// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// 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_FORWARD_LIST
#define _LIBCPP_FORWARD_LIST

/*
    forward_list synopsis

namespace std
{

template <class T, class Allocator = allocator<T>>
class forward_list
{
public:
    typedef T         value_type;
    typedef Allocator allocator_type;

    typedef value_type&                                                reference;
    typedef const value_type&                                          const_reference;
    typedef typename allocator_traits<allocator_type>::pointer         pointer;
    typedef typename allocator_traits<allocator_type>::const_pointer   const_pointer;
    typedef typename allocator_traits<allocator_type>::size_type       size_type;
    typedef typename allocator_traits<allocator_type>::difference_type difference_type;

    typedef <details> iterator;
    typedef <details> const_iterator;

    forward_list()
        noexcept(is_nothrow_default_constructible<allocator_type>::value);
    explicit forward_list(const allocator_type& a);
    explicit forward_list(size_type n);
    explicit forward_list(size_type n, const allocator_type& a); // C++14
    forward_list(size_type n, const value_type& v);
    forward_list(size_type n, const value_type& v, const allocator_type& a);
    template <class InputIterator>
        forward_list(InputIterator first, InputIterator last);
    template <class InputIterator>
        forward_list(InputIterator first, InputIterator last, const allocator_type& a);
    template<container-compatible-range<T> R>
        forward_list(from_range_t, R&& rg, const Allocator& = Allocator()); // C++23
    forward_list(const forward_list& x);
    forward_list(const forward_list& x, const allocator_type& a);
    forward_list(forward_list&& x)
        noexcept(is_nothrow_move_constructible<allocator_type>::value);
    forward_list(forward_list&& x, const allocator_type& a);
    forward_list(initializer_list<value_type> il);
    forward_list(initializer_list<value_type> il, const allocator_type& a);

    ~forward_list();

    forward_list& operator=(const forward_list& x);
    forward_list& operator=(forward_list&& x)
        noexcept((__node_traits::propagate_on_container_move_assignment::value &&
                  is_nothrow_move_assignable<allocator_type>::value) ||
                 allocator_traits<allocator_type>::is_always_equal::value);
    forward_list& operator=(initializer_list<value_type> il);

    template <class InputIterator>
        void assign(InputIterator first, InputIterator last);
    template<container-compatible-range<T> R>
      void assign_range(R&& rg); // C++23
    void assign(size_type n, const value_type& v);
    void assign(initializer_list<value_type> il);

    allocator_type get_allocator() const noexcept;

    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;

    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    iterator       before_begin() noexcept;
    const_iterator before_begin() const noexcept;
    const_iterator cbefore_begin() const noexcept;

    bool empty() const noexcept;
    size_type max_size() const noexcept;

    reference       front();
    const_reference front() const;

    template <class... Args> reference emplace_front(Args&&... args);  // reference in C++17
    void push_front(const value_type& v);
    void push_front(value_type&& v);
    template<container-compatible-range<T> R>
      void prepend_range(R&& rg); // C++23

    void pop_front();

    template <class... Args>
        iterator emplace_after(const_iterator p, Args&&... args);
    iterator insert_after(const_iterator p, const value_type& v);
    iterator insert_after(const_iterator p, value_type&& v);
    iterator insert_after(const_iterator p, size_type n, const value_type& v);
    template <class InputIterator>
        iterator insert_after(const_iterator p,
                              InputIterator first, InputIterator last);
    template<container-compatible-range<T> R>
      iterator insert_range_after(const_iterator position, R&& rg); // C++23
    iterator insert_after(const_iterator p, initializer_list<value_type> il);

    iterator erase_after(const_iterator p);
    iterator erase_after(const_iterator first, const_iterator last);

    void swap(forward_list& x)
        noexcept(allocator_traits<allocator_type>::is_always_equal::value);  // C++17

    void resize(size_type n);
    void resize(size_type n, const value_type& v);
    void clear() noexcept;

    void splice_after(const_iterator p, forward_list& x);
    void splice_after(const_iterator p, forward_list&& x);
    void splice_after(const_iterator p, forward_list& x, const_iterator i);
    void splice_after(const_iterator p, forward_list&& x, const_iterator i);
    void splice_after(const_iterator p, forward_list& x,
                      const_iterator first, const_iterator last);
    void splice_after(const_iterator p, forward_list&& x,
                      const_iterator first, const_iterator last);
    size_type remove(const value_type& v);           // void before C++20
    template <class Predicate>
      size_type remove_if(Predicate pred);           // void before C++20
    size_type unique();                              // void before C++20
    template <class BinaryPredicate>
      size_type unique(BinaryPredicate binary_pred); // void before C++20
    void merge(forward_list& x);
    void merge(forward_list&& x);
    template <class Compare> void merge(forward_list& x, Compare comp);
    template <class Compare> void merge(forward_list&& x, Compare comp);
    void sort();
    template <class Compare> void sort(Compare comp);
    void reverse() noexcept;
};


