimized token numbers (all skeletons)

  In the generated parsers, tokens have two numbers: the "external" token
  number as returned by yylex (which starts at 257), and the "internal"
  symbol number (which starts at 3).  Each time yylex is called, a table
  lookup maps the external token number to the internal symbol number.

  When the %define variable api.token.raw is set, tokens are assigned their
  internal number, which saves one table lookup per token, and also saves
  the generation of the mapping table.

  The gain is typically moderate, but in extreme cases (very simple user
  actions), a 10% improvement can be observed.

*** Generated parsers use better types for states

  Stacks now use the best integral type for state numbers, instead of always
  using 15 bits.  As a result "small" parsers now have a smaller memory
  footprint (they use 8 bits), and there is support for large automata (16
  bits), and extra large (using int, i.e., typically 31 bits).

*** Generated parsers prefer signed integer types

  Bison skeletons now prefer signed to unsigned integer types when either
  will do, as the signed types are less error-prone and allow for better
  checking with 'gcc -fsanitize=undefined'.  Also, the types chosen are now
  portable to unusual machines where char, short and int are all the same
  width.  On non-GNU platforms this may entail including <limits.h> and (if
  available) <stdint.h> to define integer types and constants.

*** A skeleton for the D programming language

  For the last few releases, Bison has shipped a stealth experimental
  skeleton: lalr1.d.  It was first contributed by Oliver Mangold, based on
  Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
  H. S. Teoh.

  However, because nobody has committed to improving, testing, and
  documenting this skeleton, it is not clear that it will be supported in
  the future.

  The lalr1.d skeleton *is functional*, and works well, as demonstrated in
  examples/d/calc.d.  Please try it, enjoy it, and... commit to support it.

*** Debug traces in Java

  The Java backend no longer emits code and data for parser tracing if the
  %define variable parse.trace is not defined.

** Diagnostics

*** New diagnostic: -Wdangling-alias

  String literals, which allow for better error messages, are (too)
  liberally accepted by Bison, which might result in silent errors.  For
  instance

    %type <exVal> cond "condition"

  does not define "condition" as a string alias to 'cond' (nonterminal
  symbols do not have string aliases).  It is rather equivalent to

    %nterm <exVal> cond
    %token <exVal> "condition"

  i.e., it gives the type 'exVal' to the "condition" token, which was
  clearly not the intention.

  Also, because string aliases need not be defined, typos such as "baz"
  instead of "bar" will be not reported.

  The option -Wdangling-alias catches these situations.  On

    %token BAR "bar"
    %type <ival> foo "foo"
    %%
    foo: "baz" {}

  bison -Wdangling-alias reports

    warning: string literal not attached to a symbol
          | %type <ival> foo "foo"
          |                  ^~~~~
    warning: string literal not attached to a symbol
          | foo: "baz" {}
          |      ^~~~~

   The -Wall option does not (yet?) include -Wdangling-alias.

*** Better POSIX Yacc compatibility diagnostics

  POSIX Yacc restricts %type to nonterminals.  This is now diagnosed by
  -Wyacc.

    %token TOKEN1
    %type  <ival> TOKEN1 TOKEN2 't'
    %token TOKEN2
    %%
    expr:

  gives with -Wyacc

    input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
        2 | %type  <ival> TOKEN1 TOKEN2 't'
          |               ^~~~~~
    input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
        2 | %type  <ival> TOKEN1 TOKEN2 't'
          |                             ^~~
    input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
        2 | %type  <ival> TOKEN1 TOKEN2 't'
          |                      ^~~~~~

*** Diagnostics with insertion

  The diagnostics now display the suggestion below the underlined source.
  Replacement for undeclared symbols are now also suggested.

    $ cat /tmp/foo.y
    %%
    list: lis '.' |

    $ bison -Wall foo.y
    foo.y:2.7-9: error: symbol 'lis' is used, but is not defined as a token and has no rules; did you mean 'list'?
        2 | list: lis '.' |
          |       ^~~
          |       list
    foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
        2 | list: lis '.' |
          |                ^
          |                %empty
    foo.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]

*** Diagnostics about long lines

  Quoted sources may now be truncated to fit the screen.  For instance, on a
  30-column wide terminal:

    $ cat foo.y
    %token FOO                       FOO                         FOO
    %%
    exp: FOO
    $ bison foo.y
    foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
        1 | …         FOO                  …
          |           ^~~
    foo.y:1.8-10:      previous declaration
        1 | %token FOO                     …
          |        ^~~
    foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
        1 | …         FOO
          |           ^~~
    foo.y:1.8-10:      previous declaration
        1 | %token FOO                     …
          |        ^~~

** Changes

*** Debugging glr.c and glr.cc

  The glr.c skeleton always had asserts to check its own behavior (not the
  user's).  These assertions are now under the control of the parse.assert
  %define variable (disabled by default).

*** Clean up

  Several new compiler warnings in the generated output have been avoided.
  Some unused features are no longer emitted.  Cleaner generated code in
  general.

** Bug Fixes

  Portability issues in the test suite.

  In theory, parsers using %nonassoc could crash when reporting verbose
  error messages. This unlikely bug has been fixed.

  In Java, %define api.prefix was ignored.  It now behaves as expected.

* Noteworthy changes in release 3.4.2 (2019-09-12) [stable]

** Bug fixes

  In some cases, when warnings are disabled, bison could emit tons of white
  spaces as diagnostics.

  When running out of memory, bison could crash (found by fuzzing).

  When defining twice the EOF token, bison would crash.

  New warnings from recent compilers have been addressed in the generated
  parsers (yacc.c, glr.c, glr.cc).

  When lone carriage-return characters appeared in the input file,
  diagnostics could hang forever.

* Noteworthy changes in release 3.4.1 (2019-05-22) [stable]

** Bug fixes

  Portability fixes.

* Noteworthy changes in release 3.4 (2019-05-19) [stable]

** Deprecated features

  The %pure-parser directive is deprecated in favor of '%define api.pure'
  since Bison 2.3b (2008-05-27), but no warning was issued; there is one
  now.  Note that since Bison 2.7 you are strongly encouraged to use
  '%define api.pure full' instead of '%define api.pure'.

** New features

*** Colored diagnostics

  As an experimental feature, diagnostics are now colored, controlled by the
  new options --color and --style.

  To use them, install the libtextstyle library before configuring Bison.
  It is available from

    https://alpha.gnu.org/gnu/gettext/

  for instance

    https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz

  The option --color supports the following arguments:
    - always, yes: Enable colors.
    - never, no: Disable colors.
    - auto, tty (default): Enable colors if the output device is a tty.

  To customize the styles, create a CSS file similar to

    /* bison-bw.css */
    .warning   { }
    .error     { font-weight: 800; text-decoration: underline; }
    .note      { }

  then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
  environment variable to "bison-bw.css".

*** Disabling output

  When given -fsyntax-only, the diagnostics are reported, but no output is
  generated.

  The name of this option is somewhat misleading as bison does more than
  just checking the syntax: every stage is run (including checking for
  conflicts for instance), except the generation of the output files.

*** Include the generated header (yacc.c)

  Before, when --defines is used, bison generated a header, and pasted an
  exact copy of it into the generated parser implementation file.  If the
  header name is not "y.tab.h", it is now #included instead of being
  duplicated.

  To use an '#include' even if the header name is "y.tab.h" (which is what
  happens with --yacc, or when using the Autotools' ylwrap), define
  api.header.include to the exact argument to pass to #include.  For
  instance:

    %define api.header.include {"parse.h"}

  or

    %define api.header.include {<parser/parse.h>}

*** api.location.type is now supported in C (yacc.c, glr.c)

  The %define variable api.location.type defines the name of the type to use
  for locations.  When defined, Bison no longer defines YYLTYPE.

  This can be used in programs with several parsers to factor their
  definition of locations: let one of them generate them, and the others
  just use them.

** Changes

*** Graphviz output

  In conformance with the recommendations of the Graphviz team, if %require
  "3.4" (or better) is specified, the option --graph generates a *.gv file
  by default, instead of *.dot.

*** Diagnostics overhaul

  Column numbers were wrong with multibyte characters, which would also
  result in skewed diagnostics with carets.  Beside, because we were
  indenting the quoted source with a single space, lines with tab characters
  were incorrectly underlined.

  To address these issues, and to be clearer, Bison now issues diagnostics
  as GCC9 does.  For instance it used to display (there's a tab before the
  opening brace):

    foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
     expr: expr '+' "number"        { $$ = $1 + $2; }
                                         ^~
  It now reports

    foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
        3 | expr: expr '+' "number" { $$ = $1 + $2; }
          |                                     ^~

  Other constructs now also have better locations, resulting in more precise
  diagnostics.

*** Fix-it hints for %empty

  Running Bison with -Wempty-rules and --update will remove incorrect %empty
  annotations, and add the missing ones.

*** Generated reports

  The format of the reports (parse.output) was improved for readability.

*** Better support for --no-line.

  When --no-line is used, the generated files are now cleaner: no lines are
  generated instead of empty lines.  Together with using api.header.include,
  that should help people saving the generated files into version control
  systems get smaller diffs.

** Documentation

  A new example in C shows an simple infix calculator with a hand-written
  scanner (examples/c/calc).

  A new example in C shows a reentrant parser (capable of recursive calls)
  built with Flex and Bison (examples/c/reccalc).

  There is a new section about the history of Yaccs and Bison.

** Bug fixes

  A few obscure bugs were fixed, including the second oldest (known) bug in
  Bison: it was there when Bison was entered in the RCS version control
  system, in December 1987.  See the NEWS of Bison 3.3 for the previous
  oldest bug.

* Noteworthy changes in release 3.3.2 (2019-02-03) [stable]

** Bug fixes

  Bison 3.3 failed to generate parsers for grammars with unused nonterminal
  symbols.

* Noteworthy changes in release 3.3.1 (2019-01-27) [stable]

** Changes

  The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
  extensions into errors.  It now makes them simple warnings (-Wyacc).

* Noteworthy changes in release 3.3 (2019-01-26) [stable]

  A new mailing list was created, Bison Announce.  It is low traffic, and is
  only about announcing new releases and important messages (e.g., polls
  about major decisions to make).

  https://lists.gnu.org/mailman/listinfo/bison-announce

** Backward incompatible changes

  Support for DJGPP, which has been unmaintained and untested for years, is
  removed.

** Deprecated features

  A new feature, --update (see below) helps adjusting existing grammars to
  deprecations.

*** Deprecated directives

  The %error-verbose directive is deprecated in favor of '%define
  parse.error verbose' since Bison 3.0, but no warning was issued.

  The '%name-prefix "xx"' directive is deprecated in favor of '%define
  api.prefix {xx}' since Bison 3.0, but no warning was issued.  These
  directives are slightly different, you might need to adjust your code.
  %name-prefix renames only symbols with external linkage, while api.prefix
  also renames types and macros, including YYDEBUG, YYTOKENTYPE,
  yytokentype, YYSTYPE, YYLTYPE, etc.

  Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
  {xx}' will typically have to update YY_DECL from

    #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)

  to

    #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)

*** Deprecated %define variable names

  The following variables, mostly related to parsers in Java, have been
  renamed for consistency.  Backward compatibility is ensured, but upgrading
  is recommended.

    abstract           -> api.parser.abstract
    annotations        -> api.parser.annotations
    extends            -> api.parser.extends
    final              -> api.parser.final
    implements         -> api.parser.implements
    parser_class_name  -> api.parser.class
    public             -> api.parser.public
    strictfp           -> api.parser.strictfp

** New features

*** Generation of fix-its for IDEs/Editors

  When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
  bison now generates machine readable editing instructions to fix some
  issues.  Currently, this is mostly limited to updating deprecated
  directives and removing duplicates.  For instance:

    $ cat foo.y
    %error-verbose
    %define parser_class_name "Parser"
    %define api.parser.class "Parser"
    %%
    exp:;

  See the "fix-it:" lines below:

    $ bison -ffixit foo.y
    foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
     %error-verbose
     ^~~~~~~~~~~~~~
    fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
    foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
     %define parser_class_name "Parser"
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
    foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
     %define api.parser.class "Parser"
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    foo.y:2.1-34:     previous definition
     %define parser_class_name "Parser"
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    fix-it:"foo.y":{3:1-3:34}:""
    foo.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]

  This uses the same output format as GCC and Clang.

*** Updating grammar files

  Fixes can be applied on the fly.  The previous example ends with the
  suggestion to re-run bison with the option -u/--update, which results in a
  cleaner grammar file.

    $ bison --update foo.y
    [...]
    bison: file 'foo.y' was updated (backup: 'foo.y~')

    $ cat foo.y
    %define parse.error verbose
    %define api.parser.class {Parser}
    %%
    exp:;

*** Bison is now relocatable

  If you pass '--enable-relocatable' to 'configure', Bison is relocatable.

  A relocatable program can be moved or copied to a different location on
  the file system.  It can also be used through mount points for network
  sharing.  It is possible to make symbolic links to the installed and moved
  programs, and invoke them through the symbolic link.

*** %expect and %expect-rr modifiers on individual rules

  One can now document (and check) which rules participate in shift/reduce
  and reduce/reduce conflicts.  This is particularly important GLR parsers,
  where conflicts are a normal occurrence.  For example,

      %glr-parser
      %expect 1
      %%

      ...

      argument_list:
        arguments %expect 1
      | arguments ','
      | %empty
      ;

      arguments:
        expression
      | argument_list ',' expression
      ;

      ...

  Looking at the output from -v, one can see that the shift-reduce conflict
  here is due to the fact that the parser does not know whether to reduce
  arguments to argument_list until it sees the token _after_ the following
  ','.  By marking the rule with %expect 1 (because there is a conflict in
  one state), we document the source of the 1 overall shift-reduce conflict.

