intermediate changes

ref:cde9a383711a11544ce7e107a78147fb96cc4029

1 files changed, 658 insertions, 0 deletions

diff --git a/contrib/libs/cxxsupp/libcxx/include/span b/contrib/libs/cxxsupp/libcxx/include/span
new file mode 100644
index 0000000000..5d57bd693c
--- /dev/null
+++ b/contrib/libs/cxxsupp/libcxx/include/span
@@ -0,0 +1,658 @@
+// -*- C++ -*-
+//===------------------------------ span ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_SPAN
+#define _LIBCPP_SPAN
+
+/*
+    span synopsis
+
+namespace std {
+
+// constants
+inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
+
+// [views.span], class template span
+template <class ElementType, size_t Extent = dynamic_extent>
+    class span;
+
+template<class ElementType, size_t Extent>
+  inline constexpr bool ranges::enable_view<span<ElementType, Extent>> = true;
+
+template<class ElementType, size_t Extent>
+    inline constexpr bool ranges::enable_borrowed_range<span<ElementType, Extent>> = true;
+
+// [span.objectrep], views of object representation
+template <class ElementType, size_t Extent>
+    span<const byte, ((Extent == dynamic_extent) ? dynamic_extent :
+        (sizeof(ElementType) * Extent))> as_bytes(span<ElementType, Extent> s) noexcept;
+
+template <class ElementType, size_t Extent>
+    span<      byte, ((Extent == dynamic_extent) ? dynamic_extent :
+        (sizeof(ElementType) * Extent))> as_writable_bytes(span<ElementType, Extent> s) noexcept;
+
+
+template <class ElementType, size_t Extent = dynamic_extent>
+class span {
+public:
+    // constants and types
+    using element_type = ElementType;
+    using value_type = remove_cv_t<ElementType>;
+    using size_type = size_t;
+    using difference_type = ptrdiff_t;
+    using pointer = element_type*;
+    using const_pointer = const element_type*;
+    using reference = element_type&;
+    using const_reference = const element_type&;
+    using iterator = implementation-defined;
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    static constexpr size_type extent = Extent;
+
+    // [span.cons], span constructors, copy, assignment, and destructor
+    constexpr span() noexcept;
+    template <class It>
+    constexpr explicit(Extent != dynamic_extent) span(It first, size_type count);
+    template <class It, class End>
+    constexpr explicit(Extent != dynamic_extent) span(It first, End last);
+    template <size_t N>
+        constexpr span(element_type (&arr)[N]) noexcept;
+    template <size_t N>
+        constexpr span(array<value_type, N>& arr) noexcept;
+    template <size_t N>
+        constexpr span(const array<value_type, N>& arr) noexcept;
+    template<class R>
+      constexpr explicit(Extent != dynamic_extent) span(R&& r);
+    constexpr span(const span& other) noexcept = default;
+    template <class OtherElementType, size_t OtherExtent>
+        constexpr explicit(Extent != dynamic_extent) span(const span<OtherElementType, OtherExtent>& s) noexcept;
+    ~span() noexcept = default;
+    constexpr span& operator=(const span& other) noexcept = default;
+
+    // [span.sub], span subviews
+    template <size_t Count>
+        constexpr span<element_type, Count> first() const;
+    template <size_t Count>
+        constexpr span<element_type, Count> last() const;
+    template <size_t Offset, size_t Count = dynamic_extent>
+        constexpr span<element_type, see below> subspan() const;
+
+    constexpr span<element_type, dynamic_extent> first(size_type count) const;
+    constexpr span<element_type, dynamic_extent> last(size_type count) const;
+    constexpr span<element_type, dynamic_extent> subspan(size_type offset, size_type count = dynamic_extent) const;
+
+    // [span.obs], span observers
+    constexpr size_type size() const noexcept;
+    constexpr size_type size_bytes() const noexcept;
+    constexpr bool empty() const noexcept;
+
+    // [span.elem], span element access
+    constexpr reference operator[](size_type idx) const;
+    constexpr reference front() const;
+    constexpr reference back() const;
+    constexpr pointer data() const noexcept;
+
+    // [span.iterators], span iterator support
+    constexpr iterator begin() const noexcept;
+    constexpr iterator end() const noexcept;
+    constexpr reverse_iterator rbegin() const noexcept;
+    constexpr reverse_iterator rend() const noexcept;
+
+private:
+    pointer data_;    // exposition only
+    size_type size_;  // exposition only
+};
+
+template<class It, class EndOrSize>
+    span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>;
