debug/forward_list

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// <forward_list> -*- C++ -*-
 
// Copyright (C) 2010-2026 Free Software Foundation, Inc.
//
// 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 debug/forward_list
 *  This file is a GNU debug extension to the Standard C++ Library.
 */
 
#ifndef _GLIBCXX_DEBUG_FORWARD_LIST
#define _GLIBCXX_DEBUG_FORWARD_LIST 1
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#include <bits/c++config.h>
namespace std _GLIBCXX_VISIBILITY(default) { namespace __debug {
  template<typename _Tp, typename _Allocator> class forward_list;
} } // namespace std::__debug
 
#include <forward_list>
#include <debug/safe_sequence.h>
#include <debug/safe_container.h>
#include <debug/safe_iterator.h>
 
// Special validity check for forward_list ranges.
#define __glibcxx_check_valid_fl_range(_First,_Last,_Dist)              \
_GLIBCXX_DEBUG_VERIFY(_First._M_valid_range(_Last, _Dist, false),       \
                      _M_message(__gnu_debug::__msg_valid_range)        \
                      ._M_iterator(_First, #_First)                     \
                      ._M_iterator(_Last, #_Last))
 
namespace __gnu_debug
{
  /// Special iterators swap and invalidation for forward_list because of the
  /// before_begin iterator.
  template<typename _SafeSequence>
    class _Safe_forward_list
    : public _Safe_sequence<_SafeSequence>
    {
      const _SafeSequence*
      _M_this() const noexcept
      { return static_cast<const _SafeSequence*>(this); }
 
      static void
      _S_swap_aux(const _Safe_forward_list& __lhs,
                  _Safe_iterator_base*& __lhs_iterators,
                  const _Safe_forward_list& __rhs,
                  _Safe_iterator_base*& __rhs_iterators);
 
      void _M_swap_single(const _Safe_forward_list&) const noexcept;
 
    protected:
      void
      _M_invalidate_all() const
      {
        using _Base_const_iterator = __decltype(_M_this()->_M_base().cend());
        this->_M_invalidate_if([this](_Base_const_iterator __it)
        {
          return __it != _M_this()->_M_base().cbefore_begin()
            && __it != _M_this()->_M_base().cend(); });
      }
 
      void
      _M_swap(const _Safe_forward_list&) const noexcept;
    };
 
   template<typename _SafeSequence>
    void
    _Safe_forward_list<_SafeSequence>::
    _S_swap_aux(const _Safe_forward_list& __lhs,
                _Safe_iterator_base*& __lhs_iterators,
                const _Safe_forward_list& __rhs,
                _Safe_iterator_base*& __rhs_iterators)
    {
      using const_iterator = typename _SafeSequence::const_iterator;
      _Safe_iterator_base* __bbegin_its = 0;
      _Safe_iterator_base* __last_bbegin = 0;
 
      for (_Safe_iterator_base* __iter = __lhs_iterators; __iter;)
        {
          // Even iterator is cast to const_iterator, not a problem.
          _Safe_iterator_base* __victim_base = __iter;
          const_iterator* __victim =
            static_cast<const_iterator*>(__victim_base);
          __iter = __iter->_M_next;
          if (__victim->base() == __rhs._M_this()->_M_base().cbefore_begin())
            {
              __victim->_M_unlink();
              if (__lhs_iterators == __victim_base)
                __lhs_iterators = __victim_base->_M_next;
              if (__bbegin_its)
                {
                  __victim_base->_M_next = __bbegin_its;
                  __bbegin_its->_M_prior = __victim_base;
                }
              else
                __last_bbegin = __victim_base;
              __bbegin_its = __victim_base;
            }
          else
            __victim_base->_M_sequence = std::__addressof(__lhs);
        }
 
      if (__bbegin_its)
        {
          if (__rhs_iterators)
            {
              __rhs_iterators->_M_prior = __last_bbegin;
              __last_bbegin->_M_next = __rhs_iterators;
            }
          __rhs_iterators = __bbegin_its;
        }
    }
 
   template<typename _SafeSequence>
    void
    _Safe_forward_list<_SafeSequence>::
    _M_swap_single(const _Safe_forward_list& __other) const noexcept
    {
      std::swap(_M_this()->_M_iterators, __other._M_iterators);
      std::swap(_M_this()->_M_const_iterators, __other._M_const_iterators);
      // Useless, always 1 on forward_list
      //std::swap(_M_this()->_M_version, __other._M_version);
      _Safe_iterator_base* __this_its = _M_this()->_M_iterators;
      _S_swap_aux(__other, __other._M_iterators,
                  *_M_this(), _M_this()->_M_iterators);
      _Safe_iterator_base* __this_const_its = _M_this()->_M_const_iterators;
      _S_swap_aux(__other, __other._M_const_iterators,
                  *_M_this(), _M_this()->_M_const_iterators);
      _S_swap_aux(*_M_this(), __this_its,
                  __other, __other._M_iterators);
      _S_swap_aux(*_M_this(), __this_const_its,
                  __other, __other._M_const_iterators);
    }
 