  In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
  conflicts.  In this case, we mark each of the conflicting rules.  For
  example,

      %glr-parser
      %expect-rr 1

      %%

      stmt:
        target_list '=' expr ';'
      | expr_list ';'
      ;

      target_list:
        target
      | target ',' target_list
      ;

      target:
        ID %expect-rr 1
      ;

      expr_list:
        expr
      | expr ',' expr_list
      ;

      expr:
        ID %expect-rr 1
      | ...
      ;

  In a statement such as

      x, y = 3, 4;

  the parser must reduce x to a target or an expr, but does not know which
  until it sees the '='.  So we notate the two possible reductions to
  indicate that each conflicts in one rule.

  This feature needs user feedback, and might evolve in the future.

*** C++: Actual token constructors

  When variants and token constructors are enabled, in addition to the
  type-safe named token constructors (make_ID, make_INT, etc.), we now
  generate genuine constructors for symbol_type.

  For instance with these declarations

    %token           ':'
       <std::string> ID
       <int>         INT;

  you may use these constructors:

    symbol_type (int token, const std::string&);
    symbol_type (int token, const int&);
    symbol_type (int token);

  Correct matching between token types and value types is checked via
  'assert'; for instance, 'symbol_type (ID, 42)' would abort.  Named
  constructors are preferable, as they offer better type safety (for
  instance 'make_ID (42)' would not even compile), but symbol_type
  constructors may help when token types are discovered at run-time, e.g.,

     [a-z]+   {
                if (auto i = lookup_keyword (yytext))
                  return yy::parser::symbol_type (i);
                else
                  return yy::parser::make_ID (yytext);
              }

*** C++: Variadic emplace

  If your application requires C++11 and you don't use symbol constructors,
  you may now use a variadic emplace for semantic values:

    %define api.value.type variant
    %token <std::pair<int, int>> PAIR

  in your scanner:

    int yylex (parser::semantic_type *lvalp)
    {
      lvalp->emplace <std::pair<int, int>> (1, 2);
      return parser::token::PAIR;
    }

*** C++: Syntax error exceptions in GLR

  The glr.cc skeleton now supports syntax_error exceptions thrown from user
  actions, or from the scanner.

*** More POSIX Yacc compatibility warnings

  More Bison specific directives are now reported with -y or -Wyacc.  This
  change was ready since the release of Bison 3.0 in September 2015.  It was
  delayed because Autoconf used to define YACC as `bison -y`, which resulted
  in numerous warnings for Bison users that use the GNU Build System.

  If you still experience that problem, either redefine YACC as `bison -o
  y.tab.c`, or pass -Wno-yacc to Bison.

*** The tables yyrhs and yyphrs are back

  Because no Bison skeleton uses them, these tables were removed (no longer
  passed to the skeletons, not even computed) in 2008.  However, some users
  have expressed interest in being able to use them in their own skeletons.

** Bug fixes

*** Incorrect number of reduce-reduce conflicts

  On a grammar such as

     exp: "num" | "num" | "num"

  bison used to report a single RR conflict, instead of two.  This is now
  fixed.  This was the oldest (known) bug in Bison: it was there when Bison
  was entered in the RCS version control system, in December 1987.

  Some grammar files might have to adjust their %expect-rr.

*** Parser directives that were not careful enough

  Passing invalid arguments to %nterm, for instance character literals, used
  to result in unclear error messages.

** Documentation

  The examples/ directory (installed in .../share/doc/bison/examples) has
  been restructured per language for clarity.  The examples come with a
  README and a Makefile.  Not only can they be used to toy with Bison, they
  can also be starting points for your own grammars.

  There is now a Java example, and a simple example in C based on Flex and
  Bison (examples/c/lexcalc/).

** Changes

*** Parsers in C++

  They now use noexcept and constexpr.  Please, report missing annotations.

*** Symbol Declarations

  The syntax of the variation directives to declare symbols was overhauled
  for more consistency, and also better POSIX Yacc compliance (which, for
  instance, allows "%type" without actually providing a type).  The %nterm
  directive, supported by Bison since its inception, is now documented and
  officially supported.

  The syntax is now as follows:

    %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
    %left  TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
    %type  TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
    %nterm TAG? ID+ ( TAG ID+ )*

  where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
  such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
  ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
  literal such as ‘"number"’.  The post-fix quantifiers are ‘?’ (zero or
  one), ‘*’ (zero or more) and ‘+’ (one or more).

* Noteworthy changes in release 3.2.4 (2018-12-24) [stable]

** Bug fixes

  Fix the move constructor of symbol_type.

  Always provide a copy constructor for symbol_type, even in modern C++.

* Noteworthy changes in release 3.2.3 (2018-12-18) [stable]

** Bug fixes

  Properly support token constructors in C++ with types that include commas
  (e.g., std::pair<int, int>).  A regression introduced in Bison 3.2.

* Noteworthy changes in release 3.2.2 (2018-11-21) [stable]

** Bug fixes

  C++ portability issues.

* Noteworthy changes in release 3.2.1 (2018-11-09) [stable]

** Bug fixes

  Several portability issues have been fixed in the build system, in the
  test suite, and in the generated parsers in C++.

* Noteworthy changes in release 3.2 (2018-10-29) [stable]

** Backward incompatible changes

  Support for DJGPP, which has been unmaintained and untested for years, is
  obsolete.  Unless there is activity to revive it, it will be removed.

** Changes

  %printers should use yyo rather than yyoutput to denote the output stream.

  Variant-based symbols in C++ should use emplace() rather than build().

  In C++ parsers, parser::operator() is now a synonym for the parser::parse.

** Documentation

  A new section, "A Simple C++ Example", is a tutorial for parsers in C++.

  A comment in the generated code now emphasizes that users should not
  depend upon non-documented implementation details, such as macros starting
  with YY_.

** New features

*** C++: Support for move semantics (lalr1.cc)

  The lalr1.cc skeleton now fully supports C++ move semantics, while
  maintaining compatibility with C++98.  You may now store move-only types
  when using Bison's variants.  For instance:

    %code {
      #include <memory>
      #include <vector>
    }

    %skeleton "lalr1.cc"
    %define api.value.type variant

    %%

    %token <int> INT "int";
    %type <std::unique_ptr<int>> int;
    %type <std::vector<std::unique_ptr<int>>> list;

    list:
      %empty    {}
    | list int  { $$ = std::move($1); $$.emplace_back(std::move($2)); }

    int: "int"  { $$ = std::make_unique<int>($1); }

*** C++: Implicit move of right-hand side values (lalr1.cc)

  In modern C++ (C++11 and later), you should always use 'std::move' with
  the values of the right-hand side symbols ($1, $2, etc.), as they will be
  popped from the stack anyway.  Using 'std::move' is mandatory for
  move-only types such as unique_ptr, and it provides a significant speedup
  for large types such as std::string, or std::vector, etc.

  If '%define api.value.automove' is set, every occurrence '$n' is replaced
  by 'std::move ($n)'.  The second rule in the previous grammar can be
  simplified to:

    list: list int  { $$ = $1; $$.emplace_back($2); }

  With automove enabled, the semantic values are no longer lvalues, so do
  not use the swap idiom:

    list: list int  { std::swap($$, $1); $$.emplace_back($2); }

  This idiom is anyway obsolete: it is preferable to move than to swap.

  A warning is issued when automove is enabled, and a value is used several
  times.

    input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
    exp: "twice" exp   { $$ = $2 + $2; }
                                   ^^

  Enabling api.value.automove does not require support for modern C++.  The
  generated code is valid C++98/03, but will use copies instead of moves.

  The new examples/c++/variant-11.yy shows these features in action.

*** C++: The implicit default semantic action is always run

  When variants are enabled, the default action was not run, so

    exp: "number"

  was equivalent to

    exp: "number"  {}

  It now behaves like in all the other cases, as

    exp: "number"  { $$ = $1; }

  possibly using std::move if automove is enabled.

  We do not expect backward compatibility issues.  However, beware of
  forward compatibility issues: if you rely on default actions with
  variants, be sure to '%require "3.2"' to avoid older versions of Bison to
  generate incorrect parsers.

*** C++: Renaming location.hh

  When both %defines and %locations are enabled, Bison generates a
  location.hh file.  If you don't use locations outside of the parser, you
  may avoid its creation with:

    %define api.location.file none

  However this file is useful if, for instance, your parser builds an AST
  decorated with locations: you may use Bison's location independently of
  Bison's parser.  You can now give it another name, for instance:

    %define api.location.file "my-location.hh"

  This name can have directory components, and even be absolute.  The name
  under which the location file is included is controlled by
  api.location.include.

  This way it is possible to have several parsers share the same location
  file.

  For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:

    %locations
    %define api.namespace {foo}
    %define api.location.file "include/ast/loc.hh"
    %define api.location.include {<ast/loc.hh>}

  and use it in src/bar/parser.hh:

    %locations
    %define api.namespace {bar}
    %code requires {#include <ast/loc.hh>}
    %define api.location.type {bar::location}

  Absolute file names are supported, so in your Makefile, passing the flag
  -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
  safe.

*** C++: stack.hh and position.hh are deprecated

  When asked to generate a header file (%defines), the lalr1.cc skeleton
  generates a stack.hh file.  This file had no interest for users; it is now
  made useless: its content is included in the parser definition.  It is
  still generated for backward compatibility.

  When in addition to %defines, location support is requested (%locations),
  the file position.hh is also generated.  It is now also useless: its
  content is now included in location.hh.

  These files are no longer generated when your grammar file requires at
  least Bison 3.2 (%require "3.2").

** Bug fixes

  Portability issues on MinGW and VS2015.

  Portability issues in the test suite.

  Portability/warning issues with Flex.

* Noteworthy changes in release 3.1 (2018-08-27) [stable]

** Backward incompatible changes

  Compiling Bison now requires a C99 compiler---as announced during the
  release of Bison 3.0, five years ago.  Generated parsers do not require a
  C99 compiler.

  Support for DJGPP, which has been unmaintained and untested for years, is
  obsolete. Unless there is activity to revive it, the next release of Bison
  will have it removed.

** New features

*** Typed midrule actions

  Because their type is unknown to Bison, the values of midrule actions are
  not treated like the others: they don't have %printer and %destructor
  support.  It also prevents C++ (Bison) variants to handle them properly.

  Typed midrule actions address these issues.  Instead of:

    exp: { $<ival>$ = 1; } { $<ival>$ = 2; }   { $$ = $<ival>1 + $<ival>2; }

  write:

    exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; }   { $$ = $1 + $2; }

*** Reports include the type of the symbols

  The sections about terminal and nonterminal symbols of the '*.output' file
  now specify their declared type.  For instance, for:

    %token <ival> NUM

  the report now shows '<ival>':

    Terminals, with rules where they appear

    NUM <ival> (258) 5

*** Diagnostics about useless rules

  In the following grammar, the 'exp' nonterminal is trivially useless.  So,
  of course, its rules are useless too.

    %%
    input: '0' | exp
    exp: exp '+' exp | exp '-' exp | '(' exp ')'

  Previously all the useless rules were reported, including those whose
  left-hand side is the 'exp' nonterminal:

    warning: 1 nonterminal useless in grammar [-Wother]
    warning: 4 rules useless in grammar [-Wother]
    2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
     input: '0' | exp
                  ^^^
    2.14-16: warning: rule useless in grammar [-Wother]
     input: '0' | exp
                  ^^^
    3.6-16: warning: rule useless in grammar [-Wother]
     exp: exp '+' exp | exp '-' exp | '(' exp ')'
          ^^^^^^^^^^^
    3.20-30: warning: rule useless in grammar [-Wother]
     exp: exp '+' exp | exp '-' exp | '(' exp ')'
                        ^^^^^^^^^^^
    3.34-44: warning: rule useless in grammar [-Wother]
     exp: exp '+' exp | exp '-' exp | '(' exp ')'
                                      ^^^^^^^^^^^

  Now, rules whose left-hand side symbol is useless are no longer reported
  as useless.  The locations of the errors have also been adjusted to point
  to the first use of the nonterminal as a left-hand side of a rule:

    warning: 1 nonterminal useless in grammar [-Wother]
    warning: 4 rules useless in grammar [-Wother]
    3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
     exp: exp '+' exp | exp '-' exp | '(' exp ')'
     ^^^
    2.14-16: warning: rule useless in grammar [-Wother]
     input: '0' | exp
                  ^^^

*** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)

  When compiled with exceptions disabled, the generated parsers no longer
  uses try/catch clauses.

  Currently only GCC and Clang are supported.

** Documentation

*** A demonstration of variants

  A new example was added (installed in .../share/doc/bison/examples),
  'variant.yy', which shows how to use (Bison) variants in C++.

  The other examples were made nicer to read.

*** Some features are no longer 'experimental'

  The following features, mature enough, are no longer flagged as
  experimental in the documentation: push parsers, default %printer and
  %destructor (typed: <*> and untyped: <>), %define api.value.type union and
  variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
  semantic predicates (%?).

** Bug fixes

*** GLR: Predicates support broken by #line directives

  Predicates (%?) in GLR such as

    widget:
      %? {new_syntax} 'w' id new_args
    | %?{!new_syntax} 'w' id old_args

  were issued with #lines in the middle of C code.

*** Printer and destructor with broken #line directives

  The #line directives were not properly escaped when emitting the code for
  %printer/%destructor, which resulted in compiler errors if there are
  backslashes or double-quotes in the grammar file name.

*** Portability on ICC

  The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
  Generated parsers now work around this.

*** Various

  There were several small fixes in the test suite and in the build system,
  many warnings in bison and in the generated parsers were eliminated.  The
  documentation also received its share of minor improvements.

  Useless code was removed from C++ parsers, and some of the generated
  constructors are more 'natural'.