template <class InputIterator, class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
    forward_list(InputIterator, InputIterator, Allocator = Allocator())
    -> forward_list<typename iterator_traits<InputIterator>::value_type, Allocator>;  // C++17

template<ranges::input_range R, class Allocator = allocator<ranges::range_value_t<R>>>
  forward_list(from_range_t, R&&, Allocator = Allocator())
      -> forward_list<ranges::range_value_t<R>, Allocator>; // C++23

template <class T, class Allocator>
    bool operator==(const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y);

template <class T, class Allocator>
    bool operator< (const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y); // removed in C++20

template <class T, class Allocator>
    bool operator!=(const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y); // removed in C++20

template <class T, class Allocator>
    bool operator> (const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y); // removed in C++20

template <class T, class Allocator>
    bool operator>=(const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y); // removed in C++20

template <class T, class Allocator>
    bool operator<=(const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y); // removed in C++20

template<class T, class Allocator>
    synth-three-way-result<T> operator<=>(const forward_list<T, Allocator>& x,
                                          const forward_list<T, Allocator>& y); // since C++20

template <class T, class Allocator>
    void swap(forward_list<T, Allocator>& x, forward_list<T, Allocator>& y)
         noexcept(noexcept(x.swap(y)));

template <class T, class Allocator, class U>
    typename forward_list<T, Allocator>::size_type
    erase(forward_list<T, Allocator>& c, const U& value);       // C++20
template <class T, class Allocator, class Predicate>
    typename forward_list<T, Allocator>::size_type
    erase_if(forward_list<T, Allocator>& c, Predicate pred);    // C++20

}  // std

*/

#if __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
#  include <__cxx03/forward_list>
#else
#  include <__algorithm/comp.h>
#  include <__algorithm/lexicographical_compare.h>
#  include <__algorithm/lexicographical_compare_three_way.h>
#  include <__algorithm/min.h>
#  include <__assert>
#  include <__config>
#  include <__cstddef/nullptr_t.h>
#  include <__iterator/distance.h>
#  include <__iterator/iterator_traits.h>
#  include <__iterator/move_iterator.h>
#  include <__iterator/next.h>
#  include <__memory/addressof.h>
#  include <__memory/allocation_guard.h>
#  include <__memory/allocator.h>
#  include <__memory/allocator_traits.h>
#  include <__memory/compressed_pair.h>
#  include <__memory/construct_at.h>
#  include <__memory/pointer_traits.h>
#  include <__memory/swap_allocator.h>
#  include <__memory_resource/polymorphic_allocator.h>
#  include <__new/launder.h>
#  include <__ranges/access.h>
#  include <__ranges/concepts.h>
#  include <__ranges/container_compatible_range.h>
#  include <__ranges/from_range.h>
#  include <__type_traits/conditional.h>
#  include <__type_traits/container_traits.h>
#  include <__type_traits/enable_if.h>
#  include <__type_traits/is_allocator.h>
#  include <__type_traits/is_const.h>
#  include <__type_traits/is_nothrow_assignable.h>
#  include <__type_traits/is_nothrow_constructible.h>
#  include <__type_traits/is_pointer.h>
#  include <__type_traits/is_same.h>
#  include <__type_traits/is_swappable.h>
#  include <__type_traits/type_identity.h>
#  include <__utility/forward.h>
#  include <__utility/move.h>
#  include <__utility/swap.h>
#  include <limits>
#  include <version>

// standard-mandated includes

// [iterator.range]
#  include <__iterator/access.h>
#  include <__iterator/data.h>
#  include <__iterator/empty.h>
#  include <__iterator/reverse_access.h>
#  include <__iterator/size.h>

// [forward.list.syn]
#  include <compare>
#  include <initializer_list>

#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#    pragma GCC system_header
#  endif

_LIBCPP_PUSH_MACROS
#  include <__undef_macros>

_LIBCPP_BEGIN_NAMESPACE_STD

template <class _Tp, class _VoidPtr>
struct __forward_list_node;
template <class _NodePtr>
struct __forward_begin_node;

template <class>
struct __forward_list_node_value_type;

template <class _Tp, class _VoidPtr>
struct __forward_list_node_value_type<__forward_list_node<_Tp, _VoidPtr> > {
  typedef _Tp type;
};

template <class _NodePtr>
struct __forward_node_traits {
  typedef __remove_cv_t<typename pointer_traits<_NodePtr>::element_type> __node_type;
  typedef typename __forward_list_node_value_type<__node_type>::type __node_value_type;
  typedef _NodePtr __node_pointer;
  typedef __forward_begin_node<_NodePtr> __begin_node;
  typedef __rebind_pointer_t<_NodePtr, __begin_node> __begin_node_pointer;
  typedef __rebind_pointer_t<_NodePtr, void> __void_pointer;