+
+template<class T, size_t N>
+    span(T (&)[N]) -> span<T, N>;
+
+template<class T, size_t N>
+    span(array<T, N>&) -> span<T, N>;
+
+template<class T, size_t N>
+    span(const array<T, N>&) -> span<const T, N>;
+
+template<class R>
+    span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>;
+
+} // namespace std
+
+*/
+
+#include <__config>
+#include <__debug>
+#include <__iterator/concepts.h>
+#include <__iterator/wrap_iter.h>
+#include <__ranges/concepts.h>
+#include <__ranges/data.h>
+#include <__ranges/enable_borrowed_range.h>
+#include <__ranges/enable_view.h>
+#include <__ranges/size.h>
+#include <array>        // for array
+#include <cstddef>      // for byte
+#include <iterator>     // for iterators
+#include <limits>
+#include <type_traits>  // for remove_cv, etc
+#include <version>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
+template <typename _Tp, size_t _Extent = dynamic_extent> class span;
+
+
+template <class _Tp>
+struct __is_std_array : false_type {};
+
+template <class _Tp, size_t _Sz>
+struct __is_std_array<array<_Tp, _Sz>> : true_type {};
+
+template <class _Tp>
+struct __is_std_span : false_type {};
+
+template <class _Tp, size_t _Sz>
+struct __is_std_span<span<_Tp, _Sz>> : true_type {};
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+template <class _Range, class _ElementType>
+concept __span_compatible_range =
+  ranges::contiguous_range<_Range> &&
+  ranges::sized_range<_Range> &&
+  (ranges::borrowed_range<_Range> || is_const_v<_ElementType>) &&
+  !__is_std_span<remove_cvref_t<_Range>>::value  &&
+  !__is_std_array<remove_cvref_t<_Range>>::value &&
+  !is_array_v<remove_cvref_t<_Range>> &&
+  is_convertible_v<remove_reference_t<ranges::range_reference_t<_Range>>(*)[], _ElementType(*)[]>;
+#else
+template <class _Tp, class _ElementType, class = void>
+struct __is_span_compatible_container : public false_type {};
+
+template <class _Tp, class _ElementType>
+struct __is_span_compatible_container<_Tp, _ElementType,
+        void_t<
+        // is not a specialization of span
+            enable_if_t<!__is_std_span<_Tp>::value, nullptr_t>,
+        // is not a specialization of array
+            enable_if_t<!__is_std_array<_Tp>::value, nullptr_t>,
+        // is_array_v<Container> is false,
+            enable_if_t<!is_array_v<_Tp>, nullptr_t>,
+        // data(cont) and size(cont) are well formed
+            decltype(data(declval<_Tp>())),
+            decltype(size(declval<_Tp>())),
+        // remove_pointer_t<decltype(data(cont))>(*)[] is convertible to ElementType(*)[]
+            enable_if_t<
+                is_convertible_v<remove_pointer_t<decltype(data(declval<_Tp &>()))>(*)[],
+                                 _ElementType(*)[]>,
+                nullptr_t>
+        >>
+    : public true_type {};
+#endif
+
+template <typename _Tp, size_t _Extent>
+class _LIBCPP_TEMPLATE_VIS span {
+public:
+//  constants and types
+    using element_type           = _Tp;
+    using value_type             = remove_cv_t<_Tp>;
+    using size_type              = size_t;
+    using difference_type        = ptrdiff_t;
+    using pointer                = _Tp *;
+    using const_pointer          = const _Tp *;
+    using reference              = _Tp &;
+    using const_reference        = const _Tp &;
+#if (_LIBCPP_DEBUG_LEVEL == 2) || defined(_LIBCPP_ABI_SPAN_POINTER_ITERATORS)
+    using iterator               = pointer;
+#else
+    using iterator               = __wrap_iter<pointer>;
+#endif
+    using reverse_iterator       = _VSTD::reverse_iterator<iterator>;
+
+    static constexpr size_type extent = _Extent;
+
+// [span.cons], span constructors, copy, assignment, and destructor
+    template <size_t _Sz = _Extent, enable_if_t<_Sz == 0, nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr} {}
+
+    constexpr span           (const span&) noexcept = default;
+    constexpr span& operator=(const span&) noexcept = default;
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+    template <class _It,
+              enable_if_t<contiguous_iterator<_It> &&
+                              is_convertible_v<remove_reference_t<iter_reference_t<_It>>(*)[], element_type (*)[]>,
+                          nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit span(_It __first, size_type __count)
+        : __data{_VSTD::to_address(__first)} {
+      (void)__count;
+      _LIBCPP_ASSERT(_Extent == __count, "size mismatch in span's constructor (iterator, len)");
+    }
+
+    template <