* Noteworthy changes in release 3.0.5 (2018-05-27) [stable]

** Bug fixes

*** C++: Fix support of 'syntax_error'

  One incorrect 'inline' resulted in linking errors about the constructor of
  the syntax_error exception.

*** C++: Fix warnings

  GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
  warnings about yyformat being possibly null.  It also warned about the
  deprecated implicit definition of copy constructors when there's a
  user-defined (copy) assignment operator.

*** Location of errors

  In C++ parsers, out-of-bounds errors can happen when a rule with an empty
  ride-hand side raises a syntax error.  The behavior of the default parser
  (yacc.c) in such a condition was undefined.

  Now all the parsers match the behavior of glr.c: @$ is used as the
  location of the error.  This handles gracefully rules with and without
  rhs.

*** Portability fixes in the test suite

  On some platforms, some Java and/or C++ tests were failing.

* Noteworthy changes in release 3.0.4 (2015-01-23) [stable]

** Bug fixes

*** C++ with Variants (lalr1.cc)

  Fix a compiler warning when no %destructor use $$.

*** Test suites

  Several portability issues in tests were fixed.

* Noteworthy changes in release 3.0.3 (2015-01-15) [stable]

** Bug fixes

*** C++ with Variants (lalr1.cc)

  Problems with %destructor and '%define parse.assert' have been fixed.

*** Named %union support (yacc.c, glr.c)

  Bison 3.0 introduced a regression on named %union such as

    %union foo { int ival; };

  The possibility to use a name was introduced "for Yacc compatibility".
  It is however not required by POSIX Yacc, and its usefulness is not clear.

*** %define api.value.type union with %defines (yacc.c, glr.c)

  The C parsers were broken when %defines was used together with "%define
  api.value.type union".

*** Redeclarations are reported in proper order

  On

    %token FOO "foo"
    %printer {} "foo"
    %printer {} FOO

  bison used to report:

    foo.yy:2.10-11: error: %printer redeclaration for FOO
     %printer {} "foo"
              ^^
    foo.yy:3.10-11:     previous declaration
     %printer {} FOO
              ^^

  Now, the "previous" declaration is always the first one.


** Documentation

  Bison now installs various files in its docdir (which defaults to
  '/usr/local/share/doc/bison'), including the three fully blown examples
  extracted from the documentation:

   - rpcalc
     Reverse Polish Calculator, a simple introductory example.
   - mfcalc
     Multi-function Calc, a calculator with memory and functions and located
     error messages.
   - calc++
     a calculator in C++ using variant support and token constructors.

* Noteworthy changes in release 3.0.2 (2013-12-05) [stable]

** Bug fixes

*** Generated source files when errors are reported

  When warnings are issued and -Werror is set, bison would still generate
  the source files (*.c, *.h...).  As a consequence, some runs of "make"
  could fail the first time, but not the second (as the files were generated
  anyway).

  This is fixed: bison no longer generates this source files, but, of
  course, still produces the various reports (*.output, *.xml, etc.).

*** %empty is used in reports

  Empty right-hand sides are denoted by '%empty' in all the reports (text,
  dot, XML and formats derived from it).

*** YYERROR and variants

  When C++ variant support is enabled, an error triggered via YYERROR, but
  not caught via error recovery, resulted in a double deletion.

* Noteworthy changes in release 3.0.1 (2013-11-12) [stable]

** Bug fixes

*** Errors in caret diagnostics

  On some platforms, some errors could result in endless diagnostics.

*** Fixes of the -Werror option

  Options such as "-Werror -Wno-error=foo" were still turning "foo"
  diagnostics into errors instead of warnings.  This is fixed.

  Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
  leaves "foo" diagnostics as warnings.  Similarly, with "-Werror=foo
  -Wno-error", "foo" diagnostics are now errors.

*** GLR Predicates

  As demonstrated in the documentation, one can now leave spaces between
  "%?" and its "{".

*** Installation

  The yacc.1 man page is no longer installed if --disable-yacc was
  specified.

*** Fixes in the test suite

  Bugs and portability issues.

* Noteworthy changes in release 3.0 (2013-07-25) [stable]

** WARNING: Future backward-incompatibilities!

  Like other GNU packages, Bison will start using some of the C99 features
  for its own code, especially the definition of variables after statements.
  The generated C parsers still aim at C90.

** Backward incompatible changes

*** Obsolete features

  Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.

  Support for yystype and yyltype is removed (deprecated in Bison 1.875):
  use YYSTYPE and YYLTYPE.

  Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
  1.875): use %lex-param, %parse-param, or %param.

  Missing semicolons at the end of actions are no longer added (as announced
  in the release 2.5).

*** Use of YACC='bison -y'

  TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
  Bison extensions.

  Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
  Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
  'y.tab.h' and 'y.output') to be generated from 'foo.y'.

  To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
  implementation of Yacc, was using Bison as 'bison -y'.  While it does
  ensure compatible output file names, it also enables warnings for
  incompatibilities with POSIX Yacc.  In other words, 'bison -y' triggers
  warnings for Bison extensions.

  Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
  (which also generates 'y.tab.h' and 'y.output' when needed).
  Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
  flags (YFLAGS, or AM_YFLAGS with Automake).

** Bug fixes

*** The epilogue is no longer affected by internal #defines (glr.c)

  The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
  generated code.  These weren't properly undefined before the inclusion of
  the user epilogue, so functions such as the following were butchered by the
  preprocessor expansion:

    int yylex (YYSTYPE *yylval);

  This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
  identifiers for user-provided variables.

*** stdio.h is no longer needed when locations are enabled (yacc.c)

  Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
  locations are enabled.  This is fixed.

*** Warnings about useless %pure-parser/%define api.pure are restored

** Diagnostics reported by Bison

  Most of these features were contributed by Théophile Ranquet and Victor
  Santet.

*** Carets

  Version 2.7 introduced caret errors, for a prettier output.  These are now
  activated by default.  The old format can still be used by invoking Bison
  with -fno-caret (or -fnone).

  Some error messages that reproduced excerpts of the grammar are now using
  the caret information only.  For instance on:

    %%
    exp: 'a' | 'a';

  Bison 2.7 reports:

    in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
    in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]

  Now bison reports:

    in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
    in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
     exp: 'a' | 'a';
                ^^^

  and "bison -fno-caret" reports:

    in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
    in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]

*** Enhancements of the -Werror option

  The -Werror=CATEGORY option is now recognized, and will treat specified
  warnings as errors. The warnings need not have been explicitly activated
  using the -W option, this is similar to what GCC 4.7 does.

  For example, given the following command line, Bison will treat both
  warnings related to POSIX Yacc incompatibilities and S/R conflicts as
  errors (and only those):

    $ bison -Werror=yacc,error=conflicts-sr input.y

  If no categories are specified, -Werror will make all active warnings into
  errors. For example, the following line does the same the previous example:

    $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y

  (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)

  Note that the categories in this -Werror option may not be prefixed with
  "no-". However, -Wno-error[=CATEGORY] is valid.

  Note that -y enables -Werror=yacc. Therefore it is now possible to require
  Yacc-like behavior (e.g., always generate y.tab.c), but to report
  incompatibilities as warnings: "-y -Wno-error=yacc".

*** The display of warnings is now richer

  The option that controls a given warning is now displayed:

    foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]

  In the case of warnings treated as errors, the prefix is changed from
  "warning: " to "error: ", and the suffix is displayed, in a manner similar
  to GCC, as [-Werror=CATEGORY].

  For instance, where the previous version of Bison would report (and exit
  with failure):

    bison: warnings being treated as errors
    input.y:1.1: warning: stray ',' treated as white space

  it now reports:

    input.y:1.1: error: stray ',' treated as white space [-Werror=other]

*** Deprecated constructs

  The new 'deprecated' warning category flags obsolete constructs whose
  support will be discontinued.  It is enabled by default.  These warnings
  used to be reported as 'other' warnings.

*** Useless semantic types

  Bison now warns about useless (uninhabited) semantic types.  Since
  semantic types are not declared to Bison (they are defined in the opaque
  %union structure), it is %printer/%destructor directives about useless
  types that trigger the warning:

    %token <type1> term
    %type  <type2> nterm
    %printer    {} <type1> <type3>
    %destructor {} <type2> <type4>
    %%
    nterm: term { $$ = $1; };

    3.28-34: warning: type <type3> is used, but is not associated to any symbol
    4.28-34: warning: type <type4> is used, but is not associated to any symbol

*** Undefined but unused symbols

  Bison used to raise an error for undefined symbols that are not used in
  the grammar.  This is now only a warning.

    %printer    {} symbol1
    %destructor {} symbol2
    %type <type>   symbol3
    %%
    exp: "a";

*** Useless destructors or printers

  Bison now warns about useless destructors or printers.  In the following
  example, the printer for <type1>, and the destructor for <type2> are
  useless: all symbols of <type1> (token1) already have a printer, and all
  symbols of type <type2> (token2) already have a destructor.

    %token <type1> token1
           <type2> token2
           <type3> token3
           <type4> token4
    %printer    {} token1 <type1> <type3>
    %destructor {} token2 <type2> <type4>

*** Conflicts

  The warnings and error messages about shift/reduce and reduce/reduce
  conflicts have been normalized.  For instance on the following foo.y file:

    %glr-parser
    %%
    exp: exp '+' exp | '0' | '0';

  compare the previous version of bison:

    $ bison foo.y
    foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
    $ bison -Werror foo.y
    bison: warnings being treated as errors
    foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce

  with the new behavior:

    $ bison foo.y
    foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
    foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
    $ bison -Werror foo.y
    foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
    foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]

  When %expect or %expect-rr is used, such as with bar.y:

    %expect 0
    %glr-parser
    %%
    exp: exp '+' exp | '0' | '0';

  Former behavior:

    $ bison bar.y
    bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
    bar.y: expected 0 shift/reduce conflicts
    bar.y: expected 0 reduce/reduce conflicts

  New one:

    $ bison bar.y
    bar.y: error: shift/reduce conflicts: 1 found, 0 expected
    bar.y: error: reduce/reduce conflicts: 2 found, 0 expected

** Incompatibilities with POSIX Yacc

  The 'yacc' category is no longer part of '-Wall', enable it explicitly
  with '-Wyacc'.

** Additional yylex/yyparse arguments

  The new directive %param declares additional arguments to both yylex and
  yyparse.  The %lex-param, %parse-param, and %param directives support one
  or more arguments.  Instead of

    %lex-param   {arg1_type *arg1}
    %lex-param   {arg2_type *arg2}
    %parse-param {arg1_type *arg1}
    %parse-param {arg2_type *arg2}

  one may now declare

    %param {arg1_type *arg1} {arg2_type *arg2}

** Types of values for %define variables

  Bison used to make no difference between '%define foo bar' and '%define
  foo "bar"'.  The former is now called a 'keyword value', and the latter a
  'string value'.  A third kind was added: 'code values', such as '%define
  foo {bar}'.

  Keyword variables are used for fixed value sets, e.g.,

    %define lr.type lalr

  Code variables are used for value in the target language, e.g.,

    %define api.value.type {struct semantic_type}

  String variables are used remaining cases, e.g. file names.

** Variable api.token.prefix

  The variable api.token.prefix changes the way tokens are identified in
  the generated files.  This is especially useful to avoid collisions
  with identifiers in the target language.  For instance

    %token FILE for ERROR
    %define api.token.prefix {TOK_}
    %%
    start: FILE for ERROR;

  will generate the definition of the symbols TOK_FILE, TOK_for, and
  TOK_ERROR in the generated sources.  In particular, the scanner must
  use these prefixed token names, although the grammar itself still
  uses the short names (as in the sample rule given above).

** Variable api.value.type

  This new %define variable supersedes the #define macro YYSTYPE.  The use
  of YYSTYPE is discouraged.  In particular, #defining YYSTYPE *and* either
  using %union or %defining api.value.type results in undefined behavior.

  Either define api.value.type, or use "%union":

    %union
    {
      int ival;
      char *sval;
    }
    %token <ival> INT "integer"
    %token <sval> STRING "string"
    %printer { fprintf (yyo, "%d", $$); } <ival>
    %destructor { free ($$); } <sval>

    /* In yylex().  */
    yylval.ival = 42; return INT;
    yylval.sval = "42"; return STRING;

  The %define variable api.value.type supports both keyword and code values.

  The keyword value 'union' means that the user provides genuine types, not
  union member names such as "ival" and "sval" above (WARNING: will fail if
  -y/--yacc/%yacc is enabled).

    %define api.value.type union
    %token <int> INT "integer"
    %token <char *> STRING "string"
    %printer { fprintf (yyo, "%d", $$); } <int>
    %destructor { free ($$); } <char *>

    /* In yylex().  */
    yylval.INT = 42; return INT;
    yylval.STRING = "42"; return STRING;

  The keyword value variant is somewhat equivalent, but for C++ special
  provision is made to allow classes to be used (more about this below).

    %define api.value.type variant
    %token <int> INT "integer"
    %token <std::string> STRING "string"

  Code values (in braces) denote user defined types.  This is where YYSTYPE
  used to be used.

    %code requires
    {
      struct my_value
      {
        enum
        {
          is_int, is_string
        } kind;
        union
        {
          int ival;
          char *sval;
        } u;
      };
    }
    %define api.value.type {struct my_value}
    %token <u.ival> INT "integer"
    %token <u.sval> STRING "string"
    %printer { fprintf (yyo, "%d", $$); } <u.ival>
    %destructor { free ($$); } <u.sval>

    /* In yylex().  */
    yylval.u.ival = 42; return INT;
    yylval.u.sval = "42"; return STRING;

** Variable parse.error

  This variable controls the verbosity of error messages.  The use of the
  %error-verbose directive is deprecated in favor of "%define parse.error
  verbose".