// TODO(LLVM 22): Remove this check
#  ifndef _LIBCPP_ABI_FORWARD_LIST_REMOVE_NODE_POINTER_UB
  static_assert(sizeof(__begin_node_pointer) == sizeof(__node_pointer) && _LIBCPP_ALIGNOF(__begin_node_pointer) ==
                    _LIBCPP_ALIGNOF(__node_pointer),
                "It looks like you are using std::forward_list with a fancy pointer type that thas a different "
                "representation depending on whether it points to a forward_list base pointer or a forward_list node "
                "pointer (both of which are implementation details of the standard library). This means that your ABI "
                "is being broken between LLVM 19 and LLVM 20. If you don't care about your ABI being broken, define "
                "the _LIBCPP_ABI_FORWARD_LIST_REMOVE_NODE_POINTER_UB macro to silence this diagnostic.");
#  endif

  _LIBCPP_HIDE_FROM_ABI static __begin_node_pointer __as_iter_node(__node_pointer __p) {
    return static_cast<__begin_node_pointer>(static_cast<__void_pointer>(__p));
  }
};

template <class _NodePtr>
struct __forward_begin_node {
  typedef _NodePtr pointer;
  typedef __rebind_pointer_t<_NodePtr, __forward_begin_node> __begin_node_pointer;

  pointer __next_;

  _LIBCPP_HIDE_FROM_ABI __forward_begin_node() : __next_(nullptr) {}
  _LIBCPP_HIDE_FROM_ABI explicit __forward_begin_node(pointer __n) : __next_(__n) {}

  _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __next_as_begin() const {
    return static_cast<__begin_node_pointer>(__next_);
  }
};

template <class _Tp, class _VoidPtr>
using __begin_node_of _LIBCPP_NODEBUG =
    __forward_begin_node<__rebind_pointer_t<_VoidPtr, __forward_list_node<_Tp, _VoidPtr> > >;

template <class _Tp, class _VoidPtr>
struct __forward_list_node : public __begin_node_of<_Tp, _VoidPtr> {
  typedef _Tp value_type;
  typedef __begin_node_of<_Tp, _VoidPtr> _Base;
  typedef typename _Base::pointer _NodePtr;

  // We allow starting the lifetime of nodes without initializing the value held by the node,
  // since that is handled by the list itself in order to be allocator-aware.
#  ifndef _LIBCPP_CXX03_LANG

private:
  union {
    _Tp __value_;
  };

public:
  _LIBCPP_HIDE_FROM_ABI _Tp& __get_value() { return __value_; }
#  else

private:
  _ALIGNAS_TYPE(_Tp) char __buffer_[sizeof(_Tp)];

public:
  _LIBCPP_HIDE_FROM_ABI _Tp& __get_value() { return *std::__launder(reinterpret_cast<_Tp*>(&__buffer_)); }
#  endif

  _LIBCPP_HIDE_FROM_ABI explicit __forward_list_node(_NodePtr __next) : _Base(__next) {}
  _LIBCPP_HIDE_FROM_ABI ~__forward_list_node() {}
};

template <class _Tp, class _Alloc = allocator<_Tp> >
class forward_list;
template <class _NodeConstPtr>
class __forward_list_const_iterator;

template <class _NodePtr>
class __forward_list_iterator {
  typedef __forward_node_traits<_NodePtr> __traits;
  typedef typename __traits::__node_pointer __node_pointer;
  typedef typename __traits::__begin_node_pointer __begin_node_pointer;
  typedef typename __traits::__void_pointer __void_pointer;

  __begin_node_pointer __ptr_;

  _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __get_begin() const {
    return static_cast<__begin_node_pointer>(static_cast<__void_pointer>(__ptr_));
  }
  _LIBCPP_HIDE_FROM_ABI __node_pointer __get_unsafe_node_pointer() const {
    return static_cast<__node_pointer>(static_cast<__void_pointer>(__ptr_));
  }

  _LIBCPP_HIDE_FROM_ABI explicit __forward_list_iterator(nullptr_t) _NOEXCEPT : __ptr_(nullptr) {}

  _LIBCPP_HIDE_FROM_ABI explicit __forward_list_iterator(__begin_node_pointer __p) _NOEXCEPT : __ptr_(__p) {}

