inplace_vector

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// Sequence container with fixed capacity -*- C++ -*-
 
// Copyright The GNU Toolchain Authors.
//
// This file is part of the GNU ISO C++ Library.  This library 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, or (at your option)
// any later version.
 
// This library 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.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/** @file include/inplace_vector
 *  This is a Standard C++ Library header.
 *  @ingroup sequences
 */
 
#ifndef _GLIBCXX_INPLACE_VECTOR
#define _GLIBCXX_INPLACE_VECTOR 1
 
#pragma GCC system_header
 
#define __glibcxx_want_hardened_inplace_vector
#define __glibcxx_want_inplace_vector
#include <bits/version.h>
 
#ifdef __glibcxx_inplace_vector // C++ >= 26
#include <compare>
#include <initializer_list>
#include <optional>
#include <bits/stdexcept_throw.h>
#include <bits/range_access.h>
#include <bits/ranges_base.h> // borrowed_iterator_t, __detail::__container_compatible_range, __static_sized_range
#include <bits/ranges_util.h> // subrange
#include <bits/ranges_uninitialized.h>
#include <bits/stl_construct.h>
#include <bits/stl_uninitialized.h>
#include <bits/stl_algo.h> // rotate
#include <bits/erase_if.h>
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
_GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
  // [indirect], class template indirect
  template<typename _Tp, size_t _Nm>
    class inplace_vector
    {
    public:
 
      // types:
      using value_type             = _Tp;
      using pointer                = _Tp*;
      using const_pointer          = const _Tp*;
      using reference              = value_type&;
      using const_reference        = const value_type&;
      using size_type              = size_t;
      using difference_type        = ptrdiff_t;
      using iterator
        = __gnu_cxx::__normal_iterator<_Tp*, inplace_vector>;
      using const_iterator
        = __gnu_cxx::__normal_iterator<const _Tp*, inplace_vector>;
      using reverse_iterator       = std::reverse_iterator<iterator>;
      using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
      // [containers.sequences.inplace.vector.cons], construct/copy/destroy
      constexpr
      inplace_vector() noexcept
      { _M_init(); }
 
      constexpr explicit
      inplace_vector(size_type __n)
      {
        _M_init();
        _S_reserve(__n);
        std::uninitialized_value_construct_n(data(), __n);
        _M_size = __n;
      }
 
      constexpr
      inplace_vector(size_type __n, const _Tp& __value)
      {
        _M_init();
        _S_reserve(__n);
        std::uninitialized_fill_n(data(), __n, __value);
        _M_size = __n;
      }
 
      template<__any_input_iterator _InputIterator>
        constexpr
        inplace_vector(_InputIterator __first, _InputIterator __last)
        : inplace_vector()
        {
          if (const auto __n = _S_distance(__first, __last))
            {
              _S_reserve(__n);
              std::uninitialized_copy(__first, __last, data());
              _M_size = __n;
            }
          else
            {
              while (__first != __last)
                emplace_back(*__first++);
            }
        }
 
      template <__detail::__container_compatible_range<_Tp> _Rg>
        constexpr
        inplace_vector(from_range_t, _Rg&& __rg)
        : inplace_vector()
        { 
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 4396. Improve inplace_vector(from_range_t, R&& rg)
          if constexpr (ranges::__static_sized_range<_Rg>)
            static_assert(ranges::size(__rg) <= _Nm);
 
          append_range(__rg);
        }
 
      constexpr
      inplace_vector(initializer_list<_Tp> __il)
      {
        _M_init();
        _S_reserve(__il.size());
        std::uninitialized_copy(__il.begin(), __il.end(), data());
        _M_size = __il.size();
      }
 
      inplace_vector(const inplace_vector&)
        requires is_trivially_copy_constructible_v<_Tp>
        = default;
 
      constexpr
      inplace_vector(const inplace_vector& __other)
      noexcept(is_nothrow_copy_constructible_v<_Tp>)
      {
        _M_init();
        std::uninitialized_copy(__other.begin(), __other.end(), data());
        _M_size = __other.size();
      }
 
      inplace_vector(inplace_vector&&)
        requires is_trivially_move_constructible_v<_Tp>
        = default;
 
      constexpr
      inplace_vector(inplace_vector&& __other)
      noexcept(is_nothrow_move_constructible_v<_Tp>)
      {
        _M_init();
        std::uninitialized_move(__other.begin(), __other.end(), data());
        _M_size = __other.size();
      }
 