** Renamed %define variables

  The following variables have been renamed for consistency.  Backward
  compatibility is ensured, but upgrading is recommended.

    lr.default-reductions      -> lr.default-reduction
    lr.keep-unreachable-states -> lr.keep-unreachable-state
    namespace                  -> api.namespace
    stype                      -> api.value.type

** Semantic predicates

  Contributed by Paul Hilfinger.

  The new, experimental, semantic-predicate feature allows actions of the
  form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
  YYERROR) if the expression evaluates to 0, and are evaluated immediately
  in GLR parsers, rather than being deferred.  The result is that they allow
  the programmer to prune possible parses based on the values of run-time
  expressions.

** The directive %expect-rr is now an error in non GLR mode

  It used to be an error only if used in non GLR mode, _and_ if there are
  reduce/reduce conflicts.

** Tokens are numbered in their order of appearance

  Contributed by Valentin Tolmer.

  With '%token A B', A had a number less than the one of B.  However,
  precedence declarations used to generate a reversed order.  This is now
  fixed, and introducing tokens with any of %token, %left, %right,
  %precedence, or %nonassoc yields the same result.

  When mixing declarations of tokens with a literal character (e.g., 'a') or
  with an identifier (e.g., B) in a precedence declaration, Bison numbered
  the literal characters first.  For example

    %right A B 'c' 'd'

  would lead to the tokens declared in this order: 'c' 'd' A B.  Again, the
  input order is now preserved.

  These changes were made so that one can remove useless precedence and
  associativity declarations (i.e., map %nonassoc, %left or %right to
  %precedence, or to %token) and get exactly the same output.

** Useless precedence and associativity

  Contributed by Valentin Tolmer.

  When developing and maintaining a grammar, useless associativity and
  precedence directives are common.  They can be a nuisance: new ambiguities
  arising are sometimes masked because their conflicts are resolved due to
  the extra precedence or associativity information.  Furthermore, it can
  hinder the comprehension of a new grammar: one will wonder about the role
  of a precedence, where in fact it is useless.  The following changes aim
  at detecting and reporting these extra directives.

*** Precedence warning category

  A new category of warning, -Wprecedence, was introduced. It flags the
  useless precedence and associativity directives.

*** Useless associativity

  Bison now warns about symbols with a declared associativity that is never
  used to resolve conflicts.  In that case, using %precedence is sufficient;
  the parsing tables will remain unchanged.  Solving these warnings may raise
  useless precedence warnings, as the symbols no longer have associativity.
  For example:

    %left '+'
    %left '*'
    %%
    exp:
      "number"
    | exp '+' "number"
    | exp '*' exp
    ;

  will produce a

    warning: useless associativity for '+', use %precedence [-Wprecedence]
     %left '+'
           ^^^

*** Useless precedence

  Bison now warns about symbols with a declared precedence and no declared
  associativity (i.e., declared with %precedence), and whose precedence is
  never used.  In that case, the symbol can be safely declared with %token
  instead, without modifying the parsing tables.  For example:

    %precedence '='
    %%
    exp: "var" '=' "number";

  will produce a

    warning: useless precedence for '=' [-Wprecedence]
     %precedence '='
                 ^^^

*** Useless precedence and associativity

  In case of both useless precedence and associativity, the issue is flagged
  as follows:

    %nonassoc '='
    %%
    exp: "var" '=' "number";

  The warning is:

    warning: useless precedence and associativity for '=' [-Wprecedence]
     %nonassoc '='
               ^^^

** Empty rules

  With help from Joel E. Denny and Gabriel Rassoul.

  Empty rules (i.e., with an empty right-hand side) can now be explicitly
  marked by the new %empty directive.  Using %empty on a non-empty rule is
  an error.  The new -Wempty-rule warning reports empty rules without
  %empty.  On the following grammar:

    %%
    s: a b c;
    a: ;
    b: %empty;
    c: 'a' %empty;

  bison reports:

    3.4-5: warning: empty rule without %empty [-Wempty-rule]
     a: {}
        ^^
    5.8-13: error: %empty on non-empty rule
     c: 'a' %empty {};
            ^^^^^^

** Java skeleton improvements

  The constants for token names were moved to the Lexer interface.  Also, it
  is possible to add code to the parser's constructors using "%code init"
  and "%define init_throws".
  Contributed by Paolo Bonzini.

  The Java skeleton now supports push parsing.
  Contributed by Dennis Heimbigner.

** C++ skeletons improvements

*** The parser header is no longer mandatory (lalr1.cc, glr.cc)

  Using %defines is now optional.  Without it, the needed support classes
  are defined in the generated parser, instead of additional files (such as
  location.hh, position.hh and stack.hh).

*** Locations are no longer mandatory (lalr1.cc, glr.cc)

  Both lalr1.cc and glr.cc no longer require %location.

*** syntax_error exception (lalr1.cc)

  The C++ parser features a syntax_error exception, which can be
  thrown from the scanner or from user rules to raise syntax errors.
  This facilitates reporting errors caught in sub-functions (e.g.,
  rejecting too large integral literals from a conversion function
  used by the scanner, or rejecting invalid combinations from a
  factory invoked by the user actions).

*** %define api.value.type variant

  This is based on a submission from Michiel De Wilde.  With help
  from Théophile Ranquet.

  In this mode, complex C++ objects can be used as semantic values.  For
  instance:

    %token <::std::string> TEXT;
    %token <int> NUMBER;
    %token SEMICOLON ";"
    %type <::std::string> item;
    %type <::std::list<std::string>> list;
    %%
    result:
      list  { std::cout << $1 << std::endl; }
    ;

    list:
      %empty        { /* Generates an empty string list. */ }
    | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
    ;

    item:
      TEXT    { std::swap ($$, $1); }
    | NUMBER  { $$ = string_cast ($1); }
    ;

*** %define api.token.constructor

  When variants are enabled, Bison can generate functions to build the
  tokens.  This guarantees that the token type (e.g., NUMBER) is consistent
  with the semantic value (e.g., int):

    parser::symbol_type yylex ()
    {
      parser::location_type loc = ...;
      ...
      return parser::make_TEXT ("Hello, world!", loc);
      ...
      return parser::make_NUMBER (42, loc);
      ...
      return parser::make_SEMICOLON (loc);
      ...
    }

*** C++ locations

  There are operator- and operator-= for 'location'.  Negative line/column
  increments can no longer underflow the resulting value.

* Noteworthy changes in release 2.7.1 (2013-04-15) [stable]

** Bug fixes

*** Fix compiler attribute portability (yacc.c)

  With locations enabled, __attribute__ was used unprotected.

*** Fix some compiler warnings (lalr1.cc)

* Noteworthy changes in release 2.7 (2012-12-12) [stable]

** Bug fixes

  Warnings about uninitialized yylloc in yyparse have been fixed.

  Restored C90 compliance (yet no report was ever made).

** Diagnostics are improved

  Contributed by Théophile Ranquet.

*** Changes in the format of error messages

  This used to be the format of many error reports:

    input.y:2.7-12: %type redeclaration for exp
    input.y:1.7-12: previous declaration

  It is now:

    input.y:2.7-12: error: %type redeclaration for exp
    input.y:1.7-12:     previous declaration

*** New format for error reports: carets

  Caret errors have been added to Bison:

    input.y:2.7-12: error: %type redeclaration for exp
     %type <sval> exp
           ^^^^^^
    input.y:1.7-12:     previous declaration
     %type <ival> exp
           ^^^^^^

  or

    input.y:3.20-23: error: ambiguous reference: '$exp'
     exp: exp '+' exp { $exp = $1 + $3; };
                        ^^^^
    input.y:3.1-3:       refers to: $exp at $$
     exp: exp '+' exp { $exp = $1 + $3; };
     ^^^
    input.y:3.6-8:       refers to: $exp at $1
     exp: exp '+' exp { $exp = $1 + $3; };
          ^^^
    input.y:3.14-16:     refers to: $exp at $3
     exp: exp '+' exp { $exp = $1 + $3; };
                  ^^^

  The default behavior for now is still not to display these unless
  explicitly asked with -fcaret (or -fall). However, in a later release, it
  will be made the default behavior (but may still be deactivated with
  -fno-caret).

** New value for %define variable: api.pure full

  The %define variable api.pure requests a pure (reentrant) parser. However,
  for historical reasons, using it in a location-tracking Yacc parser
  resulted in a yyerror function that did not take a location as a
  parameter. With this new value, the user may request a better pure parser,
  where yyerror does take a location as a parameter (in location-tracking
  parsers).

  The use of "%define api.pure true" is deprecated in favor of this new
  "%define api.pure full".

** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)

  The %define variable api.location.type defines the name of the type to use
  for locations.  When defined, Bison no longer generates the position.hh
  and location.hh files, nor does the parser will include them: the user is
  then responsible to define her type.

  This can be used in programs with several parsers to factor their location
  and position files: let one of them generate them, and the others just use
  them.

  This feature was actually introduced, but not documented, in Bison 2.5,
  under the name "location_type" (which is maintained for backward
  compatibility).

  For consistency, lalr1.java's %define variables location_type and
  position_type are deprecated in favor of api.location.type and
  api.position.type.

** Exception safety (lalr1.cc)

  The parse function now catches exceptions, uses the %destructors to
  release memory (the lookahead symbol and the symbols pushed on the stack)
  before re-throwing the exception.

  This feature is somewhat experimental.  User feedback would be
  appreciated.

** Graph improvements in DOT and XSLT

  Contributed by Théophile Ranquet.

  The graphical presentation of the states is more readable: their shape is
  now rectangular, the state number is clearly displayed, and the items are
  numbered and left-justified.

  The reductions are now explicitly represented as transitions to other
  diamond shaped nodes.

  These changes are present in both --graph output and xml2dot.xsl XSLT
  processing, with minor (documented) differences.

** %language is no longer an experimental feature.

  The introduction of this feature, in 2.4, was four years ago. The
  --language option and the %language directive are no longer experimental.

** Documentation

  The sections about shift/reduce and reduce/reduce conflicts resolution
  have been fixed and extended.

  Although introduced more than four years ago, XML and Graphviz reports
  were not properly documented.

  The translation of midrule actions is now described.

* Noteworthy changes in release 2.6.5 (2012-11-07) [stable]

  We consider compiler warnings about Bison generated parsers to be bugs.
  Rather than working around them in your own project, please consider
  reporting them to us.

** Bug fixes

  Warnings about uninitialized yylval and/or yylloc for push parsers with a
  pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
  3.2.

  Other issues in the test suite have been addressed.

  Null characters are correctly displayed in error messages.

  When possible, yylloc is correctly initialized before calling yylex.  It
  is no longer necessary to initialize it in the %initial-action.

* Noteworthy changes in release 2.6.4 (2012-10-23) [stable]

  Bison 2.6.3's --version was incorrect.  This release fixes this issue.

* Noteworthy changes in release 2.6.3 (2012-10-22) [stable]

** Bug fixes

  Bugs and portability issues in the test suite have been fixed.

  Some errors in translations have been addressed, and --help now directs
  users to the appropriate place to report them.

  Stray Info files shipped by accident are removed.

  Incorrect definitions of YY_, issued by yacc.c when no parser header is
  generated, are removed.

  All the generated headers are self-contained.

** Header guards (yacc.c, glr.c, glr.cc)

  In order to avoid collisions, the header guards are now
  YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
  For instance the header generated from

    %define api.prefix "calc"
    %defines "lib/parse.h"

  will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.

** Fix compiler warnings in the generated parser (yacc.c, glr.c)

  The compilation of pure parsers (%define api.pure) can trigger GCC
  warnings such as:

    input.c: In function 'yyparse':
    input.c:1503:12: warning: 'yylval' may be used uninitialized in this
                              function [-Wmaybe-uninitialized]
       *++yyvsp = yylval;
                ^

  This is now fixed; pragmas to avoid these warnings are no longer needed.

  Warnings from clang ("equality comparison with extraneous parentheses" and
  "function declared 'noreturn' should not return") have also been
  addressed.

* Noteworthy changes in release 2.6.2 (2012-08-03) [stable]

** Bug fixes

  Buffer overruns, complaints from Flex, and portability issues in the test
  suite have been fixed.

** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)

  Trailing end-of-lines in %parse-param or %lex-param would result in
  invalid C++.  This is fixed.

** Spurious spaces and end-of-lines

  The generated files no longer end (nor start) with empty lines.

* Noteworthy changes in release 2.6.1 (2012-07-30) [stable]

 Bison no longer executes user-specified M4 code when processing a grammar.

** Future Changes

  In addition to the removal of the features announced in Bison 2.6, the
  next major release will remove the "Temporary hack for adding a semicolon
  to the user action", as announced in the release 2.5.  Instead of:

    exp: exp "+" exp { $$ = $1 + $3 };

  write:

    exp: exp "+" exp { $$ = $1 + $3; };

** Bug fixes

*** Type names are now properly escaped.

*** glr.cc: set_debug_level and debug_level work as expected.

*** Stray @ or $ in actions

  While Bison used to warn about stray $ or @ in action rules, it did not
  for other actions such as printers, destructors, or initial actions.  It
  now does.

** Type names in actions

  For consistency with rule actions, it is now possible to qualify $$ by a
  type-name in destructors, printers, and initial actions.  For instance:

    %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;

  will display two values for each typed and untyped symbol (provided
  that YYSTYPE has both "ival" and "fval" fields).

* Noteworthy changes in release 2.6 (2012-07-19) [stable]

** Future changes

  The next major release of Bison will drop support for the following
  deprecated features.  Please report disagreements to bug-bison@gnu.org.

*** K&R C parsers

  Support for generating parsers in K&R C will be removed.  Parsers
  generated for C support ISO C90, and are tested with ISO C99 and ISO C11
  compilers.