  /* Special forward_list _M_swap version that does not swap the
   * before-begin ownership.*/
   template<typename _SafeSequence>
    void
    _Safe_forward_list<_SafeSequence>::
    _M_swap(const _Safe_forward_list& __other) const noexcept
    {
      // We need to lock both sequences to swap
      using namespace __gnu_cxx;
      __mutex *__this_mutex = &_M_this()->_M_get_mutex();
      __mutex *__other_mutex = &__other._M_get_mutex();
      if (__this_mutex == __other_mutex)
        {
          __scoped_lock __lock(*__this_mutex);
          _M_swap_single(__other);
        }
      else
        {
          __scoped_lock __l1(__this_mutex < __other_mutex
                             ? *__this_mutex : *__other_mutex);
          __scoped_lock __l2(__this_mutex < __other_mutex
                             ? *__other_mutex : *__this_mutex);
          _M_swap_single(__other);
        }
    }
}
 
namespace std _GLIBCXX_VISIBILITY(default)
{
namespace __debug
{
  /// Class std::forward_list with safety/checking/debug instrumentation.
  template<typename _Tp, typename _Alloc = std::allocator<_Tp> >
    class forward_list
    : public __gnu_debug::_Safe_container<
        forward_list<_Tp, _Alloc>, _Alloc, __gnu_debug::_Safe_forward_list>,
      public _GLIBCXX_STD_C::forward_list<_Tp, _Alloc>
    {
      typedef _GLIBCXX_STD_C::forward_list<_Tp, _Alloc>         _Base;
      typedef __gnu_debug::_Safe_container<
        forward_list, _Alloc, __gnu_debug::_Safe_forward_list>  _Safe;
 
      typedef typename _Base::iterator          _Base_iterator;
      typedef typename _Base::const_iterator    _Base_const_iterator;
 
      template<typename _ItT, typename _SeqT, typename _CatT>
        friend class ::__gnu_debug::_Safe_iterator;
 
      // Reference wrapper for base class. See PR libstdc++/90102.
      struct _Base_ref
      {
        _Base_ref(const _Base& __r) : _M_ref(__r) { }
 
        const _Base& _M_ref;
      };
 
    public:
      typedef typename _Base::reference         reference;
      typedef typename _Base::const_reference   const_reference;
 
      typedef __gnu_debug::_Safe_iterator<
        _Base_iterator, forward_list>           iterator;
      typedef __gnu_debug::_Safe_iterator<
        _Base_const_iterator, forward_list>     const_iterator;
 
      typedef typename _Base::size_type         size_type;
      typedef typename _Base::difference_type   difference_type;
 
      typedef _Tp                               value_type;
      typedef typename _Base::allocator_type    allocator_type;
      typedef typename _Base::pointer           pointer;
      typedef typename _Base::const_pointer     const_pointer;
 
      // 23.2.3.1 construct/copy/destroy:
 
      forward_list() = default;
 
      explicit
      forward_list(const allocator_type& __al) noexcept
      : _Base(__al) { }
 
      forward_list(const forward_list& __list, const allocator_type& __al)
      : _Base(__list, __al)
      { }
 
      forward_list(forward_list&& __list, const allocator_type& __al)
        noexcept(
          std::is_nothrow_constructible<_Base,
            _Base, const allocator_type&>::value )
      : _Safe(std::move(__list), __al),
        _Base(std::move(__list), __al)
      { }
 
      explicit
      forward_list(size_type __n, const allocator_type& __al = allocator_type())
      : _Base(__n, __al)
      { }
 
      forward_list(size_type __n, const __type_identity_t<_Tp>& __value,
                   const allocator_type& __al = allocator_type())
      : _Base(__n, __value, __al)
      { }
 
      template<typename _InputIterator,
               typename = std::_RequireInputIter<_InputIterator>>
        forward_list(_InputIterator __first, _InputIterator __last,
                     const allocator_type& __al = allocator_type())
        : _Base(__gnu_debug::__base(
                  __glibcxx_check_valid_constructor_range(__first, __last)),
                __gnu_debug::__base(__last), __al)
        { }
 