  _LIBCPP_HIDE_FROM_ABI explicit __forward_list_iterator(__node_pointer __p) _NOEXCEPT
      : __ptr_(__traits::__as_iter_node(__p)) {}

  template <class, class>
  friend class forward_list;
  template <class>
  friend class __forward_list_const_iterator;

public:
  typedef forward_iterator_tag iterator_category;
  typedef typename __traits::__node_value_type value_type;
  typedef value_type& reference;
  typedef typename pointer_traits<__node_pointer>::difference_type difference_type;
  typedef __rebind_pointer_t<__node_pointer, value_type> pointer;

  _LIBCPP_HIDE_FROM_ABI __forward_list_iterator() _NOEXCEPT : __ptr_(nullptr) {}

  _LIBCPP_HIDE_FROM_ABI reference operator*() const { return __get_unsafe_node_pointer()->__get_value(); }
  _LIBCPP_HIDE_FROM_ABI pointer operator->() const {
    return pointer_traits<pointer>::pointer_to(__get_unsafe_node_pointer()->__get_value());
  }

  _LIBCPP_HIDE_FROM_ABI __forward_list_iterator& operator++() {
    __ptr_ = __traits::__as_iter_node(__ptr_->__next_);
    return *this;
  }
  _LIBCPP_HIDE_FROM_ABI __forward_list_iterator operator++(int) {
    __forward_list_iterator __t(*this);
    ++(*this);
    return __t;
  }

  friend _LIBCPP_HIDE_FROM_ABI bool operator==(const __forward_list_iterator& __x, const __forward_list_iterator& __y) {
    return __x.__ptr_ == __y.__ptr_;
  }
  friend _LIBCPP_HIDE_FROM_ABI bool operator!=(const __forward_list_iterator& __x, const __forward_list_iterator& __y) {
    return !(__x == __y);
  }
};

template <class _NodeConstPtr>
class __forward_list_const_iterator {
  static_assert(!is_const<typename pointer_traits<_NodeConstPtr>::element_type>::value, "");
  typedef _NodeConstPtr _NodePtr;

  typedef __forward_node_traits<_NodePtr> __traits;
  typedef typename __traits::__node_type __node_type;
  typedef typename __traits::__node_pointer __node_pointer;
  typedef typename __traits::__begin_node_pointer __begin_node_pointer;
  typedef typename __traits::__void_pointer __void_pointer;

  __begin_node_pointer __ptr_;

  _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __get_begin() const {
    return static_cast<__begin_node_pointer>(static_cast<__void_pointer>(__ptr_));
  }
  _LIBCPP_HIDE_FROM_ABI __node_pointer __get_unsafe_node_pointer() const {
    return static_cast<__node_pointer>(static_cast<__void_pointer>(__ptr_));
  }

  _LIBCPP_HIDE_FROM_ABI explicit __forward_list_const_iterator(nullptr_t) _NOEXCEPT : __ptr_(nullptr) {}

  _LIBCPP_HIDE_FROM_ABI explicit __forward_list_const_iterator(__begin_node_pointer __p) _NOEXCEPT : __ptr_(__p) {}

  _LIBCPP_HIDE_FROM_ABI explicit __forward_list_const_iterator(__node_pointer __p) _NOEXCEPT
      : __ptr_(__traits::__as_iter_node(__p)) {}

  template <class, class>
  friend class forward_list;

public:
  typedef forward_iterator_tag iterator_category;
  typedef typename __traits::__node_value_type value_type;
  typedef const value_type& reference;
  typedef typename pointer_traits<__node_pointer>::difference_type difference_type;
  typedef __rebind_pointer_t<__node_pointer, const value_type> pointer;

  _LIBCPP_HIDE_FROM_ABI __forward_list_const_iterator() _NOEXCEPT : __ptr_(nullptr) {}
  _LIBCPP_HIDE_FROM_ABI __forward_list_const_iterator(__forward_list_iterator<__node_pointer> __p) _NOEXCEPT
      : __ptr_(__p.__ptr_) {}

  _LIBCPP_HIDE_FROM_ABI reference operator*() const { return __get_unsafe_node_pointer()->__get_value(); }
  _LIBCPP_HIDE_FROM_ABI pointer operator->() const {
    return pointer_traits<pointer>::pointer_to(__get_unsafe_node_pointer()->__get_value());
  }