      ~inplace_vector()
        requires is_trivially_destructible_v<_Tp>
        = default;
 
      constexpr
      ~inplace_vector()
      { clear(); }
 
      inplace_vector&
      operator=(const inplace_vector&)
        requires is_trivially_copy_assignable_v<_Tp>
                   && is_trivially_copy_constructible_v<_Tp>
                   && is_trivially_destructible_v<_Tp>
        = default;
 
      constexpr inplace_vector&
      operator=(const inplace_vector& __other)
      noexcept(is_nothrow_copy_assignable_v<_Tp>
                 && is_nothrow_copy_constructible_v<_Tp>)
      {
        if (std::addressof(__other) != this) [[likely]]
          assign(__other.begin(), __other.end());
        return *this;
      }
 
      inplace_vector&
      operator=(inplace_vector&&)
        requires is_trivially_move_assignable_v<_Tp>
                   && is_trivially_move_constructible_v<_Tp>
                   && is_trivially_destructible_v<_Tp>
        = default;
 
      constexpr inplace_vector&
      operator=(inplace_vector&& __other)
      noexcept(is_nothrow_move_assignable_v<_Tp>
                 && is_nothrow_move_constructible_v<_Tp>)
      {
        if (std::addressof(__other) != this) [[likely]]
          assign(std::make_move_iterator(__other.begin()),
                 std::make_move_iterator(__other.end()));
        return *this;
      }
 
      constexpr inplace_vector&
      operator=(initializer_list<_Tp> __il)
      {
        assign(__il.begin(), __il.end());
        return *this;
      }
 
      template<__any_input_iterator _InputIterator>
        constexpr void
        assign(_InputIterator __first, _InputIterator __last)
        {
          if (const auto __n = _S_distance(__first, __last))
            {
              _S_reserve(__n);
              if (_M_size <= __n)
                {
                  for (size_t __i = 0; __i < _M_size; ++__i, (void)++__first)
                    _M_elems[__i] = *__first;
                  std::uninitialized_copy(__first, __last, end());
                }
              else
                std::destroy(std::copy(__first, __last, begin()), end());
              _M_size = __n;
            }
          else
            {
              size_t __i = 0;
              for (;__first != __last && __i < _M_size; ++__first)
                _M_elems[__i++] = *__first;
              if (__first == __last)
                {
                  std::_Destroy_n(data() + __i, _M_size - __i);
                  _M_size = __i;
                }
              else
                {
                  while (__first != __last)
                    emplace_back(*__first++);
                }
            }
        }
 
      template<__detail::__container_compatible_range<_Tp> _Rg>
        constexpr void
        assign_range(_Rg&& __rg)
        {
          if constexpr (ranges::forward_range<_Rg> || ranges::sized_range<_Rg>)
            {
              const auto __len = ranges::distance(__rg);
              if (__len > _Nm)
                __throw_bad_alloc();
 
              const size_t __sz = size_t(__len);
              if (__sz <= size())
                {
                  ranges::copy_n(ranges::begin(__rg), __sz, data());
                  std::destroy(data() + __sz, data() + _M_size);
                }
              else
                {
                  auto [__in, __out] = ranges::copy_n(
                                         ranges::begin(__rg), _M_size,
                                         data());
                  ranges::uninitialized_copy(
                    std::move(__in), ranges::end(__rg),
                    __out, unreachable_sentinel);
                }
              _M_size = __sz;
            }
          else
            {
              auto __in = ranges::begin(__rg);
              auto __end = ranges::end(__rg);
              size_type __n = 0;
              for (; __n < _M_size && __in != __end; ++__in)
                _M_elems[__n++] = *__in;
 
              if (__in == __end)
                {
                  std::destroy(data() + __n, data() + _M_size);
                  _M_size = __n;
                  return;
                }
              else if (__n < _Nm)
                {
                  auto __res = ranges::uninitialized_copy(
                                std::move(__in), __end,
                                data() + __n, data() + _Nm);
                  _M_size = __res.out - data();
                  if (__res.in == ranges::end(__rg))
                    return;
                }
              __throw_bad_alloc();
            }
        }
 
      constexpr void
      assign(size_type __n, const _Tp& __u)
      {
        _S_reserve(__n);
        if (_M_size <= __n)
          std::uninitialized_fill_n(std::fill_n(data(), _M_size, __u),
                                    __n - _M_size, __u);
        else
          std::destroy_n(std::fill_n(data(), __n, __u), _M_size - __n);
        _M_size = __n;
      }
 
      constexpr void
      assign(initializer_list<_Tp> __il)
      { assign(__il.begin(), __il.end()); }
 
      // iterators
      [[nodiscard]]
      constexpr iterator
      begin() noexcept { return iterator(data()); }
 
      [[nodiscard]]
      constexpr const_iterator
      begin() const noexcept { return const_iterator(data()); }
 
      [[nodiscard]]
      constexpr iterator
      end() noexcept
      { return iterator(data() + _M_size); }
 
      [[nodiscard]]
      constexpr const_iterator
      end() const noexcept
      { return const_iterator(data() + _M_size); }
 
      [[nodiscard]]
      constexpr reverse_iterator
      rbegin() noexcept
      { return reverse_iterator(end()); }
 