*** Features deprecated since Bison 1.875

  The definitions of yystype and yyltype will be removed; use YYSTYPE and
  YYLTYPE.

  YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
  %lex-param, will no longer be supported.

  Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
  %error-verbose.

*** The generated header will be included (yacc.c)

  Instead of duplicating the content of the generated header (definition of
  YYSTYPE, yyparse declaration etc.), the generated parser will include it,
  as is already the case for GLR or C++ parsers.  This change is deferred
  because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
  it.

** Generated Parser Headers

*** Guards (yacc.c, glr.c, glr.cc)

  The generated headers are now guarded, as is already the case for C++
  parsers (lalr1.cc).  For instance, with --defines=foo.h:

    #ifndef YY_FOO_H
    # define YY_FOO_H
    ...
    #endif /* !YY_FOO_H  */

*** New declarations (yacc.c, glr.c)

  The generated header now declares yydebug and yyparse.  Both honor
  --name-prefix=bar_, and yield

    int bar_parse (void);

  rather than

    #define yyparse bar_parse
    int yyparse (void);

  in order to facilitate the inclusion of several parser headers inside a
  single compilation unit.

*** Exported symbols in C++

  The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
  header, are removed, as they prevent the possibility of including several
  generated headers from a single compilation unit.

*** YYLSP_NEEDED

  For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
  longer defined.

** New %define variable: api.prefix

  Now that the generated headers are more complete and properly protected
  against multiple inclusions, constant names, such as YYSTYPE are a
  problem.  While yyparse and others are properly renamed by %name-prefix,
  YYSTYPE, YYDEBUG and others have never been affected by it.  Because it
  would introduce backward compatibility issues in projects not expecting
  YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
  it is deprecated in favor of a new %define variable: api.prefix.

  The following examples compares both:

    %name-prefix "bar_"               | %define api.prefix "bar_"
    %token <ival> FOO                   %token <ival> FOO
    %union { int ival; }                %union { int ival; }
    %%                                  %%
    exp: 'a';                           exp: 'a';

  bison generates:

    #ifndef BAR_FOO_H                   #ifndef BAR_FOO_H
    # define BAR_FOO_H                  # define BAR_FOO_H

    /* Enabling traces.  */             /* Enabling traces.  */
    # ifndef YYDEBUG                  | # ifndef BAR_DEBUG
                                      > #  if defined YYDEBUG
                                      > #   if YYDEBUG
                                      > #    define BAR_DEBUG 1
                                      > #   else
                                      > #    define BAR_DEBUG 0
                                      > #   endif
                                      > #  else
    #  define YYDEBUG 0               | #   define BAR_DEBUG 0
                                      > #  endif
    # endif                           | # endif

    # if YYDEBUG                      | # if BAR_DEBUG
    extern int bar_debug;               extern int bar_debug;
    # endif                             # endif

    /* Tokens.  */                      /* Tokens.  */
    # ifndef YYTOKENTYPE              | # ifndef BAR_TOKENTYPE
    #  define YYTOKENTYPE             | #  define BAR_TOKENTYPE
       enum yytokentype {             |    enum bar_tokentype {
         FOO = 258                           FOO = 258
       };                                  };
    # endif                             # endif

    #if ! defined YYSTYPE \           | #if ! defined BAR_STYPE \
     && ! defined YYSTYPE_IS_DECLARED |  && ! defined BAR_STYPE_IS_DECLARED
    typedef union YYSTYPE             | typedef union BAR_STYPE
    {                                   {
     int ival;                           int ival;
    } YYSTYPE;                        | } BAR_STYPE;
    # define YYSTYPE_IS_DECLARED 1    | # define BAR_STYPE_IS_DECLARED 1
    #endif                              #endif

    extern YYSTYPE bar_lval;          | extern BAR_STYPE bar_lval;

    int bar_parse (void);               int bar_parse (void);

    #endif /* !BAR_FOO_H  */            #endif /* !BAR_FOO_H  */

* Noteworthy changes in release 2.5.1 (2012-06-05) [stable]

** Future changes:

  The next major release will drop support for generating parsers in K&R C.

** yacc.c: YYBACKUP works as expected.

** glr.c improvements:

*** Location support is eliminated when not requested:

  GLR parsers used to include location-related code even when locations were
  not requested, and therefore not even usable.

*** __attribute__ is preserved:

  __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
  when -std is passed to GCC).

** lalr1.java: several fixes:

  The Java parser no longer throws ArrayIndexOutOfBoundsException if the
  first token leads to a syntax error.  Some minor clean ups.

** Changes for C++:

*** C++11 compatibility:

  C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
  or higher.

*** Header guards

  The header files such as "parser.hh", "location.hh", etc. used a constant
  name for preprocessor guards, for instance:

    #ifndef BISON_LOCATION_HH
    # define BISON_LOCATION_HH
    ...
    #endif // !BISON_LOCATION_HH

  The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
  case characters are converted to upper case, and series of
  non-alphanumerical characters are converted to an underscore.

  With "bison -o lang++/parser.cc", "location.hh" would now include:

    #ifndef YY_LANG_LOCATION_HH
    # define YY_LANG_LOCATION_HH
    ...
    #endif // !YY_LANG_LOCATION_HH

*** C++ locations:

  The position and location constructors (and their initialize methods)
  accept new arguments for line and column.  Several issues in the
  documentation were fixed.

** liby is no longer asking for "rpl_fprintf" on some platforms.

** Changes in the manual:

*** %printer is documented

  The "%printer" directive, supported since at least Bison 1.50, is finally
  documented.  The "mfcalc" example is extended to demonstrate it.

  For consistency with the C skeletons, the C++ parsers now also support
  "yyoutput" (as an alias to "debug_stream ()").

*** Several improvements have been made:

  The layout for grammar excerpts was changed to a more compact scheme.
  Named references are motivated.  The description of the automaton
  description file (*.output) is updated to the current format.  Incorrect
  index entries were fixed.  Some other errors were fixed.

** Building bison:

*** Conflicting prototypes with recent/modified Flex.

  Fixed build problems with the current, unreleased, version of Flex, and
  some modified versions of 2.5.35, which have modified function prototypes.

*** Warnings during the build procedure have been eliminated.

*** Several portability problems in the test suite have been fixed:

  This includes warnings with some compilers, unexpected behavior of tools
  such as diff, warning messages from the test suite itself, etc.

*** The install-pdf target works properly:

  Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
  halts in the middle of its course.

* Changes in version 2.5 (2011-05-14):

** Grammar symbol names can now contain non-initial dashes:

  Consistently with directives (such as %error-verbose) and with
  %define variables (e.g. push-pull), grammar symbol names may contain
  dashes in any position except the beginning.  This is a GNU
  extension over POSIX Yacc.  Thus, use of this extension is reported
  by -Wyacc and rejected in Yacc mode (--yacc).

** Named references:

  Historically, Yacc and Bison have supported positional references
  ($n, $$) to allow access to symbol values from inside of semantic
  actions code.

  Starting from this version, Bison can also accept named references.
  When no ambiguity is possible, original symbol names may be used
  as named references:

    if_stmt : "if" cond_expr "then" then_stmt ';'
    { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }

  In the more common case, explicit names may be declared:

    stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
    { $res = mk_if_stmt($cond, $then, $else); }

  Location information is also accessible using @name syntax.  When
  accessing symbol names containing dots or dashes, explicit bracketing
  ($[sym.1]) must be used.

  These features are experimental in this version.  More user feedback
  will help to stabilize them.
  Contributed by Alex Rozenman.

** IELR(1) and canonical LR(1):

  IELR(1) is a minimal LR(1) parser table generation algorithm.  That
  is, given any context-free grammar, IELR(1) generates parser tables
  with the full language-recognition power of canonical LR(1) but with
  nearly the same number of parser states as LALR(1).  This reduction
  in parser states is often an order of magnitude.  More importantly,
  because canonical LR(1)'s extra parser states may contain duplicate
  conflicts in the case of non-LR(1) grammars, the number of conflicts
  for IELR(1) is often an order of magnitude less as well.  This can
  significantly reduce the complexity of developing of a grammar.

  Bison can now generate IELR(1) and canonical LR(1) parser tables in
  place of its traditional LALR(1) parser tables, which remain the
  default.  You can specify the type of parser tables in the grammar
  file with these directives:

    %define lr.type lalr
    %define lr.type ielr
    %define lr.type canonical-lr

  The default-reduction optimization in the parser tables can also be
  adjusted using "%define lr.default-reductions".  For details on both
  of these features, see the new section "Tuning LR" in the Bison
  manual.

  These features are experimental.  More user feedback will help to
  stabilize them.

** LAC (Lookahead Correction) for syntax error handling

  Contributed by Joel E. Denny.

  Canonical LR, IELR, and LALR can suffer from a couple of problems
  upon encountering a syntax error.  First, the parser might perform
  additional parser stack reductions before discovering the syntax
  error.  Such reductions can perform user semantic actions that are
  unexpected because they are based on an invalid token, and they
  cause error recovery to begin in a different syntactic context than
  the one in which the invalid token was encountered.  Second, when
  verbose error messages are enabled (with %error-verbose or the
  obsolete "#define YYERROR_VERBOSE"), the expected token list in the
  syntax error message can both contain invalid tokens and omit valid
  tokens.

  The culprits for the above problems are %nonassoc, default
  reductions in inconsistent states, and parser state merging.  Thus,
  IELR and LALR suffer the most.  Canonical LR can suffer only if
  %nonassoc is used or if default reductions are enabled for
  inconsistent states.

  LAC is a new mechanism within the parsing algorithm that solves
  these problems for canonical LR, IELR, and LALR without sacrificing
  %nonassoc, default reductions, or state merging.  When LAC is in
  use, canonical LR and IELR behave almost exactly the same for both
  syntactically acceptable and syntactically unacceptable input.
  While LALR still does not support the full language-recognition
  power of canonical LR and IELR, LAC at least enables LALR's syntax
  error handling to correctly reflect LALR's language-recognition
  power.

  Currently, LAC is only supported for deterministic parsers in C.
  You can enable LAC with the following directive:

    %define parse.lac full

  See the new section "LAC" in the Bison manual for additional
  details including a few caveats.

  LAC is an experimental feature.  More user feedback will help to
  stabilize it.

** %define improvements:

*** Can now be invoked via the command line:

  Each of these command-line options

    -D NAME[=VALUE]
    --define=NAME[=VALUE]

    -F NAME[=VALUE]
    --force-define=NAME[=VALUE]

  is equivalent to this grammar file declaration

    %define NAME ["VALUE"]

  except that the manner in which Bison processes multiple definitions
  for the same NAME differs.  Most importantly, -F and --force-define
  quietly override %define, but -D and --define do not.  For further
  details, see the section "Bison Options" in the Bison manual.

*** Variables renamed:

  The following %define variables

    api.push_pull
    lr.keep_unreachable_states

  have been renamed to

    api.push-pull
    lr.keep-unreachable-states

  The old names are now deprecated but will be maintained indefinitely
  for backward compatibility.

*** Values no longer need to be quoted in the grammar file:

  If a %define value is an identifier, it no longer needs to be placed
  within quotations marks.  For example,

    %define api.push-pull "push"

  can be rewritten as

    %define api.push-pull push

*** Unrecognized variables are now errors not warnings.

*** Multiple invocations for any variable is now an error not a warning.

** Unrecognized %code qualifiers are now errors not warnings.

** Character literals not of length one:

  Previously, Bison quietly converted all character literals to length
  one.  For example, without warning, Bison interpreted the operators in
  the following grammar to be the same token:

    exp: exp '++'
       | exp '+' exp
       ;

  Bison now warns when a character literal is not of length one.  In
  some future release, Bison will start reporting an error instead.

** Destructor calls fixed for lookaheads altered in semantic actions:

  Previously for deterministic parsers in C, if a user semantic action
  altered yychar, the parser in some cases used the old yychar value to
  determine which destructor to call for the lookahead upon a syntax
  error or upon parser return.  This bug has been fixed.

** C++ parsers use YYRHSLOC:

  Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
  macro and use it in the default YYLLOC_DEFAULT.  You are encouraged
  to use it.  If, for instance, your location structure has "first"
  and "last" members, instead of

    # define YYLLOC_DEFAULT(Current, Rhs, N)                             \
      do                                                                 \
        if (N)                                                           \
          {                                                              \
            (Current).first = (Rhs)[1].location.first;                   \
            (Current).last  = (Rhs)[N].location.last;                    \
          }                                                              \
        else                                                             \
          {                                                              \
            (Current).first = (Current).last = (Rhs)[0].location.last;   \
          }                                                              \
      while (false)

  use:

    # define YYLLOC_DEFAULT(Current, Rhs, N)                             \
      do                                                                 \
        if (N)                                                           \
          {                                                              \
            (Current).first = YYRHSLOC (Rhs, 1).first;                   \
            (Current).last  = YYRHSLOC (Rhs, N).last;                    \
          }                                                              \
        else                                                             \
          {                                                              \
            (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last;   \
          }                                                              \
      while (false)

** YYLLOC_DEFAULT in C++:

  The default implementation of YYLLOC_DEFAULT used to be issued in
  the header file.  It is now output in the implementation file, after
  the user %code sections so that its #ifndef guard does not try to
  override the user's YYLLOC_DEFAULT if provided.

** YYFAIL now produces warnings and Java parsers no longer implement it:

  YYFAIL has existed for many years as an undocumented feature of
  deterministic parsers in C generated by Bison.  More recently, it was
  a documented feature of Bison's experimental Java parsers.  As
  promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
  semantic action now produces a deprecation warning, and Java parsers
  no longer implement YYFAIL at all.  For further details, including a
  discussion of how to suppress C preprocessor warnings about YYFAIL
  being unused, see the Bison 2.4.2 NEWS entry.