#if __glibcxx_containers_ranges // C++ >= 23
      template<__detail::__container_compatible_range<_Tp> _Rg>
        forward_list(from_range_t, _Rg&& __rg, const _Alloc& __a = _Alloc())
        : _Base(std::from_range, std::forward<_Rg>(__rg), __a)
        { }
#endif
 
      forward_list(const forward_list&) = default;
 
      forward_list(forward_list&&) = default;
 
      forward_list(std::initializer_list<_Tp> __il,
                   const allocator_type& __al = allocator_type())
      : _Base(__il, __al)
      { }
 
      ~forward_list() = default;
 
      forward_list(_Base_ref __x) : _Base(__x._M_ref) { }
 
      forward_list&
      operator=(const forward_list&) = default;
 
      forward_list&
      operator=(forward_list&&) = default;
 
      forward_list&
      operator=(std::initializer_list<_Tp> __il)
      {
        _Base::operator=(__il);
        this->_M_invalidate_all();
        return *this;
      }
 
      template<typename _InputIterator,
               typename = std::_RequireInputIter<_InputIterator>>
        void
        assign(_InputIterator __first, _InputIterator __last)
        {
          typename __gnu_debug::_Distance_traits<_InputIterator>::__type __dist;
          __glibcxx_check_valid_range2(__first, __last, __dist);
 
          if (__dist.second >= __gnu_debug::__dp_sign)
            _Base::assign(__gnu_debug::__unsafe(__first),
                          __gnu_debug::__unsafe(__last));
          else
            _Base::assign(__first, __last);
 
          this->_M_invalidate_all();
        }
 
#if __glibcxx_containers_ranges // C++ >= 23
      template<__detail::__container_compatible_range<_Tp> _Rg>
        void
        assign_range(_Rg&& __rg)
        {
          // Have to reimplement this function, so that we use the debug
          // version of erase_after, which invalidates the correct iterators.
 
          static_assert(assignable_from<_Tp&, ranges::range_reference_t<_Rg>>);
 
          auto __first = ranges::begin(__rg);
          const auto __last = ranges::end(__rg);
          auto __prev = _Base::before_begin();
          auto __curr = _Base::begin();
          const auto __end = _Base::end();
 
          while (__curr != __end && __first != __last)
            {
              *__curr = *__first;
              __prev = __curr;
              ++__first;
              ++__curr;
            }
 
          if (__curr != __end)
            erase_after(const_iterator(__prev, this), end());
          else
            _Base::insert_range_after(__prev,
                                      ranges::subrange(std::move(__first),
                                                       __last));
        }
#endif
 
      void
      assign(size_type __n, const _Tp& __val)
      {
        _Base::assign(__n, __val);
        this->_M_invalidate_all();
      }
 
      void
      assign(std::initializer_list<_Tp> __il)
      {
        _Base::assign(__il);
        this->_M_invalidate_all();
      }
 
      using _Base::get_allocator;
 
      // iterators:
 
      [[__nodiscard__]]
      iterator
      before_begin() noexcept
      { return { _Base::before_begin(), this }; }
 
      [[__nodiscard__]]
      const_iterator
      before_begin() const noexcept
      { return { _Base::before_begin(), this }; }
 
      [[__nodiscard__]]
      iterator
      begin() noexcept
      { return { _Base::begin(), this }; }
 
      [[__nodiscard__]]
      const_iterator
      begin() const noexcept
      { return { _Base::begin(), this }; }
 
      [[__nodiscard__]]
      iterator
      end() noexcept
      { return { _Base::end(), this }; }
 
      [[__nodiscard__]]
      const_iterator
      end() const noexcept
      { return { _Base::end(), this }; }
 
      [[__nodiscard__]]
      const_iterator
      cbegin() const noexcept
      { return { _Base::cbegin(), this }; }
 
      [[__nodiscard__]]
      const_iterator
      cbefore_begin() const noexcept
      { return { _Base::cbefore_begin(), this }; }
 
      [[__nodiscard__]]
      const_iterator
      cend() const noexcept
      { return { _Base::cend(), this }; }
 
      using _Base::empty;
      using _Base::max_size;
 