      [[nodiscard]]
      constexpr const_reverse_iterator
      rbegin() const noexcept
      { return const_reverse_iterator(end()); }
 
      [[nodiscard]]
      constexpr reverse_iterator
      rend() noexcept { return reverse_iterator(begin()); }
 
      [[nodiscard]]
      constexpr const_reverse_iterator
      rend() const noexcept { return const_reverse_iterator(begin()); }
 
      [[nodiscard]]
      constexpr const_iterator
      cbegin() const noexcept { return begin(); }
 
      [[nodiscard]]
      constexpr const_iterator
      cend() const noexcept { return end(); }
 
      [[nodiscard]]
      constexpr const_reverse_iterator
      crbegin() const noexcept { return rbegin(); }
 
      [[nodiscard]]
      constexpr const_reverse_iterator
      crend() const noexcept { return rend(); }
 
      // [containers.sequences.inplace.vector.members] size/capacity
      [[nodiscard]]
      constexpr bool
      empty() const noexcept { return _M_size == 0; }
 
      [[nodiscard]]
      constexpr size_type
      size() const noexcept
      {
        if (_M_size > _Nm)
          __builtin_unreachable();
        return _M_size;
      }
 
      [[nodiscard]]
      static constexpr size_type
      max_size() noexcept { return _Nm; }
 
      [[nodiscard]]
      static constexpr size_type
      capacity() noexcept { return _Nm; }
 
      constexpr void
      resize(size_type __n)
      {
        _S_reserve(__n);
        if (__n > _M_size)
          std::uninitialized_value_construct_n(data() + _M_size, __n - _M_size);
        else if (__n < _M_size)
          std::destroy_n(data() + __n, _M_size - __n);
        _M_size = __n;
      }
 
      constexpr void
      resize(size_type __n, const _Tp& __c)
      {
        _S_reserve(__n);
        if (__n > _M_size)
          std::uninitialized_fill_n(data() + _M_size, __n - _M_size, __c);
        else if (__n < _M_size)
          std::destroy_n(data() + __n, _M_size - __n);
        _M_size = __n;
      }
 
      static constexpr void
      reserve(size_type __n)
      { _S_reserve(__n); }
 
      static constexpr void
      shrink_to_fit() { }
 
      // element access
      [[nodiscard]]
      constexpr reference
      operator[](size_type __n)
      {
        __glibcxx_requires_subscript(__n);
        return _M_elems[__n];
      }
 
      [[nodiscard]]
      constexpr const_reference
      operator[](size_type __n) const
      {
        __glibcxx_requires_subscript(__n);
        return _M_elems[__n];
      }
 
      [[nodiscard]]
      constexpr const_reference
      at(size_type __n) const
      {
        if (__n >= _M_size)
          std::__throw_out_of_range_fmt(__N("inplace_vector::at: __n "
                                            "(which is %zu) "
                                            ">= size() (which is %zu)"),
                                        __n, _M_size);
        return _M_elems[__n];
      }
 
      [[nodiscard]]
      constexpr reference
      at(size_type __n)
      {
        if (__n >= _M_size)
          std::__throw_out_of_range_fmt(__N("inplace_vector::at: __n "
                                            "(which is %zu) "
                                            ">= size() (which is %zu)"),
                                        __n, _M_size);
        return _M_elems[__n];
      }
 
      [[nodiscard]]
      constexpr reference
      front()
      {
        __glibcxx_requires_nonempty();
        return _M_elems[0];
      }
 
      [[nodiscard]]
      constexpr const_reference
      front() const
      {
        __glibcxx_requires_nonempty();
        return _M_elems[0];
      }
 
      [[nodiscard]]
      constexpr reference
      back()
      {
        __glibcxx_requires_nonempty();
        return _M_elems[_M_size - 1];
      }
 
      [[nodiscard]]
      constexpr const_reference
      back() const
      {
        __glibcxx_requires_nonempty();
        return _M_elems[_M_size - 1];
      }
 
      // [containers.sequences.inplace.vector.data], data access
 
      [[nodiscard]]
      constexpr _Tp*
      data() noexcept
      { return static_cast<pointer>(_M_elems); }
 
      [[nodiscard]]
      constexpr const _Tp*
      data() const noexcept
      { return static_cast<const_pointer>(_M_elems); }
 