  _LIBCPP_HIDE_FROM_ABI __forward_list_const_iterator& operator++() {
    __ptr_ = __traits::__as_iter_node(__ptr_->__next_);
    return *this;
  }
  _LIBCPP_HIDE_FROM_ABI __forward_list_const_iterator operator++(int) {
    __forward_list_const_iterator __t(*this);
    ++(*this);
    return __t;
  }

  friend _LIBCPP_HIDE_FROM_ABI bool
  operator==(const __forward_list_const_iterator& __x, const __forward_list_const_iterator& __y) {
    return __x.__ptr_ == __y.__ptr_;
  }
  friend _LIBCPP_HIDE_FROM_ABI bool
  operator!=(const __forward_list_const_iterator& __x, const __forward_list_const_iterator& __y) {
    return !(__x == __y);
  }
};

template <class _Tp, class _Alloc>
class __forward_list_base {
protected:
  typedef _Tp value_type;
  typedef _Alloc allocator_type;

  typedef typename allocator_traits<allocator_type>::void_pointer void_pointer;
  typedef __forward_list_node<value_type, void_pointer> __node_type;
  typedef __begin_node_of<value_type, void_pointer> __begin_node;
  typedef __rebind_alloc<allocator_traits<allocator_type>, __node_type> __node_allocator;
  typedef allocator_traits<__node_allocator> __node_traits;
  typedef typename __node_traits::pointer __node_pointer;

  typedef __rebind_alloc<allocator_traits<allocator_type>, __begin_node> __begin_node_allocator;
  typedef typename allocator_traits<__begin_node_allocator>::pointer __begin_node_pointer;

  _LIBCPP_COMPRESSED_PAIR(__begin_node, __before_begin_, __node_allocator, __alloc_);

  _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __before_begin() _NOEXCEPT {
    return pointer_traits<__begin_node_pointer>::pointer_to(__before_begin_);
  }
  _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __before_begin() const _NOEXCEPT {
    return pointer_traits<__begin_node_pointer>::pointer_to(const_cast<__begin_node&>(__before_begin_));
  }

  typedef __forward_list_iterator<__node_pointer> iterator;
  typedef __forward_list_const_iterator<__node_pointer> const_iterator;

  _LIBCPP_HIDE_FROM_ABI __forward_list_base() _NOEXCEPT_(is_nothrow_default_constructible<__node_allocator>::value)
      : __before_begin_(__begin_node()) {}
  _LIBCPP_HIDE_FROM_ABI explicit __forward_list_base(const allocator_type& __a)
      : __before_begin_(__begin_node()), __alloc_(__node_allocator(__a)) {}
  _LIBCPP_HIDE_FROM_ABI explicit __forward_list_base(const __node_allocator& __a)
      : __before_begin_(__begin_node()), __alloc_(__a) {}

public:
#  ifndef _LIBCPP_CXX03_LANG
  _LIBCPP_HIDE_FROM_ABI
  __forward_list_base(__forward_list_base&& __x) noexcept(is_nothrow_move_constructible<__node_allocator>::value);
  _LIBCPP_HIDE_FROM_ABI __forward_list_base(__forward_list_base&& __x, const allocator_type& __a);
#  endif // _LIBCPP_CXX03_LANG

  __forward_list_base(const __forward_list_base&)            = delete;
  __forward_list_base& operator=(const __forward_list_base&) = delete;

  _LIBCPP_HIDE_FROM_ABI ~__forward_list_base();

protected:
  _LIBCPP_HIDE_FROM_ABI void __copy_assign_alloc(const __forward_list_base& __x) {
    __copy_assign_alloc(__x, integral_constant<bool, __node_traits::propagate_on_container_copy_assignment::value>());
  }

  _LIBCPP_HIDE_FROM_ABI void __move_assign_alloc(__forward_list_base& __x)
      _NOEXCEPT_(!__node_traits::propagate_on_container_move_assignment::value ||
                 is_nothrow_move_assignable<__node_allocator>::value) {
    __move_assign_alloc(__x, integral_constant<bool, __node_traits::propagate_on_container_move_assignment::value>());
  }

  template <class... _Args>
  _LIBCPP_HIDE_FROM_ABI __node_pointer __create_node(__node_pointer __next, _Args&&... __args) {
    __allocation_guard<__node_allocator> __guard(__alloc_, 1);
    // Begin the lifetime of the node itself. Note that this doesn't begin the lifetime of the value
    // held inside the node, since we need to use the allocator's construct() method for that.
    //
    // We don't use the allocator's construct() method to construct the node itself since the
    // Cpp17FooInsertable named requirements don't require the allocator's construct() method
    // to work on anything other than the value_type.
    std::__construct_at(std::addressof(*__guard.__get()), __next);

    // Now construct the value_type using the allocator's construct() method.
    __node_traits::construct(__alloc_, std::addressof(__guard.__get()->__get_value()), std::forward<_Args>(__args)...);
    return __guard.__release_ptr();
  }