      // element access:
 
      [[__nodiscard__]]
      reference
      front()
      {
        __glibcxx_check_nonempty();
        return _Base::front();
      }
 
      [[__nodiscard__]]
      const_reference
      front() const
      {
        __glibcxx_check_nonempty();
        return _Base::front();
      }
 
      // modifiers:
 
      using _Base::emplace_front;
      using _Base::push_front;
 
#if __glibcxx_containers_ranges // C++ >= 23
      using _Base::prepend_range;
#endif
 
      void
      pop_front()
      {
        __glibcxx_check_nonempty();
        this->_M_invalidate_if([this](_Base_const_iterator __it)
          { return __it == this->_M_base().cbegin(); });
        _Base::pop_front();
      }
 
      template<typename... _Args>
        iterator
        emplace_after(const_iterator __pos, _Args&&... __args)
        {
          __glibcxx_check_insert_after(__pos);
          return { _Base::emplace_after(__pos.base(),
                                        std::forward<_Args>(__args)...),
                   this };
        }
 
      iterator
      insert_after(const_iterator __pos, const _Tp& __val)
      {
        __glibcxx_check_insert_after(__pos);
        return { _Base::insert_after(__pos.base(), __val), this };
      }
 
      iterator
      insert_after(const_iterator __pos, _Tp&& __val)
      {
        __glibcxx_check_insert_after(__pos);
        return { _Base::insert_after(__pos.base(), std::move(__val)), this };
      }
 
      iterator
      insert_after(const_iterator __pos, size_type __n, const _Tp& __val)
      {
        __glibcxx_check_insert_after(__pos);
        return { _Base::insert_after(__pos.base(), __n, __val), this };
      }
 
      template<typename _InputIterator,
               typename = std::_RequireInputIter<_InputIterator>>
        iterator
        insert_after(const_iterator __pos,
                     _InputIterator __first, _InputIterator __last)
        {
          typename __gnu_debug::_Distance_traits<_InputIterator>::__type __dist;
          __glibcxx_check_insert_range_after(__pos, __first, __last, __dist);
 
          if (__dist.second >= __gnu_debug::__dp_sign)
            return
              {
                _Base::insert_after(__pos.base(),
                                    __gnu_debug::__unsafe(__first),
                                    __gnu_debug::__unsafe(__last)),
                this
              };
          else
            return { _Base::insert_after(__pos.base(), __first, __last), this };
        }
 
      iterator
      insert_after(const_iterator __pos, std::initializer_list<_Tp> __il)
      {
        __glibcxx_check_insert_after(__pos);
        return { _Base::insert_after(__pos.base(), __il), this };
      }
 
#if __glibcxx_containers_ranges // C++ >= 23
      template<__detail::__container_compatible_range<_Tp> _Rg>
        iterator
        insert_range_after(const_iterator __position, _Rg&& __rg)
        {
          auto __ret
            = _Base::insert_range_after(__position.base(),
                                        std::forward<_Rg>(__rg));
          return { __ret, this };
        }
#endif
 
      iterator
      erase_after(const_iterator __pos)
      {
        __glibcxx_check_erase_after(__pos);
 
        _Base_const_iterator __next = std::next(__pos.base());
        this->_M_invalidate_if([__next](_Base_const_iterator __it)
                               { return __it == __next; });
        return { _Base::erase_after(__pos.base()), this };
      }
 
      iterator
      erase_after(const_iterator __pos, const_iterator __last)
      {
        __glibcxx_check_erase_range_after(__pos, __last);
        for (_Base_const_iterator __victim = std::next(__pos.base());
            __victim != __last.base(); ++__victim)
          {
            _GLIBCXX_DEBUG_VERIFY(__victim != _Base::cend(),
                                  _M_message(__gnu_debug::__msg_valid_range2)
                                  ._M_sequence(*this, "this")
                                  ._M_iterator(__pos, "pos")
                                  ._M_iterator(__last, "last"));
            this->_M_invalidate_if([__victim](_Base_const_iterator __it)
              { return __it == __victim; });
          }
 
        return { _Base::erase_after(__pos.base(), __last.base()), this };
      }
 
      void
      swap(forward_list& __list)
        noexcept( noexcept(declval<_Base&>().swap(__list)) )
      {
        _Safe::_M_swap(__list);
        _Base::swap(__list);
      }
 
      void
      resize(size_type __sz)
      {
        const forward_list* __this = this;
        __this->_M_detach_singular();
 