      // [containers.sequences.inplace.vector.modifiers], modifiers
      template<typename... _Args>
        constexpr _Tp&
        emplace_back(_Args&&... __args)
        {
          if (_M_size >= _Nm)
            __throw_bad_alloc();
          return unchecked_emplace_back(std::forward<_Args>(__args)...);
        }
 
      constexpr _Tp&
      push_back(const _Tp& __x)
      { return emplace_back(__x); }
 
      constexpr _Tp&
      push_back(_Tp&& __x)
      { return emplace_back(std::move(__x)); }
 
      template<__detail::__container_compatible_range<_Tp> _Rg>
        constexpr void
        append_range(_Rg&& __rg)
        {
          if constexpr (ranges::forward_range<_Rg> || ranges::sized_range<_Rg>)
            {
              const auto __len = ranges::distance(__rg);
              if (__len > (_Nm - size()))
                __throw_bad_alloc();
 
              const size_t __sz = size_t(__len);
              // Bounded on output range due PR121143
              ranges::uninitialized_copy(
                ranges::begin(__rg), unreachable_sentinel,
                data() + _M_size, data() + _M_size + __sz);
              _M_size += size_type(__sz);
            }
          else
            {
              ranges::subrange<pointer> __tail(data() + _M_size, data() + _Nm);
              auto [__in, __out] = ranges::uninitialized_copy(__rg, __tail);
              _M_size = __out - data();
              if (__in != ranges::end(__rg))
                __throw_bad_alloc();
            }
        }
 
      constexpr void
      pop_back()
      {
        __glibcxx_requires_nonempty();
        --_M_size;
        _M_elems[_M_size].~_Tp();
      }
 
      template<typename... _Args>
        constexpr optional<_Tp&>
        try_emplace_back(_Args&&... __args)
        {
          if (_M_size >= _Nm) [[unlikely]]
            return nullopt;
          return optional<_Tp&>(in_place,
                   unchecked_emplace_back(std::forward<_Args>(__args)...));
        }
 
      constexpr optional<_Tp&>
      try_push_back(const _Tp& __x)
      {
        if (_M_size >= _Nm) [[unlikely]]
          return nullopt;
        return optional<_Tp&>(in_place, unchecked_emplace_back(__x));
      }
 
      constexpr optional<_Tp&>
      try_push_back(_Tp&& __x)
      {
        if (_M_size >= _Nm) [[unlikely]]
          return nullopt;
        return optional<_Tp&>(in_place, unchecked_emplace_back(std::move(__x)));
      }
 
      template<typename... _Args>
        constexpr _Tp&
        unchecked_emplace_back(_Args&&... __args)
        {
          __glibcxx_assert(_M_size < _Nm);
          auto __p = std::construct_at(data() + _M_size,
                                       std::forward<_Args>(__args)...);
          ++_M_size;
          return *__p;
        }
 
      constexpr _Tp&
      unchecked_push_back(const _Tp& __x)
      { return unchecked_emplace_back(__x); }
 
      constexpr _Tp&
      unchecked_push_back(_Tp&& __x)
      { return unchecked_emplace_back(std::move(__x)); }
 
      template<typename... _Args>
        constexpr iterator
        emplace(const_iterator __position, _Args&&... __args)
        {
          size_t __b = __position - cbegin(); // elements before position
          __glibcxx_assert(__b <= _M_size);
          if (_M_size >= _Nm)
            __throw_bad_alloc();
          iterator __pos = begin() + __b;
          std::construct_at(data() + _M_size, std::forward<_Args>(__args)...);
          if (_M_size++)
            std::rotate(__pos, end() - 1, end());
          return __pos;
        }
 
      constexpr iterator
      insert(const_iterator __position, const _Tp& __x)
      { return emplace(__position, __x); }
 
      constexpr iterator
      insert(const_iterator __position, _Tp&& __x)
      { return emplace(__position, std::move(__x)); }
 
      constexpr iterator
      insert(const_iterator __position, size_type __n, const _Tp& __x)
      {
        size_t __b = __position - cbegin(); // elements before position
        __glibcxx_assert(__b <= _M_size);
        if ((_Nm - _M_size) < __n)
          __throw_bad_alloc();
        iterator __pos = begin() + __b;
        std::uninitialized_fill_n(data() + _M_size, __n, __x);
        if (std::__exchange(_M_size, _M_size + __n))
          std::rotate(__pos, end() - __n, end());
        return __pos;
      }
 
      template<__any_input_iterator _InputIterator>
        constexpr iterator
        insert(const_iterator __position, _InputIterator __first,
               _InputIterator __last)
        {
          size_t __b = __position - cbegin(); // elements before position
          __glibcxx_assert(__b <= _M_size);
          iterator __pos = begin() + __b;
          const size_t __s = _M_size;
          if (const auto __n = _S_distance(__first, __last))
            {
              if ((_Nm - _M_size) < __n)
                __throw_bad_alloc();
              std::uninitialized_copy(__first, __last, data() + _M_size);
              _M_size += __n;
            }
          else
            {
              while (__first != __last)
                emplace_back(*__first++);
            }
          if (__s)
            std::rotate(__pos, begin() + __s, end());
          return __pos;
        }
 
      template<__detail::__container_compatible_range<_Tp> _Rg>
        constexpr iterator
        insert_range(const_iterator __position, _Rg&& __rg)
        {
          iterator __pos = begin() + (__position - cbegin());
          const auto __end = end();
          if constexpr (ranges::forward_range<_Rg> || ranges::sized_range<_Rg>)
            {
              const auto __len = ranges::distance(__rg);
              if (__len > (_Nm - size()))
                __throw_bad_alloc();
              if (!__len) [[unlikely]]
                return __pos;
 