** Temporary hack for adding a semicolon to the user action:

  Previously, Bison appended a semicolon to every user action for
  reductions when the output language defaulted to C (specifically, when
  neither %yacc, %language, %skeleton, or equivalent command-line
  options were specified).  This allowed actions such as

    exp: exp "+" exp { $$ = $1 + $3 };

  instead of

    exp: exp "+" exp { $$ = $1 + $3; };

  As a first step in removing this misfeature, Bison now issues a
  warning when it appends a semicolon.  Moreover, in cases where Bison
  cannot easily determine whether a semicolon is needed (for example, an
  action ending with a cpp directive or a braced compound initializer),
  it no longer appends one.  Thus, the C compiler might now complain
  about a missing semicolon where it did not before.  Future releases of
  Bison will cease to append semicolons entirely.

** Verbose syntax error message fixes:

  When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
  specified, syntax error messages produced by the generated parser
  include the unexpected token as well as a list of expected tokens.
  The effect of %nonassoc on these verbose messages has been corrected
  in two ways, but a more complete fix requires LAC, described above:

*** When %nonassoc is used, there can exist parser states that accept no
    tokens, and so the parser does not always require a lookahead token
    in order to detect a syntax error.  Because no unexpected token or
    expected tokens can then be reported, the verbose syntax error
    message described above is suppressed, and the parser instead
    reports the simpler message, "syntax error".  Previously, this
    suppression was sometimes erroneously triggered by %nonassoc when a
    lookahead was actually required.  Now verbose messages are
    suppressed only when all previous lookaheads have already been
    shifted or discarded.

*** Previously, the list of expected tokens erroneously included tokens
    that would actually induce a syntax error because conflicts for them
    were resolved with %nonassoc in the current parser state.  Such
    tokens are now properly omitted from the list.

*** Expected token lists are still often wrong due to state merging
    (from LALR or IELR) and default reductions, which can both add
    invalid tokens and subtract valid tokens.  Canonical LR almost
    completely fixes this problem by eliminating state merging and
    default reductions.  However, there is one minor problem left even
    when using canonical LR and even after the fixes above.  That is,
    if the resolution of a conflict with %nonassoc appears in a later
    parser state than the one at which some syntax error is
    discovered, the conflicted token is still erroneously included in
    the expected token list.  Bison's new LAC implementation,
    described above, eliminates this problem and the need for
    canonical LR.  However, LAC is still experimental and is disabled
    by default.

** Java skeleton fixes:

*** A location handling bug has been fixed.

*** The top element of each of the value stack and location stack is now
    cleared when popped so that it can be garbage collected.

*** Parser traces now print the top element of the stack.

** -W/--warnings fixes:

*** Bison now properly recognizes the "no-" versions of categories:

  For example, given the following command line, Bison now enables all
  warnings except warnings for incompatibilities with POSIX Yacc:

    bison -Wall,no-yacc gram.y

*** Bison now treats S/R and R/R conflicts like other warnings:

  Previously, conflict reports were independent of Bison's normal
  warning system.  Now, Bison recognizes the warning categories
  "conflicts-sr" and "conflicts-rr".  This change has important
  consequences for the -W and --warnings command-line options.  For
  example:

    bison -Wno-conflicts-sr gram.y  # S/R conflicts not reported
    bison -Wno-conflicts-rr gram.y  # R/R conflicts not reported
    bison -Wnone            gram.y  # no conflicts are reported
    bison -Werror           gram.y  # any conflict is an error

  However, as before, if the %expect or %expect-rr directive is
  specified, an unexpected number of conflicts is an error, and an
  expected number of conflicts is not reported, so -W and --warning
  then have no effect on the conflict report.

*** The "none" category no longer disables a preceding "error":

  For example, for the following command line, Bison now reports
  errors instead of warnings for incompatibilities with POSIX Yacc:

    bison -Werror,none,yacc gram.y

*** The "none" category now disables all Bison warnings:

  Previously, the "none" category disabled only Bison warnings for
  which there existed a specific -W/--warning category.  However,
  given the following command line, Bison is now guaranteed to
  suppress all warnings:

    bison -Wnone gram.y

** Precedence directives can now assign token number 0:

  Since Bison 2.3b, which restored the ability of precedence
  directives to assign token numbers, doing so for token number 0 has
  produced an assertion failure.  For example:

    %left END 0

  This bug has been fixed.

* Changes in version 2.4.3 (2010-08-05):

** Bison now obeys -Werror and --warnings=error for warnings about
   grammar rules that are useless in the parser due to conflicts.

** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
   been fixed.

** Failures in the test suite for GCC 4.5 have been fixed.

** Failures in the test suite for some versions of Sun Studio C++ have
   been fixed.

** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
   warnings about undefined %prec identifiers will not be converted to
   errors in Bison 2.5.  They will remain warnings, which should be
   sufficient for POSIX while avoiding backward compatibility issues.

** Minor documentation fixes.

* Changes in version 2.4.2 (2010-03-20):

** Some portability problems that resulted in failures and livelocks
   in the test suite on some versions of at least Solaris, AIX, HP-UX,
   RHEL4, and Tru64 have been addressed.  As a result, fatal Bison
   errors should no longer cause M4 to report a broken pipe on the
   affected platforms.

** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.

  POSIX specifies that an error be reported for any identifier that does
  not appear on the LHS of a grammar rule and that is not defined by
  %token, %left, %right, or %nonassoc.  Bison 2.3b and later lost this
  error report for the case when an identifier appears only after a
  %prec directive.  It is now restored.  However, for backward
  compatibility with recent Bison releases, it is only a warning for
  now.  In Bison 2.5 and later, it will return to being an error.
  [Between the 2.4.2 and 2.4.3 releases, it was decided that this
  warning will not be converted to an error in Bison 2.5.]

** Detection of GNU M4 1.4.6 or newer during configure is improved.

** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
   YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
   avoided.

** %code is now a permanent feature.

  A traditional Yacc prologue directive is written in the form:

    %{CODE%}

  To provide a more flexible alternative, Bison 2.3b introduced the
  %code directive with the following forms for C/C++:

    %code          {CODE}
    %code requires {CODE}
    %code provides {CODE}
    %code top      {CODE}

  These forms are now considered permanent features of Bison.  See the
  %code entries in the section "Bison Declaration Summary" in the Bison
  manual for a summary of their functionality.  See the section
  "Prologue Alternatives" for a detailed discussion including the
  advantages of %code over the traditional Yacc prologue directive.

  Bison's Java feature as a whole including its current usage of %code
  is still considered experimental.

** YYFAIL is deprecated and will eventually be removed.

  YYFAIL has existed for many years as an undocumented feature of
  deterministic parsers in C generated by Bison.  Previously, it was
  documented for Bison's experimental Java parsers.  YYFAIL is no longer
  documented for Java parsers and is formally deprecated in both cases.
  Users are strongly encouraged to migrate to YYERROR, which is
  specified by POSIX.

  Like YYERROR, you can invoke YYFAIL from a semantic action in order to
  induce a syntax error.  The most obvious difference from YYERROR is
  that YYFAIL will automatically invoke yyerror to report the syntax
  error so that you don't have to.  However, there are several other
  subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
  inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
  used.  For a more detailed discussion, see:

    http://lists.gnu.org/archive/html/bison-patches/2009-12/msg00024.html

  The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
  deterministic parsers in C will continue to implement it.  However,
  because YYFAIL is already flawed, it seems futile to try to make new
  Bison features compatible with it.  Thus, during parser generation,
  Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
  rule action.  In a later release, YYFAIL will be disabled for
  %error-verbose and "#define YYERROR_VERBOSE".  Eventually, YYFAIL will
  be removed altogether.

  There exists at least one case where Bison 2.5's YYFAIL warning will
  be a false positive.  Some projects add phony uses of YYFAIL and other
  Bison-defined macros for the sole purpose of suppressing C
  preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
  To avoid Bison's future warning, such YYFAIL uses can be moved to the
  epilogue (that is, after the second "%%") in the Bison input file.  In
  this release (2.4.2), Bison already generates its own code to suppress
  C preprocessor warnings for YYFAIL, so projects can remove their own
  phony uses of YYFAIL if compatibility with Bison releases prior to
  2.4.2 is not necessary.

** Internationalization.

  Fix a regression introduced in Bison 2.4: Under some circumstances,
  message translations were not installed although supported by the
  host system.

* Changes in version 2.4.1 (2008-12-11):

** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
   declarations have been fixed.

** Temporary hack for adding a semicolon to the user action.

  Bison used to prepend a trailing semicolon at the end of the user
  action for reductions.  This allowed actions such as

    exp: exp "+" exp { $$ = $1 + $3 };

  instead of

    exp: exp "+" exp { $$ = $1 + $3; };

  Some grammars still depend on this "feature".  Bison 2.4.1 restores
  the previous behavior in the case of C output (specifically, when
  neither %language or %skeleton or equivalent command-line options
  are used) to leave more time for grammars depending on the old
  behavior to be adjusted.  Future releases of Bison will disable this
  feature.

** A few minor improvements to the Bison manual.

* Changes in version 2.4 (2008-11-02):

** %language is an experimental feature.

  We first introduced this feature in test release 2.3b as a cleaner
  alternative to %skeleton.  Since then, we have discussed the possibility of
  modifying its effect on Bison's output file names.  Thus, in this release,
  we consider %language to be an experimental feature that will likely evolve
  in future releases.

** Forward compatibility with GNU M4 has been improved.

** Several bugs in the C++ skeleton and the experimental Java skeleton have been
  fixed.

* Changes in version 2.3b (2008-05-27):

** The quotes around NAME that used to be required in the following directive
  are now deprecated:

    %define NAME "VALUE"

** The directive "%pure-parser" is now deprecated in favor of:

    %define api.pure

  which has the same effect except that Bison is more careful to warn about
  unreasonable usage in the latter case.

** Push Parsing

  Bison can now generate an LALR(1) parser in C with a push interface.  That
  is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
  push one token at a time to the parser using "yypush_parse", which will
  return to the caller after processing each token.  By default, the push
  interface is disabled.  Either of the following directives will enable it:

    %define api.push_pull "push" // Just push; does not require yylex.
    %define api.push_pull "both" // Push and pull; requires yylex.

  See the new section "A Push Parser" in the Bison manual for details.

  The current push parsing interface is experimental and may evolve.  More user
  feedback will help to stabilize it.

** The -g and --graph options now output graphs in Graphviz DOT format,
  not VCG format.  Like --graph, -g now also takes an optional FILE argument
  and thus cannot be bundled with other short options.

** Java

  Bison can now generate an LALR(1) parser in Java.  The skeleton is
  "data/lalr1.java".  Consider using the new %language directive instead of
  %skeleton to select it.

  See the new section "Java Parsers" in the Bison manual for details.

  The current Java interface is experimental and may evolve.  More user
  feedback will help to stabilize it.
  Contributed by Paolo Bonzini.

** %language

  This new directive specifies the programming language of the generated
  parser, which can be C (the default), C++, or Java.  Besides the skeleton
  that Bison uses, the directive affects the names of the generated files if
  the grammar file's name ends in ".y".

** XML Automaton Report

  Bison can now generate an XML report of the LALR(1) automaton using the new
  "--xml" option.  The current XML schema is experimental and may evolve.  More
  user feedback will help to stabilize it.
  Contributed by Wojciech Polak.

** The grammar file may now specify the name of the parser header file using
  %defines.  For example:

    %defines "parser.h"

** When reporting useless rules, useless nonterminals, and unused terminals,
  Bison now employs the terms "useless in grammar" instead of "useless",
  "useless in parser" instead of "never reduced", and "unused in grammar"
  instead of "unused".

** Unreachable State Removal

  Previously, Bison sometimes generated parser tables containing unreachable
  states.  A state can become unreachable during conflict resolution if Bison
  disables a shift action leading to it from a predecessor state.  Bison now:

    1. Removes unreachable states.

    2. Does not report any conflicts that appeared in unreachable states.
       WARNING: As a result, you may need to update %expect and %expect-rr
       directives in existing grammar files.

    3. For any rule used only in such states, Bison now reports the rule as
       "useless in parser due to conflicts".

  This feature can be disabled with the following directive:

    %define lr.keep_unreachable_states

  See the %define entry in the "Bison Declaration Summary" in the Bison manual
  for further discussion.

** Lookahead Set Correction in the ".output" Report

  When instructed to generate a ".output" file including lookahead sets
  (using "--report=lookahead", for example), Bison now prints each reduction's
  lookahead set only next to the associated state's one item that (1) is
  associated with the same rule as the reduction and (2) has its dot at the end
  of its RHS.  Previously, Bison also erroneously printed the lookahead set
  next to all of the state's other items associated with the same rule.  This
  bug affected only the ".output" file and not the generated parser source
  code.

** --report-file=FILE is a new option to override the default ".output" file
  name.

** The "=" that used to be required in the following directives is now
  deprecated:

    %file-prefix "parser"
    %name-prefix "c_"
    %output "parser.c"

** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"

  Bison 2.3a provided a new set of directives as a more flexible alternative to
  the traditional Yacc prologue blocks.  Those have now been consolidated into
  a single %code directive with an optional qualifier field, which identifies
  the purpose of the code and thus the location(s) where Bison should generate
  it:

    1. "%code          {CODE}" replaces "%after-header  {CODE}"
    2. "%code requires {CODE}" replaces "%start-header  {CODE}"
    3. "%code provides {CODE}" replaces "%end-header    {CODE}"
    4. "%code top      {CODE}" replaces "%before-header {CODE}"

  See the %code entries in section "Bison Declaration Summary" in the Bison
  manual for a summary of the new functionality.  See the new section "Prologue
  Alternatives" for a detailed discussion including the advantages of %code
  over the traditional Yacc prologues.