+        class _It, class _End,
+        enable_if_t<is_convertible_v<remove_reference_t<iter_reference_t<_It> > (*)[], element_type (*)[]> &&
+                        contiguous_iterator<_It> && sized_sentinel_for<_End, _It> && !is_convertible_v<_End, size_t>,
+                    nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit span(_It __first, _End __last) : __data{_VSTD::to_address(__first)} {
+      (void)__last;
+      _LIBCPP_ASSERT((__last - __first >= 0), "invalid range in span's constructor (iterator, sentinel)");
+      _LIBCPP_ASSERT(__last - __first == _Extent,
+                     "invalid range in span's constructor (iterator, sentinel): last - first != extent");
+    }
+#else
+    _LIBCPP_INLINE_VISIBILITY constexpr explicit span(pointer __ptr, size_type __count) : __data{__ptr}
+        { (void)__count; _LIBCPP_ASSERT(_Extent == __count, "size mismatch in span's constructor (ptr, len)"); }
+    _LIBCPP_INLINE_VISIBILITY constexpr explicit span(pointer __f, pointer __l) : __data{__f}
+        { (void)__l;     _LIBCPP_ASSERT(_Extent == distance(__f, __l), "size mismatch in span's constructor (ptr, ptr)"); }
+#endif
+
+    _LIBCPP_INLINE_VISIBILITY constexpr span(element_type (&__arr)[_Extent])          noexcept : __data{__arr} {}
+
+    template <class _OtherElementType,
+              enable_if_t<is_convertible_v<_OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span(array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}
+
+    template <class _OtherElementType,
+              enable_if_t<is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span(const array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+    template <__span_compatible_range<element_type> _Range>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit span(_Range&& __r) : __data{ranges::data(__r)} {
+      _LIBCPP_ASSERT(ranges::size(__r) == _Extent, "size mismatch in span's constructor (range)");
+    }
+#else
+    template <class _Container>
+    _LIBCPP_INLINE_VISIBILITY
+        constexpr explicit span(      _Container& __c,
+            enable_if_t<__is_span_compatible_container<_Container, _Tp>::value, nullptr_t> = nullptr)
+        : __data{_VSTD::data(__c)} {
+            _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (range)");
+        }
+
+    template <class _Container>
+    _LIBCPP_INLINE_VISIBILITY
+        constexpr explicit span(const _Container& __c,
+            enable_if_t<__is_span_compatible_container<const _Container, _Tp>::value, nullptr_t> = nullptr)
+        : __data{_VSTD::data(__c)} {
+            _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (range)");
+        }
+#endif
+
+    template <class _OtherElementType>
+    _LIBCPP_INLINE_VISIBILITY
+        constexpr span(const span<_OtherElementType, _Extent>& __other,
+                       enable_if_t<
+                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
+                          nullptr_t> = nullptr)
+        : __data{__other.data()} {}
+
+    template <class _OtherElementType>
+    _LIBCPP_INLINE_VISIBILITY
+        constexpr explicit span(const span<_OtherElementType, dynamic_extent>& __other,
+                       enable_if_t<
+                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
+                          nullptr_t> = nullptr) noexcept
+        : __data{__other.data()} { _LIBCPP_ASSERT(_Extent == __other.size(), "size mismatch in span's constructor (other span)"); }
+
+
+//  ~span() noexcept = default;
+
+    template <size_t _Count>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span<element_type, _Count> first() const noexcept
+    {
+        static_assert(_Count <= _Extent, "Count out of range in span::first()");
+        return span<element_type, _Count>{data(), _Count};
+    }
+
+    template <size_t _Count>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span<element_type, _Count> last() const noexcept
+    {
+        static_assert(_Count <= _Extent, "Count out of range in span::last()");
+        return span<element_type, _Count>{data() + size() - _Count, _Count};
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span<element_type, dynamic_extent> first(size_type __count) const noexcept
+    {
+        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::first(count)");
+        return {data(), __count};
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span<element_type, dynamic_extent> last(size_type __count) const noexcept
+    {
+        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::last(count)");