              const size_type __n = size_type(__len);
              const size_type __num_after = __end - __pos;
              if (__num_after >= __n)
                {
                  ranges::uninitialized_move(__end - __n, __end,
                                             __end, unreachable_sentinel);
                  _M_size += __n;
                  ranges::move_backward(__pos, __end - __n, __end);
                  ranges::copy(__rg, __pos);
                }
              else if constexpr (ranges::forward_range<_Rg>)
                {
                  auto __mid = ranges::next(ranges::begin(__rg), __num_after);
                  ranges::uninitialized_copy(__mid, ranges::end(__rg),
                                             __end, unreachable_sentinel);
                  _M_size += __n - __num_after;
                  ranges::uninitialized_move(__pos, __end,
                                             __pos + __n, unreachable_sentinel);
                  _M_size += __num_after;
                  ranges::copy(ranges::begin(__rg), __mid, __pos);
                }
              else
                {
                  ranges::uninitialized_copy(
                    ranges::begin(__rg), ranges::end(__rg),
                    __end, unreachable_sentinel);
                  _M_size += __n;
                  std::rotate(__pos, __end, end());
                }
            }
          else
            {
              append_range(__rg);
              std::rotate(__pos, __end, end());
            }
          return __pos;
        }
 
      constexpr iterator
      insert(const_iterator __position, initializer_list<_Tp> __il)
      { return insert(__position, __il.begin(), __il.end()); }
 
      constexpr iterator
      erase(const_iterator __position)
      {
        size_t __n = __position - cbegin();
        __glibcxx_assert(__n < _M_size);
        iterator __pos = begin() + __n;
        std::move(__pos + 1, end(), __pos);
        pop_back();
        return __pos;
      }
 
      constexpr iterator
      erase(const_iterator __first, const_iterator __last)
      {
        size_t __n = __first - cbegin();
        size_t __x = __last - __first;
        __glibcxx_assert(__n <= _M_size);
        __glibcxx_assert(__x <= _M_size);
        iterator __pos = begin() + __n;
        iterator __end = std::move(__pos + __x, end(), __pos);
        std::destroy_n(__end, __x);
        _M_size -= __x;
        return __pos;
      }
 
      constexpr void
      swap(inplace_vector& __x)
      noexcept(is_nothrow_swappable_v<_Tp> && is_nothrow_move_constructible_v<_Tp>)
      {
        inplace_vector* __vs[2]{ this, std::addressof(__x) };
        const auto __smaller = __vs[__x.size() < size()];
        const auto __bigger = __vs[__x.size() >= size()];
        size_type __n = __smaller->size();
        size_type __n2 = __bigger->size();
 
        if constexpr (is_nothrow_move_constructible_v<_Tp>)
          {
            for (size_type __i = __n; __i < __n2; ++__i)
              {
                std::construct_at(__smaller->data() + __i,
                                  std::move(*(__bigger->data() + __i)));
                std::destroy_at(__bigger->data() + __i);
              }
          }
        else
          {
            std::uninitialized_copy(__bigger->data() + __n,
                                    __bigger->data() + __n2,
                                    __smaller->data() + __n);
            std::destroy(__bigger->data() + __n, __bigger->data() + __n2);
          }
        __smaller->_M_size = __n2;
        __bigger->_M_size = __n;
 
        using std::swap;
        for (size_type __i = 0; __i < __n; __i++)
          swap(_M_elems[__i], __x._M_elems[__i]);
      }
 
      constexpr void
      clear() noexcept
      {
        std::destroy_n(data(), size_t(_M_size));
        _M_size = 0;
      }
 
      constexpr friend bool
      operator==(const inplace_vector& __x, const inplace_vector& __y)
      { return std::equal(__x.begin(), __x.end(), __y.begin(), __y.end()); }
 
      constexpr friend auto
      operator<=>(const inplace_vector& __x, const inplace_vector& __y)
      requires requires (const _Tp __t) {
        { __t < __t } -> __detail::__boolean_testable;
      }
      {
        return std::lexicographical_compare_three_way(__x.begin(), __x.end(),
                                                      __y.begin(), __y.end(),
                                                      __detail::__synth3way);
      }
 
      // [inplace.vector.special], specialized algorithms
      constexpr friend void
      swap(inplace_vector& __x, inplace_vector& __y)
      noexcept(is_nothrow_swappable_v<_Tp> && is_nothrow_move_constructible_v<_Tp>)
      { __x.swap(__y); }
 
    private:
      union {
        _Tp _M_elems[_Nm];
      };
 