        // if __sz < size(), invalidate all iterators in [begin+__sz, end()
        _Base_iterator __victim = _Base::begin();
        _Base_iterator __end = _Base::end();
        for (size_type __i = __sz; __victim != __end && __i > 0; --__i)
          ++__victim;
 
        for (; __victim != __end; ++__victim)
          {
            this->_M_invalidate_if([__victim](_Base_const_iterator __it)
              { return __it == __victim; });
          }
 
        __try
          {
            _Base::resize(__sz);
          }
        __catch(...)
          {
            __this->_M_revalidate_singular();
            __throw_exception_again;
          }
      }
 
      void
      resize(size_type __sz, const value_type& __val)
      {
        const forward_list* __this = this;
        __this->_M_detach_singular();
 
        // if __sz < size(), invalidate all iterators in [begin+__sz, end())
        _Base_iterator __victim = _Base::begin();
        _Base_iterator __end = _Base::end();
        for (size_type __i = __sz; __victim != __end && __i > 0; --__i)
          ++__victim;
 
        for (; __victim != __end; ++__victim)
          {
            this->_M_invalidate_if([__victim](_Base_const_iterator __it)
              { return __it == __victim; });
          }
 
        __try
          {
            _Base::resize(__sz, __val);
          }
        __catch(...)
          {
            __this->_M_revalidate_singular();
            __throw_exception_again;
          }
      }
 
      void
      clear() noexcept
      {
        _Base::clear();
        this->_M_invalidate_all();
      }
 
      // 23.2.3.5 forward_list operations:
      void
      splice_after(const_iterator __pos, forward_list&& __list)
      {
        __glibcxx_check_insert_after(__pos);
        _GLIBCXX_DEBUG_VERIFY(std::__addressof(__list) != this,
                              _M_message(__gnu_debug::__msg_self_splice)
                              ._M_sequence(*this, "this"));
        _GLIBCXX_DEBUG_VERIFY(__list.get_allocator() == this->get_allocator(),
                              _M_message(__gnu_debug::__msg_splice_alloc)
                              ._M_sequence(*this)
                              ._M_sequence(__list, "__list"));
        this->_M_transfer_from_if(__list, [&__list](_Base_const_iterator __it)
          {
            return __it != __list._M_base().cbefore_begin()
                   && __it != __list._M_base().end();
          });
        _Base::splice_after(__pos.base(), std::move(__list));
      }
 
      void
      splice_after(const_iterator __pos, forward_list& __list)
      { splice_after(__pos, std::move(__list)); }
 
      void
      splice_after(const_iterator __pos, forward_list&& __list,
                   const_iterator __i)
      {
        __glibcxx_check_insert_after(__pos);
        _GLIBCXX_DEBUG_VERIFY(__i._M_before_dereferenceable(),
                              _M_message(__gnu_debug::__msg_splice_bad)
                              ._M_iterator(__i, "__i"));
        _GLIBCXX_DEBUG_VERIFY(__i._M_attached_to(std::__addressof(__list)),
                              _M_message(__gnu_debug::__msg_splice_other)
                              ._M_iterator(__i, "__i")
                              ._M_sequence(__list, "__list"));
        _GLIBCXX_DEBUG_VERIFY(__list.get_allocator() == this->get_allocator(),
                              _M_message(__gnu_debug::__msg_splice_alloc)
                              ._M_sequence(*this)
                              ._M_sequence(__list, "__list"));
 
        // _GLIBCXX_RESOLVE_LIB_DEFECTS
        // 250. splicing invalidates iterators
        _Base_const_iterator __next = std::next(__i.base());
        this->_M_transfer_from_if(__list, [__next](_Base_const_iterator __it)
          { return __it == __next; });
        _Base::splice_after(__pos.base(), std::move(__list), __i.base());
      }
 
      void
      splice_after(const_iterator __pos, forward_list& __list,
                   const_iterator __i)
      { splice_after(__pos, std::move(__list), __i); }
 