+        return {data() + size() - __count, __count};
+    }
+
+    template <size_t _Offset, size_t _Count = dynamic_extent>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr auto subspan() const noexcept
+        -> span<element_type, _Count != dynamic_extent ? _Count : _Extent - _Offset>
+    {
+        static_assert(_Offset <= _Extent, "Offset out of range in span::subspan()");
+        static_assert(_Count == dynamic_extent || _Count <= _Extent - _Offset, "Offset + count out of range in span::subspan()");
+
+        using _ReturnType = span<element_type, _Count != dynamic_extent ? _Count : _Extent - _Offset>;
+        return _ReturnType{data() + _Offset, _Count == dynamic_extent ? size() - _Offset : _Count};
+    }
+
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span<element_type, dynamic_extent>
+       subspan(size_type __offset, size_type __count = dynamic_extent) const noexcept
+    {
+        _LIBCPP_ASSERT(__offset <= size(), "Offset out of range in span::subspan(offset, count)");
+        _LIBCPP_ASSERT(__count  <= size() || __count == dynamic_extent, "Count out of range in span::subspan(offset, count)");
+        if (__count == dynamic_extent)
+            return {data() + __offset, size() - __offset};
+        _LIBCPP_ASSERT(__count <= size() - __offset, "Offset + count out of range in span::subspan(offset, count)");
+        return {data() + __offset, __count};
+    }
+
+    _LIBCPP_INLINE_VISIBILITY constexpr size_type size()       const noexcept { return _Extent; }
+    _LIBCPP_INLINE_VISIBILITY constexpr size_type size_bytes() const noexcept { return _Extent * sizeof(element_type); }
+    _LIBCPP_INLINE_VISIBILITY constexpr bool empty()           const noexcept { return _Extent == 0; }
+
+    _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](size_type __idx) const noexcept
+    {
+        _LIBCPP_ASSERT(__idx < size(), "span<T,N>[] index out of bounds");
+        return __data[__idx];
+    }
+
+    _LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
+    {
+        _LIBCPP_ASSERT(!empty(), "span<T, N>::front() on empty span");
+        return __data[0];
+    }
+
+    _LIBCPP_INLINE_VISIBILITY constexpr reference back() const noexcept
+    {
+        _LIBCPP_ASSERT(!empty(), "span<T, N>::back() on empty span");
+        return __data[size()-1];
+    }
+
+    _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }
+
+// [span.iter], span iterator support
+    _LIBCPP_INLINE_VISIBILITY constexpr iterator                 begin() const noexcept { return iterator(data()); }
+    _LIBCPP_INLINE_VISIBILITY constexpr iterator                   end() const noexcept { return iterator(data() + size()); }
+    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator        rbegin() const noexcept { return reverse_iterator(end()); }
+    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator          rend() const noexcept { return reverse_iterator(begin()); }
+
+    _LIBCPP_INLINE_VISIBILITY span<const byte, _Extent * sizeof(element_type)> __as_bytes() const noexcept
+    { return span<const byte, _Extent * sizeof(element_type)>{reinterpret_cast<const byte *>(data()), size_bytes()}; }
+
+    _LIBCPP_INLINE_VISIBILITY span<byte, _Extent * sizeof(element_type)> __as_writable_bytes() const noexcept
+    { return span<byte, _Extent * sizeof(element_type)>{reinterpret_cast<byte *>(data()), size_bytes()}; }
+
+private:
+    pointer    __data;
+
+};
+
+
+template <typename _Tp>
+class _LIBCPP_TEMPLATE_VIS span<_Tp, dynamic_extent> {
+private:
+
+public:
+//  constants and types
+    using element_type           = _Tp;
+    using value_type             = remove_cv_t<_Tp>;
+    using size_type              = size_t;
+    using difference_type        = ptrdiff_t;
+    using pointer                = _Tp *;
+    using const_pointer          = const _Tp *;
+    using reference              = _Tp &;
+    using const_reference        = const _Tp &;
+#if (_LIBCPP_DEBUG_LEVEL == 2) || defined(_LIBCPP_ABI_SPAN_POINTER_ITERATORS)
+    using iterator               = pointer;
+#else
+    using iterator               = __wrap_iter<pointer>;
+#endif
+    using reverse_iterator       = _VSTD::reverse_iterator<iterator>;
+
+    static constexpr size_type extent = dynamic_extent;
+
+// [span.cons], span constructors, copy, assignment, and destructor
+    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr}, __size{0} {}
+
+    constexpr span           (const span&) noexcept = default;
+    constexpr span& operator=(const span&) noexcept = default;