      // Check whether integer type _UInt is wide enough to store _Nm,
      // so that we use a smaller type for _M_size when that saves space.
      template<typename _UInt, bool = (alignof(_Tp) <= sizeof(_UInt))>
        static constexpr bool __fits
          = _Nm <= __gnu_cxx::__int_traits<_UInt>::__max;
 
      // Don't bother using a smaller type if alignment of the array elements
      // means that it doesn't actually save space.
      template<typename _UInt>
        static constexpr bool __fits<_UInt, false> = false;
 
      static consteval auto __select_size_type()
      {
        if constexpr (__fits<unsigned char>)
          return (unsigned char)0;
#if __SHRT_WIDTH__ < __SIZE_WIDTH__
        else if constexpr (__fits<unsigned short>)
          return (unsigned short)0;
#endif
#if __INT_WIDTH__ < __SIZE_WIDTH__ && __INT_WIDTH__ > __SHRT_WIDTH__
        else if constexpr (__fits<unsigned int>)
          return 0u;
#endif
#if __LONG_WIDTH__ < __SIZE_WIDTH__ && __LONG_WIDTH__ > __INT_WIDTH__
        else if constexpr (__fits<unsigned long>)
          return 0ul;
#endif
        else // Just use size_t.
          return 0uz;
      }
      decltype(__select_size_type()) _M_size = 0;
 
      constexpr void
      _M_init()
      {
        if !consteval
          {
#if __glibcxx_start_lifetime_as
            std::start_lifetime_as_array<_Tp>(data(), _Nm);
#endif
          }
        else
          {
            // TODO: use new(_M_elems) _Tp[_Nm]() once PR121068 is fixed
            if constexpr (is_trivial_v<_Tp>)
              for (size_t __i = 0; __i < _Nm; ++__i)
                _M_elems[__i] = _Tp();
            else
              __builtin_unreachable(); // only trivial types are supported at compile time
          }
      }
 
      static constexpr void
      _S_reserve(size_t __n)
      {
        if (__n > _Nm)
          __throw_bad_alloc();
      }
 
      template<typename _InputIterator>
        constexpr static auto
        _S_distance(_InputIterator __first, _InputIterator __last)
        {
          if constexpr (sized_sentinel_for<_InputIterator, _InputIterator>
                          || forward_iterator<_InputIterator>)
            return (size_type)ranges::distance(__first, __last);
          else if constexpr (derived_from<__iter_category_t<_InputIterator>,
                                     forward_iterator_tag>)
            return (size_type)std::distance(__first, __last);
          else
            return false_type{};
        }
    };
 
  // specialization for zero capacity, that is required to be trivally copyable
  // and empty regardless of _Tp.
  template<typename _Tp>
    class inplace_vector<_Tp, 0>
    {
    public:
      // types:
      using value_type             = _Tp;
      using pointer                = _Tp*;
      using const_pointer          = const _Tp*;
      using reference              = value_type&;
      using const_reference        = const value_type&;
      using size_type              = size_t;
      using difference_type        = ptrdiff_t;
      using iterator
        = __gnu_cxx::__normal_iterator<_Tp*, inplace_vector>;
      using const_iterator
        = __gnu_cxx::__normal_iterator<const _Tp*, inplace_vector>;
      using reverse_iterator       = std::reverse_iterator<iterator>;
      using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
      // [containers.sequences.inplace.vector.cons], construct/copy/destroy
      inplace_vector() = default;
 
      constexpr explicit
      inplace_vector(size_type __n)
      {
        if (__n != 0)
          __throw_bad_alloc();
      }
 
      constexpr
      inplace_vector(size_type __n, const _Tp& __value)
      {
        if (__n != 0)
          __throw_bad_alloc();
      }
 
      template<__any_input_iterator _InputIterator>
        constexpr
        inplace_vector(_InputIterator __first, _InputIterator __last)
        {
          if (__first != __last)
            __throw_bad_alloc();
        }
 
      template <__detail::__container_compatible_range<_Tp> _Rg>
        constexpr
        inplace_vector(from_range_t, _Rg&& __rg)
        {
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 4396. Improve inplace_vector(from_range_t, R&& rg)
          if constexpr (ranges::__static_sized_range<_Rg>)
            static_assert(ranges::size(__rg) == 0);
 
          if (ranges::begin(__rg) != ranges::end(__rg))
            __throw_bad_alloc();
        }
 
      constexpr
      inplace_vector(initializer_list<_Tp> __il)
      {
        if (__il.size() != 0)
          __throw_bad_alloc();
      }
 
      inplace_vector(const inplace_vector&) = default;
      inplace_vector(inplace_vector&&) = default;
 
      constexpr
      ~inplace_vector() = default;
 
      inplace_vector&
      operator=(const inplace_vector&) = default;
 
      inplace_vector&
      operator=(inplace_vector&&) = default;
 
      constexpr inplace_vector&
      operator=(initializer_list<_Tp> __il)
      {
        if (__il.size() != 0)
          __throw_bad_alloc();
        return *this;
      }
 