  _LIBCPP_HIDE_FROM_ABI void __delete_node(__node_pointer __node) {
    // For the same reason as above, we use the allocator's destroy() method for the value_type,
    // but not for the node itself.
    __node_traits::destroy(__alloc_, std::addressof(__node->__get_value()));
    std::__destroy_at(std::addressof(*__node));
    __node_traits::deallocate(__alloc_, __node, 1);
  }

public:
  _LIBCPP_HIDE_FROM_ABI void swap(__forward_list_base& __x)
#  if _LIBCPP_STD_VER >= 14
      _NOEXCEPT;
#  else
      _NOEXCEPT_(!__node_traits::propagate_on_container_swap::value || __is_nothrow_swappable_v<__node_allocator>);
#  endif

protected:
  _LIBCPP_HIDE_FROM_ABI void clear() _NOEXCEPT;

private:
  _LIBCPP_HIDE_FROM_ABI void __copy_assign_alloc(const __forward_list_base&, false_type) {}
  _LIBCPP_HIDE_FROM_ABI void __copy_assign_alloc(const __forward_list_base& __x, true_type) {
    if (__alloc_ != __x.__alloc_)
      clear();
    __alloc_ = __x.__alloc_;
  }

  _LIBCPP_HIDE_FROM_ABI void __move_assign_alloc(__forward_list_base&, false_type) _NOEXCEPT {}
  _LIBCPP_HIDE_FROM_ABI void __move_assign_alloc(__forward_list_base& __x, true_type)
      _NOEXCEPT_(is_nothrow_move_assignable<__node_allocator>::value) {
    __alloc_ = std::move(__x.__alloc_);
  }
};

#  ifndef _LIBCPP_CXX03_LANG

template <class _Tp, class _Alloc>
inline __forward_list_base<_Tp, _Alloc>::__forward_list_base(__forward_list_base&& __x) noexcept(
    is_nothrow_move_constructible<__node_allocator>::value)
    : __before_begin_(std::move(__x.__before_begin_)), __alloc_(std::move(__x.__alloc_)) {
  __x.__before_begin()->__next_ = nullptr;
}

template <class _Tp, class _Alloc>
inline __forward_list_base<_Tp, _Alloc>::__forward_list_base(__forward_list_base&& __x, const allocator_type& __a)
    : __before_begin_(__begin_node()), __alloc_(__node_allocator(__a)) {
  if (__alloc_ == __x.__alloc_) {
    __before_begin()->__next_     = __x.__before_begin()->__next_;
    __x.__before_begin()->__next_ = nullptr;
  }
}

#  endif // _LIBCPP_CXX03_LANG

template <class _Tp, class _Alloc>
__forward_list_base<_Tp, _Alloc>::~__forward_list_base() {
  clear();
}

template <class _Tp, class _Alloc>
inline void __forward_list_base<_Tp, _Alloc>::swap(__forward_list_base& __x)
#  if _LIBCPP_STD_VER >= 14
    _NOEXCEPT
#  else
    _NOEXCEPT_(!__node_traits::propagate_on_container_swap::value || __is_nothrow_swappable_v<__node_allocator>)
#  endif
{
  std::__swap_allocator(__alloc_, __x.__alloc_);
  using std::swap;
  swap(__before_begin()->__next_, __x.__before_begin()->__next_);
}

template <class _Tp, class _Alloc>
void __forward_list_base<_Tp, _Alloc>::clear() _NOEXCEPT {
  for (__node_pointer __p = __before_begin()->__next_; __p != nullptr;) {
    __node_pointer __next = __p->__next_;
    __delete_node(__p);
    __p = __next;
  }
  __before_begin()->__next_ = nullptr;
}

template <class _Tp, class _Alloc /*= allocator<_Tp>*/>
class forward_list : private __forward_list_base<_Tp, _Alloc> {
  typedef __forward_list_base<_Tp, _Alloc> __base;
  typedef typename __base::__node_allocator __node_allocator;
  typedef typename __base::__node_type __node_type;
  typedef typename __base::__node_traits __node_traits;
  typedef typename __base::__node_pointer __node_pointer;
  typedef typename __base::__begin_node_pointer __begin_node_pointer;

public:
  typedef _Tp value_type;
  typedef _Alloc allocator_type;

  static_assert(__check_valid_allocator<allocator_type>::value, "");

  static_assert(is_same<value_type, typename allocator_type::value_type>::value,
                "Allocator::value_type must be same type as value_type");

  static_assert(!is_same<allocator_type, __node_allocator>::value,
                "internal allocator type must differ from user-specified type; otherwise overload resolution breaks");

  typedef value_type& reference;
  typedef const value_type& const_reference;
  typedef typename allocator_traits<allocator_type>::pointer pointer;
  typedef typename allocator_traits<allocator_type>::const_pointer const_pointer;
  typedef typename allocator_traits<allocator_type>::size_type size_type;
  typedef typename allocator_traits<allocator_type>::difference_type difference_type;

  typedef typename __base::iterator iterator;
  typedef typename __base::const_iterator const_iterator;
#  if _LIBCPP_STD_VER >= 20
  typedef size_type __remove_return_type;
#  else
  typedef void __remove_return_type;
#  endif