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+    template <class _It,
+              enable_if_t<contiguous_iterator<_It> &&
+                              is_convertible_v<remove_reference_t<iter_reference_t<_It> > (*)[], element_type (*)[]>,
+                          nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span(_It __first, size_type __count)
+        : __data{_VSTD::to_address(__first)}, __size{__count} {}
+
+    template <
+        class _It, class _End,
+        enable_if_t<is_convertible_v<remove_reference_t<iter_reference_t<_It> > (*)[], element_type (*)[]> &&
+                        contiguous_iterator<_It> && sized_sentinel_for<_End, _It> && !is_convertible_v<_End, size_t>,
+                    nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span(_It __first, _End __last)
+        : __data(_VSTD::to_address(__first)), __size(__last - __first) {}
+#else
+    _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __ptr, size_type __count) : __data{__ptr}, __size{__count} {}
+    _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __f, pointer __l) : __data{__f}, __size{static_cast<size_t>(distance(__f, __l))} {}
+#endif
+
+    template <size_t _Sz>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span(element_type (&__arr)[_Sz])          noexcept : __data{__arr}, __size{_Sz} {}
+
+    template <class _OtherElementType, size_t _Sz,
+              enable_if_t<is_convertible_v<_OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span(array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
+
+    template <class _OtherElementType, size_t _Sz,
+              enable_if_t<is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span(const array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+    template <__span_compatible_range<element_type> _Range>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span(_Range&& __r) : __data(ranges::data(__r)), __size{ranges::size(__r)} {}
+#else
+    template <class _Container>
+    _LIBCPP_INLINE_VISIBILITY
+        constexpr span(      _Container& __c,
+            enable_if_t<__is_span_compatible_container<_Container, _Tp>::value, nullptr_t> = nullptr)
+        : __data{_VSTD::data(__c)}, __size{(size_type) _VSTD::size(__c)} {}
+
+    template <class _Container>
+    _LIBCPP_INLINE_VISIBILITY
+        constexpr span(const _Container& __c,
+            enable_if_t<__is_span_compatible_container<const _Container, _Tp>::value, nullptr_t> = nullptr)
+        : __data{_VSTD::data(__c)}, __size{(size_type) _VSTD::size(__c)} {}
+#  endif
+
+    template <class _OtherElementType, size_t _OtherExtent>
+    _LIBCPP_INLINE_VISIBILITY
+        constexpr span(const span<_OtherElementType, _OtherExtent>& __other,
+                       enable_if_t<
+                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
+                          nullptr_t> = nullptr) noexcept
+        : __data{__other.data()}, __size{__other.size()} {}
+
+//    ~span() noexcept = default;
+
+    template <size_t _Count>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span<element_type, _Count> first() const noexcept
+    {
+        _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::first()");
+        return span<element_type, _Count>{data(), _Count};
+    }
+
+    template <size_t _Count>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span<element_type, _Count> last() const noexcept
+    {
+        _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::last()");
+        return span<element_type, _Count>{data() + size() - _Count, _Count};
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span<element_type, dynamic_extent> first(size_type __count) const noexcept
+    {
+        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::first(count)");
+        return {data(), __count};
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span<element_type, dynamic_extent> last (size_type __count) const noexcept
+    {
+        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::last(count)");
+        return {data() + size() - __count, __count};
+    }
+
+    template <size_t _Offset, size_t _Count = dynamic_extent>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span<element_type, _Count> subspan() const noexcept
+    {
+        _LIBCPP_ASSERT(_Offset <= size(), "Offset out of range in span::subspan()");
+        _LIBCPP_ASSERT(_Count == dynamic_extent || _Count <= size() - _Offset, "Offset + count out of range in span::subspan()");