      template<__any_input_iterator _InputIterator>
        constexpr void
        assign(_InputIterator __first, _InputIterator __last)
        {
          if (__first != __last)
            __throw_bad_alloc();
        }
 
      template<__detail::__container_compatible_range<_Tp> _Rg>
        constexpr void
        assign_range(_Rg&& __rg)
        {
          if (ranges::begin(__rg) != ranges::end(__rg))
            __throw_bad_alloc();
        }
 
      constexpr void
      assign(size_type __n, const _Tp& __u)
      {
        if (__n != 0)
          __throw_bad_alloc();
      }
 
      constexpr void
      assign(initializer_list<_Tp> __il)
      {
        if (__il.size() != 0)
          __throw_bad_alloc();
      }
 
      // iterators
      [[nodiscard]]
      constexpr iterator
      begin() noexcept { return iterator(nullptr); }
 
      [[nodiscard]]
      constexpr const_iterator
      begin() const noexcept { return const_iterator(nullptr); }
 
      [[nodiscard]]
      constexpr iterator
      end() noexcept { return iterator(nullptr); }
 
      [[nodiscard]]
      constexpr const_iterator
      end() const noexcept { return const_iterator(nullptr); }
 
      [[nodiscard]]
      constexpr reverse_iterator
      rbegin() noexcept
      { return reverse_iterator(end()); }
 
      [[nodiscard]]
      constexpr const_reverse_iterator
      rbegin() const noexcept
      { return const_reverse_iterator(end()); }
 
      [[nodiscard]]
      constexpr reverse_iterator
      rend() noexcept { return reverse_iterator(begin()); }
 
      [[nodiscard]]
      constexpr const_reverse_iterator
      rend() const noexcept { return const_reverse_iterator(begin()); }
 
      [[nodiscard]]
      constexpr const_iterator
      cbegin() const noexcept { return begin(); }
 
      [[nodiscard]]
      constexpr const_iterator
      cend() const noexcept { return end(); }
 
      [[nodiscard]]
      constexpr const_reverse_iterator
      crbegin() const noexcept { return rbegin(); }
 
      [[nodiscard]]
      constexpr const_reverse_iterator
      crend() const noexcept { return rend(); }
 
      // [containers.sequences.inplace.vector.members] size/capacity
      [[nodiscard]]
      constexpr bool
      empty() const noexcept { return true; }
 
      [[nodiscard]]
      constexpr size_type
      size() const noexcept { return 0; }
 
      [[nodiscard]]
      static constexpr size_type
      max_size() noexcept { return 0; }
 
      [[nodiscard]]
      static constexpr size_type
      capacity() noexcept { return 0; }
 
      constexpr void
      resize(size_type __n)
      {
        if (__n != 0)
          __throw_bad_alloc();
      }
 
      constexpr void
      resize(size_type __n, const _Tp&)
      {
        if (__n != 0)
          __throw_bad_alloc();
      }
 
      static constexpr void
      reserve(size_type __n)
      {
        if (__n != 0)
          __throw_bad_alloc();
      }
 
      static constexpr void
      shrink_to_fit() { }
 
      // element access
      [[nodiscard,noreturn]]
      constexpr reference
      operator[](size_type)
      { __builtin_trap(); }
 
      [[nodiscard,noreturn]]
      constexpr const_reference
      operator[](size_type) const
      { __builtin_trap(); }
 
      [[nodiscard,noreturn]]
      constexpr const_reference
      at(size_type __n) const
      {
        std::__throw_out_of_range_fmt(__N("inplace_vector::at: __n "
                                          "(which is %zu) "
                                          ">= size() (which is 0)"),
                                      __n);
      }
 
      [[nodiscard,noreturn]]
      constexpr reference
      at(size_type __n)
      {
        std::__throw_out_of_range_fmt(__N("inplace_vector::at: __n "
                                          "(which is %zu) "
                                          ">= size() (which is 0)"),
                                      __n);
      }
 
      [[nodiscard,noreturn]]
      constexpr reference
      front()
      { __builtin_trap(); }
 
      [[nodiscard,noreturn]]
      constexpr const_reference
      front() const
      { __builtin_trap(); }
 
      [[nodiscard,noreturn]]
      constexpr reference
      back()
      { __builtin_trap(); }
 
      [[nodiscard,noreturn]]
      constexpr const_reference
      back() const
      { __builtin_trap(); }
 
      // [containers.sequences.inplace.vector.data], data access
 
      [[nodiscard]]
      constexpr _Tp*
      data() noexcept
      { return nullptr; }
 
      [[nodiscard]]
      constexpr const _Tp*
      data() const noexcept
      { return nullptr; }
 
      // [containers.sequences.inplace.vector.modifiers], modifiers
      template<typename... _Args>
        [[noreturn]]
        constexpr _Tp&
        emplace_back(_Args&&...)
        { __throw_bad_alloc(); }
 