  _LIBCPP_HIDE_FROM_ABI forward_list() _NOEXCEPT_(is_nothrow_default_constructible<__node_allocator>::value) {
  } // = default;
  _LIBCPP_HIDE_FROM_ABI explicit forward_list(const allocator_type& __a);
  _LIBCPP_HIDE_FROM_ABI explicit forward_list(size_type __n);
#  if _LIBCPP_STD_VER >= 14
  _LIBCPP_HIDE_FROM_ABI explicit forward_list(size_type __n, const allocator_type& __a);
#  endif
  _LIBCPP_HIDE_FROM_ABI forward_list(size_type __n, const value_type& __v);

  template <__enable_if_t<__is_allocator<_Alloc>::value, int> = 0>
  _LIBCPP_HIDE_FROM_ABI forward_list(size_type __n, const value_type& __v, const allocator_type& __a) : __base(__a) {
    insert_after(cbefore_begin(), __n, __v);
  }

  template <class _InputIterator, __enable_if_t<__has_input_iterator_category<_InputIterator>::value, int> = 0>
  _LIBCPP_HIDE_FROM_ABI forward_list(_InputIterator __f, _InputIterator __l);

  template <class _InputIterator, __enable_if_t<__has_input_iterator_category<_InputIterator>::value, int> = 0>
  _LIBCPP_HIDE_FROM_ABI forward_list(_InputIterator __f, _InputIterator __l, const allocator_type& __a);

#  if _LIBCPP_STD_VER >= 23
  template <_ContainerCompatibleRange<_Tp> _Range>
  _LIBCPP_HIDE_FROM_ABI forward_list(from_range_t, _Range&& __range, const allocator_type& __a = allocator_type())
      : __base(__a) {
    prepend_range(std::forward<_Range>(__range));
  }
#  endif

  _LIBCPP_HIDE_FROM_ABI forward_list(const forward_list& __x);
  _LIBCPP_HIDE_FROM_ABI forward_list(const forward_list& __x, const __type_identity_t<allocator_type>& __a);

  _LIBCPP_HIDE_FROM_ABI forward_list& operator=(const forward_list& __x);

#  ifndef _LIBCPP_CXX03_LANG
  _LIBCPP_HIDE_FROM_ABI forward_list(forward_list&& __x) noexcept(is_nothrow_move_constructible<__base>::value)
      : __base(std::move(__x)) {}
  _LIBCPP_HIDE_FROM_ABI forward_list(forward_list&& __x, const __type_identity_t<allocator_type>& __a);

  _LIBCPP_HIDE_FROM_ABI forward_list(initializer_list<value_type> __il);
  _LIBCPP_HIDE_FROM_ABI forward_list(initializer_list<value_type> __il, const allocator_type& __a);

  _LIBCPP_HIDE_FROM_ABI forward_list& operator=(forward_list&& __x) noexcept(
      (__node_traits::propagate_on_container_move_assignment::value &&
       is_nothrow_move_assignable<allocator_type>::value) ||
      allocator_traits<allocator_type>::is_always_equal::value);

  _LIBCPP_HIDE_FROM_ABI forward_list& operator=(initializer_list<value_type> __il);

  _LIBCPP_HIDE_FROM_ABI void assign(initializer_list<value_type> __il);
#  endif // _LIBCPP_CXX03_LANG

  // ~forward_list() = default;

  template <class _InputIterator, __enable_if_t<__has_input_iterator_category<_InputIterator>::value, int> = 0>
  void _LIBCPP_HIDE_FROM_ABI assign(_InputIterator __f, _InputIterator __l);

#  if _LIBCPP_STD_VER >= 23
  template <_ContainerCompatibleRange<_Tp> _Range>
  _LIBCPP_HIDE_FROM_ABI void assign_range(_Range&& __range) {
    __assign_with_sentinel(ranges::begin(__range), ranges::end(__range));
  }
#  endif

  _LIBCPP_HIDE_FROM_ABI void assign(size_type __n, const value_type& __v);

  _LIBCPP_HIDE_FROM_ABI allocator_type get_allocator() const _NOEXCEPT { return allocator_type(this->__alloc_); }