  The prologue alternatives are experimental.  More user feedback will help to
  determine whether they should become permanent features.

** Revised warning: unset or unused midrule values

  Since Bison 2.2, Bison has warned about midrule values that are set but not
  used within any of the actions of the parent rule.  For example, Bison warns
  about unused $2 in:

    exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };

  Now, Bison also warns about midrule values that are used but not set.  For
  example, Bison warns about unset $$ in the midrule action in:

    exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };

  However, Bison now disables both of these warnings by default since they
  sometimes prove to be false alarms in existing grammars employing the Yacc
  constructs $0 or $-N (where N is some positive integer).

  To enable these warnings, specify the option "--warnings=midrule-values" or
  "-W", which is a synonym for "--warnings=all".

** Default %destructor or %printer with "<*>" or "<>"

  Bison now recognizes two separate kinds of default %destructor's and
  %printer's:

    1. Place "<*>" in a %destructor/%printer symbol list to define a default
       %destructor/%printer for all grammar symbols for which you have formally
       declared semantic type tags.

    2. Place "<>" in a %destructor/%printer symbol list to define a default
       %destructor/%printer for all grammar symbols without declared semantic
       type tags.

  Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
  "<*>" and "<>" combined achieve the same effect with one exception: Bison no
  longer applies any %destructor to a midrule value if that midrule value is
  not actually ever referenced using either $$ or $n in a semantic action.

  The default %destructor's and %printer's are experimental.  More user
  feedback will help to determine whether they should become permanent
  features.

  See the section "Freeing Discarded Symbols" in the Bison manual for further
  details.

** %left, %right, and %nonassoc can now declare token numbers.  This is required
  by POSIX.  However, see the end of section "Operator Precedence" in the Bison
  manual for a caveat concerning the treatment of literal strings.

** The nonfunctional --no-parser, -n, and %no-parser options have been
  completely removed from Bison.

* Changes in version 2.3a, 2006-09-13:

** Instead of %union, you can define and use your own union type
  YYSTYPE if your grammar contains at least one <type> tag.
  Your YYSTYPE need not be a macro; it can be a typedef.
  This change is for compatibility with other Yacc implementations,
  and is required by POSIX.

** Locations columns and lines start at 1.
  In accordance with the GNU Coding Standards and Emacs.

** You may now declare per-type and default %destructor's and %printer's:

  For example:

    %union { char *string; }
    %token <string> STRING1
    %token <string> STRING2
    %type  <string> string1
    %type  <string> string2
    %union { char character; }
    %token <character> CHR
    %type  <character> chr
    %destructor { free ($$); } %symbol-default
    %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
    %destructor { } <character>

  guarantees that, when the parser discards any user-defined symbol that has a
  semantic type tag other than "<character>", it passes its semantic value to
  "free".  However, when the parser discards a "STRING1" or a "string1", it
  also prints its line number to "stdout".  It performs only the second
  "%destructor" in this case, so it invokes "free" only once.

  [Although we failed to mention this here in the 2.3a release, the default
  %destructor's and %printer's were experimental, and they were rewritten in
  future versions.]

** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
  "--yacc", or "%yacc"), Bison no longer generates #define statements for
  associating token numbers with token names.  Removing the #define statements
  helps to sanitize the global namespace during preprocessing, but POSIX Yacc
  requires them.  Bison still generates an enum for token names in all cases.

** Handling of traditional Yacc prologue blocks is now more consistent but
  potentially incompatible with previous releases of Bison.

  As before, you declare prologue blocks in your grammar file with the
  "%{ ... %}" syntax.  To generate the pre-prologue, Bison concatenates all
  prologue blocks that you've declared before the first %union.  To generate
  the post-prologue, Bison concatenates all prologue blocks that you've
  declared after the first %union.

  Previous releases of Bison inserted the pre-prologue into both the header
  file and the code file in all cases except for LALR(1) parsers in C.  In the
  latter case, Bison inserted it only into the code file.  For parsers in C++,
  the point of insertion was before any token definitions (which associate
  token numbers with names).  For parsers in C, the point of insertion was
  after the token definitions.

  Now, Bison never inserts the pre-prologue into the header file.  In the code
  file, it always inserts it before the token definitions.

** Bison now provides a more flexible alternative to the traditional Yacc
  prologue blocks: %before-header, %start-header, %end-header, and
  %after-header.

  For example, the following declaration order in the grammar file reflects the
  order in which Bison will output these code blocks.  However, you are free to
  declare these code blocks in your grammar file in whatever order is most
  convenient for you:

    %before-header {
      /* Bison treats this block like a pre-prologue block: it inserts it into
       * the code file before the contents of the header file.  It does *not*
       * insert it into the header file.  This is a good place to put
       * #include's that you want at the top of your code file.  A common
       * example is '#include "system.h"'.  */
    }
    %start-header {
      /* Bison inserts this block into both the header file and the code file.
       * In both files, the point of insertion is before any Bison-generated
       * token, semantic type, location type, and class definitions.  This is a
       * good place to define %union dependencies, for example.  */
    }
    %union {
      /* Unlike the traditional Yacc prologue blocks, the output order for the
       * new %*-header blocks is not affected by their declaration position
       * relative to any %union in the grammar file.  */
    }
    %end-header {
      /* Bison inserts this block into both the header file and the code file.
       * In both files, the point of insertion is after the Bison-generated
       * definitions.  This is a good place to declare or define public
       * functions or data structures that depend on the Bison-generated
       * definitions.  */
    }
    %after-header {
      /* Bison treats this block like a post-prologue block: it inserts it into
       * the code file after the contents of the header file.  It does *not*
       * insert it into the header file.  This is a good place to declare or
       * define internal functions or data structures that depend on the
       * Bison-generated definitions.  */
    }

  If you have multiple occurrences of any one of the above declarations, Bison
  will concatenate the contents in declaration order.

  [Although we failed to mention this here in the 2.3a release, the prologue
  alternatives were experimental, and they were rewritten in future versions.]

** The option "--report=look-ahead" has been changed to "--report=lookahead".
  The old spelling still works, but is not documented and may be removed
  in a future release.

* Changes in version 2.3, 2006-06-05:

** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
  for compatibility with LALR(1) grammars.

** It is now documented that any definition of YYSTYPE or YYLTYPE should
  be to a type name that does not contain parentheses or brackets.

* Changes in version 2.2, 2006-05-19:

** The distribution terms for all Bison-generated parsers now permit
  using the parsers in nonfree programs.  Previously, this permission
  was granted only for Bison-generated LALR(1) parsers in C.

** %name-prefix changes the namespace name in C++ outputs.

** The C++ parsers export their token_type.

** Bison now allows multiple %union declarations, and concatenates
  their contents together.

** New warning: unused values
  Right-hand side symbols whose values are not used are reported,
  if the symbols have destructors.  For instance:

     exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
        | exp "+" exp
        ;

  will trigger a warning about $$ and $5 in the first rule, and $3 in
  the second ($1 is copied to $$ by the default rule).  This example
  most likely contains three errors, and could be rewritten as:

     exp: exp "?" exp ":" exp
            { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
        | exp "+" exp
            { $$ = $1 ? $1 : $3; if ($1) free ($3); }
        ;

  However, if the original actions were really intended, memory leaks
  and all, the warnings can be suppressed by letting Bison believe the
  values are used, e.g.:

     exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
        | exp "+" exp         { $$ = $1; (void) $3; }
        ;

  If there are midrule actions, the warning is issued if no action
  uses it.  The following triggers no warning: $1 and $3 are used.

     exp: exp { push ($1); } '+' exp { push ($3); sum (); };

  The warning is intended to help catching lost values and memory leaks.
  If a value is ignored, its associated memory typically is not reclaimed.

** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
  Destructors are now called when user code invokes YYABORT, YYACCEPT,
  and YYERROR, for all objects on the stack, other than objects
  corresponding to the right-hand side of the current rule.

** %expect, %expect-rr
  Incorrect numbers of expected conflicts are now actual errors,
  instead of warnings.

** GLR, YACC parsers.
  The %parse-params are available in the destructors (and the
  experimental printers) as per the documentation.

** Bison now warns if it finds a stray "$" or "@" in an action.

** %require "VERSION"
  This specifies that the grammar file depends on features implemented
  in Bison version VERSION or higher.

** lalr1.cc: The token and value types are now class members.
  The tokens were defined as free form enums and cpp macros.  YYSTYPE
  was defined as a free form union.  They are now class members:
  tokens are enumerations of the "yy::parser::token" struct, and the
  semantic values have the "yy::parser::semantic_type" type.

  If you do not want or can update to this scheme, the directive
  '%define "global_tokens_and_yystype" "1"' triggers the global
  definition of tokens and YYSTYPE.  This change is suitable both
  for previous releases of Bison, and this one.

  If you wish to update, then make sure older version of Bison will
  fail using '%require "2.2"'.

** DJGPP support added.

* Changes in version 2.1, 2005-09-16:

** The C++ lalr1.cc skeleton supports %lex-param.

** Bison-generated parsers now support the translation of diagnostics like
  "syntax error" into languages other than English.  The default
  language is still English.  For details, please see the new
  Internationalization section of the Bison manual.  Software
  distributors should also see the new PACKAGING file.  Thanks to
  Bruno Haible for this new feature.

** Wording in the Bison-generated parsers has been changed slightly to
  simplify translation.  In particular, the message "memory exhausted"
  has replaced "parser stack overflow", as the old message was not
  always accurate for modern Bison-generated parsers.

** Destructors are now called when the parser aborts, for all symbols left
  behind on the stack.  Also, the start symbol is now destroyed after a
  successful parse.  In both cases, the behavior was formerly inconsistent.

** When generating verbose diagnostics, Bison-generated parsers no longer
  quote the literal strings associated with tokens.  For example, for
  a syntax error associated with '%token NUM "number"' they might
  print 'syntax error, unexpected number' instead of 'syntax error,
  unexpected "number"'.

* Changes in version 2.0, 2004-12-25:

** Possibly-incompatible changes

  - Bison-generated parsers no longer default to using the alloca function
    (when available) to extend the parser stack, due to widespread
    problems in unchecked stack-overflow detection.  You can "#define
    YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
    the manual to determine safe values for YYMAXDEPTH in that case.

  - Error token location.
    During error recovery, the location of the syntax error is updated
    to cover the whole sequence covered by the error token: it includes
    the shifted symbols thrown away during the first part of the error
    recovery, and the lookahead rejected during the second part.

  - Semicolon changes:
    . Stray semicolons are no longer allowed at the start of a grammar.
    . Semicolons are now required after in-grammar declarations.

  - Unescaped newlines are no longer allowed in character constants or
    string literals.  They were never portable, and GCC 3.4.0 has
    dropped support for them.  Better diagnostics are now generated if
    forget a closing quote.

  - NUL bytes are no longer allowed in Bison string literals, unfortunately.

** New features

  - GLR grammars now support locations.

  - New directive: %initial-action.
    This directive allows the user to run arbitrary code (including
    initializing @$) from yyparse before parsing starts.

  - A new directive "%expect-rr N" specifies the expected number of
    reduce/reduce conflicts in GLR parsers.

  - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
    This is a GNU extension.

  - The option "--report=lookahead" was changed to "--report=look-ahead".
    [However, this was changed back after 2.3.]

  - Experimental %destructor support has been added to lalr1.cc.

  - New configure option --disable-yacc, to disable installation of the
    yacc command and -ly library introduced in 1.875 for POSIX conformance.

** Bug fixes

  - For now, %expect-count violations are now just warnings, not errors.
    This is for compatibility with Bison 1.75 and earlier (when there are
    reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
    are too many or too few shift/reduce conflicts).  However, in future
    versions of Bison we plan to improve the %expect machinery so that
    these violations will become errors again.

  - Within Bison itself, numbers (e.g., goto numbers) are no longer
    arbitrarily limited to 16-bit counts.

  - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.

* Changes in version 1.875, 2003-01-01:

** The documentation license has been upgraded to version 1.2
  of the GNU Free Documentation License.

** syntax error processing

  - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
    locations too.  This fixes bugs in error-location computation.

  - %destructor
    It is now possible to reclaim the memory associated to symbols
    discarded during error recovery.  This feature is still experimental.

  - %error-verbose
    This new directive is preferred over YYERROR_VERBOSE.

  - #defining yyerror to steal internal variables is discouraged.
    It is not guaranteed to work forever.

** POSIX conformance

  - Semicolons are once again optional at the end of grammar rules.
    This reverts to the behavior of Bison 1.33 and earlier, and improves
    compatibility with Yacc.

  - "parse error" -> "syntax error"
    Bison now uniformly uses the term "syntax error"; formerly, the code
    and manual sometimes used the term "parse error" instead.  POSIX
    requires "syntax error" in diagnostics, and it was thought better to
    be consistent.

  - The documentation now emphasizes that yylex and yyerror must be
    declared before use.  C99 requires this.

  - Bison now parses C99 lexical constructs like UCNs and
    backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.

  - File names are properly escaped in C output.  E.g., foo\bar.y is
    output as "foo\\bar.y".

  - Yacc command and library now available
    The Bison distribution now installs a "yacc" command, as POSIX requires.
    Also, Bison now installs a small library liby.a containing
    implementations of Yacc-compatible yyerror and main functions.
    This library is normally not useful, but POSIX requires it.

  - Type clashes now generate warnings, not errors.

  - If the user does not define YYSTYPE as a macro, Bison now declares it
    using typedef instead of defining it as a macro.
    For consistency, YYLTYPE is also declared instead of defined.