      [[noreturn]]
      constexpr _Tp&
      push_back(const _Tp&)
      { __throw_bad_alloc(); }
 
      [[noreturn]]
      constexpr _Tp&
      push_back(_Tp&&)
      { __throw_bad_alloc(); }
 
      template<__detail::__container_compatible_range<_Tp> _Rg>
        constexpr void
        append_range(_Rg&& __rg)
        {
          if (ranges::begin(__rg) != ranges::end(__rg))
            __throw_bad_alloc();
        }
 
      [[noreturn]]
      constexpr void
      pop_back()
      { __builtin_trap(); }
 
      template<typename... _Args>
        constexpr optional<_Tp&>
        try_emplace_back(_Args&&...)
        { return nullopt; }
 
      constexpr optional<_Tp&>
      try_push_back(const _Tp&)
      { return nullopt; }
 
      constexpr optional<_Tp&>
      try_push_back(_Tp&&)
      { return nullopt; }
 
      template<typename... _Args>
        [[noreturn]]
        constexpr _Tp&
        unchecked_emplace_back(_Args&&...)
        { __builtin_trap(); }
 
      [[noreturn]]
      constexpr _Tp&
      unchecked_push_back(const _Tp&)
      { __builtin_trap(); }
 
      [[noreturn]]
      constexpr _Tp&
      unchecked_push_back(_Tp&&)
      { __builtin_trap(); }
 
      template<typename... _Args>
        [[noreturn]]
        constexpr iterator
        emplace(const_iterator, _Args&&...)
        { __throw_bad_alloc(); }
 
      [[noreturn]]
      constexpr iterator
      insert(const_iterator, const _Tp&)
      { __throw_bad_alloc(); }
 
      [[noreturn]]
      constexpr iterator
      insert(const_iterator, _Tp&&)
      { __throw_bad_alloc(); }
 
      constexpr iterator
      insert(const_iterator, size_type __n, const _Tp&)
      {
        if (__n != 0)
          __throw_bad_alloc();
        return begin();
      }
 
      template<typename _InputIterator>
        constexpr iterator
        insert(const_iterator, _InputIterator __first, _InputIterator __last)
        {
          if (__first != __last)
            __throw_bad_alloc();
          return begin();
        }
 
      template<__detail::__container_compatible_range<_Tp> _Rg>
        constexpr iterator
        insert_range(const_iterator, _Rg&& __rg)
        {
          if (ranges::begin(__rg) != ranges::end(__rg))
            __throw_bad_alloc();
          return begin();
        }
 
      constexpr iterator
      insert(const_iterator, initializer_list<_Tp> __il)
      {
        if (__il.size() != 0)
          __throw_bad_alloc();
        return begin();
      }
 
      [[noreturn]]
      constexpr iterator
      erase(const_iterator)
      { __builtin_trap(); }
 
      constexpr iterator
      erase(const_iterator __first, const_iterator __last)
      {
        __glibcxx_assert(__first == __last);
        return begin();
      }
 
      constexpr void
      swap(inplace_vector& __x)
      noexcept
      { }
 
      constexpr void
      clear() noexcept
      { }
 
      constexpr friend bool
      operator==(const inplace_vector&, const inplace_vector&)
      { return true; }
 
      constexpr friend auto
      operator<=>(const inplace_vector&, const inplace_vector&)
      requires requires (const _Tp __t) {
        { __t < __t } -> __detail::__boolean_testable;
      }
      { return std::strong_ordering::equal; }
 
      // n.b. there is not explicit wording requiring that swap for inplace_vector,
      // with zero size, works even if element type is not swappable. However given
      // that move operations are required to be present and trivial, it makes sense
      // to support them.
      constexpr friend void
      swap(inplace_vector&, inplace_vector&) noexcept
      { }
    };
 
_GLIBCXX_END_NAMESPACE_CONTAINER
 
  template<typename _Tp, size_t _Nm, typename _Predicate>
    constexpr size_t
    erase_if(_GLIBCXX_STD_C::inplace_vector<_Tp, _Nm>& __cont,
             _Predicate __pred)
    {
      if constexpr (_Nm != 0)
        return __detail::__erase_if(__cont, __cont, std::move(__pred));
 
      return 0;
    }
 
  template<typename _Tp, size_t _Nm, typename _Up = _Tp>
    constexpr  size_t
    erase(_GLIBCXX_STD_C::inplace_vector<_Tp, _Nm>& __cont, const _Up& __value)
    { return std::erase_if(__cont, __gnu_cxx::__ops::__equal_to(__value)); }
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
 
#ifdef _GLIBCXX_DEBUG
# include <debug/inplace_vector>
#endif
 
#endif // __glibcxx_inplace_vector
#endif // _GLIBCXX_INPLACE_VECTOR