      void
      splice_after(const_iterator __pos, forward_list&& __list,
                   const_iterator __before, const_iterator __last)
      {
        typename __gnu_debug::_Distance_traits<const_iterator>::__type __dist;
        auto __listptr = std::__addressof(__list);
        __glibcxx_check_insert_after(__pos);
        __glibcxx_check_valid_fl_range(__before, __last, __dist);
        _GLIBCXX_DEBUG_VERIFY(__before._M_attached_to(__listptr),
                              _M_message(__gnu_debug::__msg_splice_other)
                              ._M_sequence(__list, "list")
                              ._M_iterator(__before, "before"));
        _GLIBCXX_DEBUG_VERIFY(__before._M_dereferenceable()
                              || __before._M_is_before_begin(),
                              _M_message(__gnu_debug::__msg_valid_range2)
                              ._M_sequence(__list, "list")
                              ._M_iterator(__before, "before")
                              ._M_iterator(__last, "last"));
        _GLIBCXX_DEBUG_VERIFY(__before != __last,
                              _M_message(__gnu_debug::__msg_valid_range2)
                              ._M_sequence(__list, "list")
                              ._M_iterator(__before, "before")
                              ._M_iterator(__last, "last"));
        _GLIBCXX_DEBUG_VERIFY(__list.get_allocator() == this->get_allocator(),
                              _M_message(__gnu_debug::__msg_splice_alloc)
                              ._M_sequence(*this)
                              ._M_sequence(__list, "__list"));
 
        for (_Base_const_iterator __tmp = std::next(__before.base());
             __tmp != __last.base(); ++__tmp)
          {
            _GLIBCXX_DEBUG_VERIFY(__tmp != __list._M_base().end(),
                                  _M_message(__gnu_debug::__msg_valid_range2)
                                  ._M_sequence(__list, "list")
                                  ._M_iterator(__before, "before")
                                  ._M_iterator(__last, "last"));
            _GLIBCXX_DEBUG_VERIFY(__listptr != this || __tmp != __pos.base(),
                                  _M_message(__gnu_debug::__msg_splice_overlap)
                                  ._M_iterator(__tmp, "position")
                                  ._M_iterator(__before, "before")
                                  ._M_iterator(__last, "last"));
            // _GLIBCXX_RESOLVE_LIB_DEFECTS
            // 250. splicing invalidates iterators
            this->_M_transfer_from_if(__list, [__tmp](_Base_const_iterator __it)
              { return __it == __tmp; });
          }
 
        _Base::splice_after(__pos.base(), std::move(__list),
                            __before.base(), __last.base());
      }
 
      void
      splice_after(const_iterator __pos, forward_list& __list,
                   const_iterator __before, const_iterator __last)
      { splice_after(__pos, std::move(__list), __before, __last); }
 
    private:
#if __cplusplus > 201703L
      using __remove_return_type = size_type;
# define _GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG \
      __attribute__((__abi_tag__("__cxx20")))
# define _GLIBCXX20_ONLY(__expr) __expr
#else
      using __remove_return_type = void;
# define _GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG
# define _GLIBCXX20_ONLY(__expr)
#endif
 
    public:
      _GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG
      __remove_return_type
      remove(const _Tp& __val)
      {
        if (!this->_M_iterators && !this->_M_const_iterators)
          return _Base::remove(__val);
 
        size_type __removed __attribute__((__unused__)) = 0;
        _Base __to_destroy(get_allocator());
        _Base_const_iterator __x = _Base::cbefore_begin();
        _Base_const_iterator __old = __x++;
        while (__x != _Base::cend())
          {
            if (*__x == __val)
              {
                _Base_const_iterator __next = std::next(__old);
                this->_M_invalidate_if([__next](_Base_const_iterator __it)
                                       { return __it == __next; });
                __to_destroy.splice_after(__to_destroy.cbefore_begin(),
                                          *this, __old);
                __x = __old;
                _GLIBCXX20_ONLY( __removed++ );
              }
 
            __old = __x++;
          }
 
        return _GLIBCXX20_ONLY( __removed );
      }
 
      template<typename _Pred>
        __remove_return_type
        remove_if(_Pred __pred)
        {
          if (!this->_M_iterators && !this->_M_const_iterators)
            return _Base::remove_if(__pred);
 
          size_type __removed __attribute__((__unused__)) = 0;
          _Base __to_destroy(get_allocator());
          _Base_iterator __x = _Base::before_begin();
          _Base_iterator __old = __x++;
          while (__x != _Base::end())
            {
              if (__pred(*__x))
                {
                  this->_M_invalidate_if([__x](_Base_const_iterator __it)
                                         { return __it == __x; });
                  __to_destroy.splice_after(__to_destroy.cbefore_begin(),
                                            *this, __old);
                  __x = __old;
                  _GLIBCXX20_ONLY( __removed++ );
                }
 