+        return span<element_type, _Count>{data() + _Offset, _Count == dynamic_extent ? size() - _Offset : _Count};
+    }
+
+    constexpr span<element_type, dynamic_extent>
+    _LIBCPP_INLINE_VISIBILITY
+    subspan(size_type __offset, size_type __count = dynamic_extent) const noexcept
+    {
+        _LIBCPP_ASSERT(__offset <= size(), "Offset out of range in span::subspan(offset, count)");
+        _LIBCPP_ASSERT(__count  <= size() || __count == dynamic_extent, "count out of range in span::subspan(offset, count)");
+        if (__count == dynamic_extent)
+            return {data() + __offset, size() - __offset};
+        _LIBCPP_ASSERT(__count <= size() - __offset, "Offset + count out of range in span::subspan(offset, count)");
+        return {data() + __offset, __count};
+    }
+
+    _LIBCPP_INLINE_VISIBILITY constexpr size_type size()       const noexcept { return __size; }
+    _LIBCPP_INLINE_VISIBILITY constexpr size_type size_bytes() const noexcept { return __size * sizeof(element_type); }
+    _LIBCPP_INLINE_VISIBILITY constexpr bool empty()           const noexcept { return __size == 0; }
+
+    _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](size_type __idx) const noexcept
+    {
+        _LIBCPP_ASSERT(__idx < size(), "span<T>[] index out of bounds");
+        return __data[__idx];
+    }
+
+    _LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
+    {
+        _LIBCPP_ASSERT(!empty(), "span<T>[].front() on empty span");
+        return __data[0];
+    }
+
+    _LIBCPP_INLINE_VISIBILITY constexpr reference back() const noexcept
+    {
+        _LIBCPP_ASSERT(!empty(), "span<T>[].back() on empty span");
+        return __data[size()-1];
+    }
+
+
+    _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }
+
+// [span.iter], span iterator support
+    _LIBCPP_INLINE_VISIBILITY constexpr iterator                 begin() const noexcept { return iterator(data()); }
+    _LIBCPP_INLINE_VISIBILITY constexpr iterator                   end() const noexcept { return iterator(data() + size()); }
+    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator        rbegin() const noexcept { return reverse_iterator(end()); }
+    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator          rend() const noexcept { return reverse_iterator(begin()); }
+
+    inline _LIBCPP_INLINE_VISIBILITY span<const byte, dynamic_extent> __as_bytes() const noexcept;
+
+    inline _LIBCPP_INLINE_VISIBILITY span<byte, dynamic_extent> __as_writable_bytes() const noexcept;
+
+private:
+    pointer   __data;
+    size_type __size;
+};
+
+template<typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY span<const byte, dynamic_extent> span<_Tp, dynamic_extent>::__as_bytes() const noexcept
+{ return {reinterpret_cast<const byte *>(data()), size_bytes()}; }
+
+template<typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY span<byte, dynamic_extent> span<_Tp, dynamic_extent>::__as_writable_bytes() const noexcept
+{ return {reinterpret_cast<byte *>(data()), size_bytes()}; }
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+template <class _Tp, size_t _Extent>
+inline constexpr bool ranges::enable_borrowed_range<span<_Tp, _Extent> > = true;
+
+template <class _ElementType, size_t _Extent>
+inline constexpr bool ranges::enable_view<span<_ElementType, _Extent>> = true;
+#endif // !defined(_LIBCPP_HAS_NO_RANGES)
+
+//  as_bytes & as_writable_bytes
+template <class _Tp, size_t _Extent>
+_LIBCPP_INLINE_VISIBILITY
+auto as_bytes(span<_Tp, _Extent> __s) noexcept
+-> decltype(__s.__as_bytes())
+{ return    __s.__as_bytes(); }
+
+template <class _Tp, size_t _Extent>
+_LIBCPP_INLINE_VISIBILITY
+auto as_writable_bytes(span<_Tp, _Extent> __s) noexcept
+-> enable_if_t<!is_const_v<_Tp>, decltype(__s.__as_writable_bytes())>
+{ return __s.__as_writable_bytes(); }
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+//  Deduction guides
+template<contiguous_iterator _It, class _EndOrSize>
+    span(_It, _EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>;
+#endif
+
+template<class _Tp, size_t _Sz>
+    span(_Tp (&)[_Sz]) -> span<_Tp, _Sz>;
+
+template<class _Tp, size_t _Sz>
+    span(array<_Tp, _Sz>&) -> span<_Tp, _Sz>;
+
+template<class _Tp, size_t _Sz>
+    span(const array<_Tp, _Sz>&) -> span<const _Tp, _Sz>;
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+template<ranges::contiguous_range _Range>
+    span(_Range&&) -> span<remove_reference_t<ranges::range_reference_t<_Range>>>;
+#endif
+
+#endif // _LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP_SPAN