** Other compatibility issues

  - %union directives can now have a tag before the "{", e.g., the
    directive "%union foo {...}" now generates the C code
    "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
    The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
    For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
    This is for compatibility with both Yacc and Bison 1.35.

  - ";" is output before the terminating "}" of an action, for
    compatibility with Bison 1.35.

  - Bison now uses a Yacc-style format for conflict reports, e.g.,
    "conflicts: 2 shift/reduce, 1 reduce/reduce".

  - "yystype" and "yyltype" are now obsolescent macros instead of being
    typedefs or tags; they are no longer documented and are planned to be
    withdrawn in a future release.

** GLR parser notes

  - GLR and inline
    Users of Bison have to decide how they handle the portability of the
    C keyword "inline".

  - "parsing stack overflow..." -> "parser stack overflow"
    GLR parsers now report "parser stack overflow" as per the Bison manual.

** %parse-param and %lex-param
  The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
  additional context to yyparse and yylex.  They suffer from several
  shortcomings:

  - a single argument only can be added,
  - their types are weak (void *),
  - this context is not passed to ancillary functions such as yyerror,
  - only yacc.c parsers support them.

  The new %parse-param/%lex-param directives provide a more precise control.
  For instance:

    %parse-param {int *nastiness}
    %lex-param   {int *nastiness}
    %parse-param {int *randomness}

  results in the following signatures:

    int yylex   (int *nastiness);
    int yyparse (int *nastiness, int *randomness);

  or, if both %pure-parser and %locations are used:

    int yylex   (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
    int yyparse (int *nastiness, int *randomness);

** Bison now warns if it detects conflicting outputs to the same file,
  e.g., it generates a warning for "bison -d -o foo.h foo.y" since
  that command outputs both code and header to foo.h.

** #line in output files
  - --no-line works properly.

** Bison can no longer be built by a K&R C compiler; it requires C89 or
  later to be built.  This change originally took place a few versions
  ago, but nobody noticed until we recently asked someone to try
  building Bison with a K&R C compiler.

* Changes in version 1.75, 2002-10-14:

** Bison should now work on 64-bit hosts.

** Indonesian translation thanks to Tedi Heriyanto.

** GLR parsers
  Fix spurious parse errors.

** Pure parsers
  Some people redefine yyerror to steal yyparse' private variables.
  Reenable this trick until an official feature replaces it.

** Type Clashes
  In agreement with POSIX and with other Yaccs, leaving a default
  action is valid when $$ is untyped, and $1 typed:

        untyped: ... typed;

  but the converse remains an error:

        typed: ... untyped;

** Values of midrule actions
  The following code:

        foo: { ... } { $$ = $1; } ...

  was incorrectly rejected: $1 is defined in the second midrule
  action, and is equal to the $$ of the first midrule action.

* Changes in version 1.50, 2002-10-04:

** GLR parsing
  The declaration
     %glr-parser
  causes Bison to produce a Generalized LR (GLR) parser, capable of handling
  almost any context-free grammar, ambiguous or not.  The new declarations
  %dprec and %merge on grammar rules allow parse-time resolution of
  ambiguities.  Contributed by Paul Hilfinger.

  Unfortunately Bison 1.50 does not work properly on 64-bit hosts
  like the Alpha, so please stick to 32-bit hosts for now.

** Output Directory
  When not in Yacc compatibility mode, when the output file was not
  specified, running "bison foo/bar.y" created "foo/bar.c".  It
  now creates "bar.c".

** Undefined token
  The undefined token was systematically mapped to 2 which prevented
  the use of 2 by the user.  This is no longer the case.

** Unknown token numbers
  If yylex returned an out of range value, yyparse could die.  This is
  no longer the case.

** Error token
  According to POSIX, the error token must be 256.
  Bison extends this requirement by making it a preference: *if* the
  user specified that one of her tokens is numbered 256, then error
  will be mapped onto another number.

** Verbose error messages
  They no longer report "..., expecting error or..." for states where
  error recovery is possible.

** End token
  Defaults to "$end" instead of "$".

** Error recovery now conforms to documentation and to POSIX
  When a Bison-generated parser encounters a syntax error, it now pops
  the stack until it finds a state that allows shifting the error
  token.  Formerly, it popped the stack until it found a state that
  allowed some non-error action other than a default reduction on the
  error token.  The new behavior has long been the documented behavior,
  and has long been required by POSIX.  For more details, please see
  Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
  <http://lists.gnu.org/archive/html/bug-bison/2002-05/msg00038.html>.

** Traces
  Popped tokens and nonterminals are now reported.

** Larger grammars
  Larger grammars are now supported (larger token numbers, larger grammar
  size (= sum of the LHS and RHS lengths), larger LALR tables).
  Formerly, many of these numbers ran afoul of 16-bit limits;
  now these limits are 32 bits on most hosts.

** Explicit initial rule
  Bison used to play hacks with the initial rule, which the user does
  not write.  It is now explicit, and visible in the reports and
  graphs as rule 0.

** Useless rules
  Before, Bison reported the useless rules, but, although not used,
  included them in the parsers.  They are now actually removed.

** Useless rules, useless nonterminals
  They are now reported, as a warning, with their locations.

** Rules never reduced
  Rules that can never be reduced because of conflicts are now
  reported.

** Incorrect "Token not used"
  On a grammar such as

    %token useless useful
    %%
    exp: '0' %prec useful;

  where a token was used to set the precedence of the last rule,
  bison reported both "useful" and "useless" as useless tokens.

** Revert the C++ namespace changes introduced in 1.31
  as they caused too many portability hassles.

** Default locations
  By an accident of design, the default computation of @$ was
  performed after another default computation was performed: @$ = @1.
  The latter is now removed: YYLLOC_DEFAULT is fully responsible of
  the computation of @$.

** Token end-of-file
  The token end of file may be specified by the user, in which case,
  the user symbol is used in the reports, the graphs, and the verbose
  error messages instead of "$end", which remains being the default.
  For instance
    %token MYEOF 0
  or
    %token MYEOF 0 "end of file"

** Semantic parser
  This old option, which has been broken for ages, is removed.

** New translations
  Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
  Croatian, thanks to Denis Lackovic.

** Incorrect token definitions
  When given
    %token 'a' "A"
  bison used to output
    #define 'a' 65

** Token definitions as enums
  Tokens are output both as the traditional #define's, and, provided
  the compiler supports ANSI C or is a C++ compiler, as enums.
  This lets debuggers display names instead of integers.

** Reports
  In addition to --verbose, bison supports --report=THINGS, which
  produces additional information:
  - itemset
    complete the core item sets with their closure
  - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
    explicitly associate lookahead tokens to items
  - solved
    describe shift/reduce conflicts solving.
    Bison used to systematically output this information on top of
    the report.  Solved conflicts are now attached to their states.

** Type clashes
  Previous versions don't complain when there is a type clash on
  the default action if the rule has a midrule action, such as in:

    %type <foo> bar
    %%
    bar: '0' {} '0';

  This is fixed.

** GNU M4 is now required when using Bison.

* Changes in version 1.35, 2002-03-25:

** C Skeleton
  Some projects use Bison's C parser with C++ compilers, and define
  YYSTYPE as a class.  The recent adjustment of C parsers for data
  alignment and 64 bit architectures made this impossible.

  Because for the time being no real solution for C++ parser
  generation exists, kludges were implemented in the parser to
  maintain this use.  In the future, when Bison has C++ parsers, this
  kludge will be disabled.

  This kludge also addresses some C++ problems when the stack was
  extended.

* Changes in version 1.34, 2002-03-12:

** File name clashes are detected
  $ bison foo.y -d -o foo.x
  fatal error: header and parser would both be named "foo.x"

** A missing ";" at the end of a rule triggers a warning
  In accordance with POSIX, and in agreement with other
  Yacc implementations, Bison will mandate this semicolon in the near
  future.  This eases the implementation of a Bison parser of Bison
  grammars by making this grammar LALR(1) instead of LR(2).  To
  facilitate the transition, this release introduces a warning.

** Revert the C++ namespace changes introduced in 1.31, as they caused too
  many portability hassles.

** DJGPP support added.

** Fix test suite portability problems.

* Changes in version 1.33, 2002-02-07:

** Fix C++ issues
  Groff could not be compiled for the definition of size_t was lacking
  under some conditions.

** Catch invalid @n
  As is done with $n.

* Changes in version 1.32, 2002-01-23:

** Fix Yacc output file names

** Portability fixes

** Italian, Dutch translations

* Changes in version 1.31, 2002-01-14:

** Many Bug Fixes

** GNU Gettext and %expect
  GNU Gettext asserts 10 s/r conflicts, but there are 7.  Now that
  Bison dies on incorrect %expectations, we fear there will be
  too many bug reports for Gettext, so _for the time being_, %expect
  does not trigger an error when the input file is named "plural.y".

** Use of alloca in parsers
  If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
  malloc exclusively.  Since 1.29, but was not NEWS'ed.

  alloca is used only when compiled with GCC, to avoid portability
  problems as on AIX.

** yyparse now returns 2 if memory is exhausted; formerly it dumped core.

** When the generated parser lacks debugging code, YYDEBUG is now 0
  (as POSIX requires) instead of being undefined.

** User Actions
  Bison has always permitted actions such as { $$ = $1 }: it adds the
  ending semicolon.  Now if in Yacc compatibility mode, the semicolon
  is no longer output: one has to write { $$ = $1; }.

** Better C++ compliance
  The output parsers try to respect C++ namespaces.
  [This turned out to be a failed experiment, and it was reverted later.]

** Reduced Grammars
  Fixed bugs when reporting useless nonterminals.

** 64 bit hosts
  The parsers work properly on 64 bit hosts.

** Error messages
  Some calls to strerror resulted in scrambled or missing error messages.

** %expect
  When the number of shift/reduce conflicts is correct, don't issue
  any warning.

** The verbose report includes the rule line numbers.

** Rule line numbers are fixed in traces.

** Swedish translation

** Parse errors
  Verbose parse error messages from the parsers are better looking.
  Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
     Now: parse error: unexpected '/', expecting "number" or '-' or '('

** Fixed parser memory leaks.
  When the generated parser was using malloc to extend its stacks, the
  previous allocations were not freed.

** Fixed verbose output file.
  Some newlines were missing.
  Some conflicts in state descriptions were missing.

** Fixed conflict report.
  Option -v was needed to get the result.

** %expect
  Was not used.
  Mismatches are errors, not warnings.

** Fixed incorrect processing of some invalid input.

** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.

** Fixed some typos in the documentation.

** %token MY_EOF 0 is supported.
  Before, MY_EOF was silently renumbered as 257.

** doc/refcard.tex is updated.

** %output, %file-prefix, %name-prefix.
  New.

** --output
  New, aliasing "--output-file".

* Changes in version 1.30, 2001-10-26:

** "--defines" and "--graph" have now an optional argument which is the
  output file name. "-d" and "-g" do not change; they do not take any
  argument.

** "%source_extension" and "%header_extension" are removed, failed
  experiment.

** Portability fixes.

* Changes in version 1.29, 2001-09-07:

** The output file does not define const, as this caused problems when used
  with common autoconfiguration schemes.  If you still use ancient compilers
  that lack const, compile with the equivalent of the C compiler option
  "-Dconst=".  Autoconf's AC_C_CONST macro provides one way to do this.

** Added "-g" and "--graph".

** The Bison manual is now distributed under the terms of the GNU FDL.

** The input and the output files has automatically a similar extension.

** Russian translation added.

** NLS support updated; should hopefully be less troublesome.

** Added the old Bison reference card.

** Added "--locations" and "%locations".

** Added "-S" and "--skeleton".

** "%raw", "-r", "--raw" is disabled.

** Special characters are escaped when output.  This solves the problems
  of the #line lines with path names including backslashes.

** New directives.
  "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
  "%debug", "%source_extension" and "%header_extension".

** @$
  Automatic location tracking.

* Changes in version 1.28, 1999-07-06:

** Should compile better now with K&R compilers.

** Added NLS.

** Fixed a problem with escaping the double quote character.

** There is now a FAQ.

* Changes in version 1.27:

** The make rule which prevented bison.simple from being created on
  some systems has been fixed.

* Changes in version 1.26:

** Bison now uses Automake.

** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.

** Token numbers now start at 257 as previously documented, not 258.

** Bison honors the TMPDIR environment variable.

** A couple of buffer overruns have been fixed.

** Problems when closing files should now be reported.

** Generated parsers should now work even on operating systems which do
  not provide alloca().

* Changes in version 1.25, 1995-10-16:

** Errors in the input grammar are not fatal; Bison keeps reading
the grammar file, and reports all the errors found in it.

** Tokens can now be specified as multiple-character strings: for
example, you could use "<=" for a token which looks like <=, instead
of choosing a name like LESSEQ.

** The %token_table declaration says to write a table of tokens (names
and numbers) into the parser file.  The yylex function can use this
table to recognize multiple-character string tokens, or for other
purposes.

** The %no_lines declaration says not to generate any #line preprocessor
directives in the parser file.

** The %raw declaration says to use internal Bison token numbers, not
Yacc-compatible token numbers, when token names are defined as macros.

** The --no-parser option produces the parser tables without including
the parser engine; a project can now use its own parser engine.
The actions go into a separate file called NAME.act, in the form of
a switch statement body.

* Changes in version 1.23:

The user can define YYPARSE_PARAM as the name of an argument to be
passed into yyparse.  The argument should have type void *.  It should
actually point to an object.  Grammar actions can access the variable
by casting it to the proper pointer type.

Line numbers in output file corrected.

* Changes in version 1.22:

--help option added.

* Changes in version 1.20:

Output file does not redefine const for C++.

-----

Copyright (C) 1995-2015, 2018-2020 Free Software Foundation, Inc.

This file is part of Bison, the GNU Parser Generator.

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.

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