              __old = __x++;
            }
 
          return _GLIBCXX20_ONLY( __removed );
        }
 
      _GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG
      __remove_return_type
      unique()
      { return unique(std::equal_to<_Tp>()); }
 
      template<typename _BinPred>
        __remove_return_type
        unique(_BinPred __binary_pred)
        {
          if (!this->_M_iterators && !this->_M_const_iterators)
            return _Base::unique(__binary_pred);
 
          _Base_const_iterator __first = _Base::cbegin();
          _Base_const_iterator __last = _Base::cend();
          if (__first == __last)
            return _GLIBCXX20_ONLY(0);
 
          size_type __removed __attribute__((__unused__)) = 0;
          _Base __to_destroy(get_allocator());
          _Base_const_iterator __next = std::next(__first);
          while (__next != __last)
            {
              if (__binary_pred(*__first, *__next))
                {
                  this->_M_invalidate_if([__next](_Base_const_iterator __it)
                                         { return __it == __next; });
                  __to_destroy.splice_after(__to_destroy.cbefore_begin(),
                                            *this, __first);
                  __next = __first;
                  _GLIBCXX20_ONLY( __removed++ );
                }
 
              __first = __next++;
            }
 
          return _GLIBCXX20_ONLY( __removed );
        }
 
#undef _GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG
#undef _GLIBCXX20_ONLY
 
      void
      merge(forward_list&& __list)
      {
        if (this != std::__addressof(__list))
        {
          __glibcxx_check_sorted(_Base::begin(), _Base::end());
          __glibcxx_check_sorted(__list._M_base().begin(),
                                 __list._M_base().end());
          this->_M_transfer_from_if(__list, [&__list](_Base_const_iterator __it)
            {
              return __it != __list._M_base().cbefore_begin()
                     && __it != __list._M_base().cend();
            });
          _Base::merge(std::move(__list));
        }
      }
 
      void
      merge(forward_list& __list)
      { merge(std::move(__list)); }
 
      template<typename _Comp>
        void
        merge(forward_list&& __list, _Comp __comp)
        {
          if (this != std::__addressof(__list))
          {
            __glibcxx_check_sorted_pred(_Base::begin(), _Base::end(), __comp);
            __glibcxx_check_sorted_pred(__list._M_base().begin(),
                                        __list._M_base().end(), __comp);
            this->_M_transfer_from_if(__list,
                                      [&__list](_Base_const_iterator __it)
              {
                return __it != __list._M_base().cbefore_begin()
                       && __it != __list._M_base().cend();
              });
            _Base::merge(std::move(__list), __comp);
          }
        }
 
      template<typename _Comp>
        void
        merge(forward_list& __list, _Comp __comp)
        { merge(std::move(__list), __comp); }
 
      using _Base::sort;
      using _Base::reverse;
 
      _Base&
      _M_base() noexcept { return *this; }
 
      const _Base&
      _M_base() const noexcept { return *this; }
    };
 
#if __cpp_deduction_guides >= 201606
  template<typename _InputIterator, typename _ValT
             = typename iterator_traits<_InputIterator>::value_type,
           typename _Allocator = allocator<_ValT>,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireAllocator<_Allocator>>
    forward_list(_InputIterator, _InputIterator, _Allocator = _Allocator())
      -> forward_list<_ValT, _Allocator>;
 
  template<typename _Tp, typename _Allocator = allocator<_Tp>,
           typename = _RequireAllocator<_Allocator>>
    forward_list(size_t, _Tp, _Allocator = _Allocator())
      -> forward_list<_Tp, _Allocator>;
 
#if __glibcxx_containers_ranges // C++ >= 23
  template<ranges::input_range _Rg,
           typename _Allocator = allocator<ranges::range_value_t<_Rg>>>
    forward_list(from_range_t, _Rg&&, _Allocator = _Allocator())
      -> forward_list<ranges::range_value_t<_Rg>, _Allocator>;
#endif
#endif
 
  template<typename _Tp, typename _Alloc>
    bool
    operator==(const forward_list<_Tp, _Alloc>& __lx,
               const forward_list<_Tp, _Alloc>& __ly)
    { return __lx._M_base() == __ly._M_base(); }
 
#if __cpp_lib_three_way_comparison
  template<typename _Tp, typename _Alloc>
    constexpr __detail::__synth3way_t<_Tp>
    operator<=>(const forward_list<_Tp, _Alloc>& __x,
                const forward_list<_Tp, _Alloc>& __y)
    { return __x._M_base() <=> __y._M_base(); }
#else
  template<typename _Tp, typename _Alloc>
    inline bool
    operator<(const forward_list<_Tp, _Alloc>& __lx,
              const forward_list<_Tp, _Alloc>& __ly)
    { return __lx._M_base() < __ly._M_base(); }
 
  template<typename _Tp, typename _Alloc>
    inline bool
    operator!=(const forward_list<_Tp, _Alloc>& __lx,
               const forward_list<_Tp, _Alloc>& __ly)
    { return !(__lx == __ly); }
 
  /// Based on operator<
  template<typename _Tp, typename _Alloc>
    inline bool
    operator>(const forward_list<_Tp, _Alloc>& __lx,
              const forward_list<_Tp, _Alloc>& __ly)
    { return (__ly < __lx); }
 
  /// Based on operator<
  template<typename _Tp, typename _Alloc>
    inline bool
    operator>=(const forward_list<_Tp, _Alloc>& __lx,
               const forward_list<_Tp, _Alloc>& __ly)
    { return !(__lx < __ly); }
 
  /// Based on operator<
  template<typename _Tp, typename _Alloc>
    inline bool
    operator<=(const forward_list<_Tp, _Alloc>& __lx,
               const forward_list<_Tp, _Alloc>& __ly)
    { return !(__ly < __lx); }
#endif // three-way comparison
 
  /// See std::forward_list::swap().
  template<typename _Tp, typename _Alloc>
    inline void
    swap(forward_list<_Tp, _Alloc>& __lx, forward_list<_Tp, _Alloc>& __ly)
    noexcept(noexcept(__lx.swap(__ly)))
    { __lx.swap(__ly); }
 
} // namespace __debug
 
#ifdef __glibcxx_erase_if // C++ >= 20 && HOSTED
_GLIBCXX_BEGIN_NAMESPACE_VERSION
  template<typename _Tp, typename _Alloc, typename _Predicate>
    inline typename __debug::forward_list<_Tp, _Alloc>::size_type
    erase_if(__debug::forward_list<_Tp, _Alloc>& __cont, _Predicate __pred)
    { return __cont.remove_if(__pred); }
 
  template<typename _Tp, typename _Alloc,
           typename _Up _GLIBCXX26_DEF_VAL_T(_Tp)>
    inline typename __debug::forward_list<_Tp, _Alloc>::size_type
    erase(__debug::forward_list<_Tp, _Alloc>& __cont, const _Up& __value)
    {
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 4135. helper lambda of std::erase for list should specify return type
      return std::erase_if(__cont, [&](const auto& __elem) -> bool {
          return __elem == __value;
      });
    }
_GLIBCXX_END_NAMESPACE_VERSION
#endif // __glibcxx_erase_if
} // namespace std
 
namespace __gnu_debug
{
  template<typename _Tp, typename _Alloc>
    struct _BeforeBeginHelper<std::__debug::forward_list<_Tp, _Alloc> >
    {
      typedef std::__debug::forward_list<_Tp, _Alloc> _Sequence;
 
      template<typename _Iterator>
        static bool
        _S_Is(const _Safe_iterator<_Iterator, _Sequence>& __it)
        {
          return
            __it.base() == __it._M_get_sequence()->_M_base().before_begin();
        }
 
      template<typename _Iterator>
        static bool
        _S_Is_Beginnest(const _Safe_iterator<_Iterator, _Sequence>& __it)
        { return _S_Is(__it); }
    };
 
  template<typename _Tp, typename _Alloc>
    struct _Sequence_traits<std::__debug::forward_list<_Tp, _Alloc> >
    {
      typedef typename std::__debug::forward_list<_Tp, _Alloc>::iterator _It;
 
      static typename _Distance_traits<_It>::__type
      _S_size(const std::__debug::forward_list<_Tp, _Alloc>& __seq)
      {
        return __seq.empty()
          ? std::make_pair(0, __dp_exact) : std::make_pair(1, __dp_sign);
      }
    };
 
#ifndef _GLIBCXX_DEBUG_PEDANTIC
  template<class _Tp, class _Alloc>
    struct _Insert_range_from_self_is_safe<
      std::__debug::forward_list<_Tp, _Alloc> >
    { enum { __value = 1 }; };
#endif
}
 
#endif