  _LIBCPP_HIDE_FROM_ABI iterator begin() _NOEXCEPT { return iterator(__base::__before_begin()->__next_); }
  _LIBCPP_HIDE_FROM_ABI const_iterator begin() const _NOEXCEPT {
    return const_iterator(__base::__before_begin()->__next_);
  }
  _LIBCPP_HIDE_FROM_ABI iterator end() _NOEXCEPT { return iterator(nullptr); }
  _LIBCPP_HIDE_FROM_ABI const_iterator end() const _NOEXCEPT { return const_iterator(nullptr); }

  _LIBCPP_HIDE_FROM_ABI const_iterator cbegin() const _NOEXCEPT {
    return const_iterator(__base::__before_begin()->__next_);
  }
  _LIBCPP_HIDE_FROM_ABI const_iterator cend() const _NOEXCEPT { return const_iterator(nullptr); }

  _LIBCPP_HIDE_FROM_ABI iterator before_begin() _NOEXCEPT { return iterator(__base::__before_begin()); }
  _LIBCPP_HIDE_FROM_ABI const_iterator before_begin() const _NOEXCEPT {
    return const_iterator(__base::__before_begin());
  }
  _LIBCPP_HIDE_FROM_ABI const_iterator cbefore_begin() const _NOEXCEPT {
    return const_iterator(__base::__before_begin());
  }

  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool empty() const _NOEXCEPT {
    return __base::__before_begin()->__next_ == nullptr;
  }
  _LIBCPP_HIDE_FROM_ABI size_type max_size() const _NOEXCEPT {
    return std::min<size_type>(__node_traits::max_size(this->__alloc_), numeric_limits<difference_type>::max());
  }

  _LIBCPP_HIDE_FROM_ABI reference front() {
    _LIBCPP_ASSERT_NON_NULL(!empty(), "forward_list::front called on an empty list");
    return __base::__before_begin()->__next_->__get_value();
  }
  _LIBCPP_HIDE_FROM_ABI const_reference front() const {
    _LIBCPP_ASSERT_NON_NULL(!empty(), "forward_list::front called on an empty list");
    return __base::__before_begin()->__next_->__get_value();
  }

#  ifndef _LIBCPP_CXX03_LANG
#    if _LIBCPP_STD_VER >= 17
  template <class... _Args>
  _LIBCPP_HIDE_FROM_ABI reference emplace_front(_Args&&... __args);
#    else
  template <class... _Args>
  _LIBCPP_HIDE_FROM_ABI void emplace_front(_Args&&... __args);
#    endif
  _LIBCPP_HIDE_FROM_ABI void push_front(value_type&& __v);
#  endif // _LIBCPP_CXX03_LANG
  _LIBCPP_HIDE_FROM_ABI void push_front(const value_type& __v);

#  if _LIBCPP_STD_VER >= 23
  template <_ContainerCompatibleRange<_Tp> _Range>
  _LIBCPP_HIDE_FROM_ABI void prepend_range(_Range&& __range) {
    insert_range_after(cbefore_begin(), std::forward<_Range>(__range));
  }
#  endif

  _LIBCPP_HIDE_FROM_ABI void pop_front();

#  ifndef _LIBCPP_CXX03_LANG
  template <class... _Args>
  _LIBCPP_HIDE_FROM_ABI iterator emplace_after(const_iterator __p, _Args&&... __args);

  _LIBCPP_HIDE_FROM_ABI iterator insert_after(const_iterator __p, value_type&& __v);
  _LIBCPP_HIDE_FROM_ABI iterator insert_after(const_iterator __p, initializer_list<value_type> __il) {
    return insert_after(__p, __il.begin(), __il.end());
  }
#  endif // _LIBCPP_CXX03_LANG
  _LIBCPP_HIDE_FROM_ABI iterator insert_after(const_iterator __p, const value_type& __v);
  _LIBCPP_HIDE_FROM_ABI iterator insert_after(const_iterator __p, size_type __n, const value_type& __v);
  template <class _InputIterator, __enable_if_t<__has_input_iterator_category<_InputIterator>::value, int> = 0>
  _LIBCPP_HIDE_FROM_ABI iterator insert_after(const_iterator __p, _InputIterator __f, _InputIterator __l);

#  if _LIBCPP_STD_VER >= 23
  template <_ContainerCompatibleRange<_Tp> _Range>
  _LIBCPP_HIDE_FROM_ABI iterator insert_range_after(const_iterator __position, _Range&& __range) {
    return __insert_after_with_sentinel(__position, ranges::begin(__range), ranges::end(__range));
  }
#  endif

  template <class _InputIterator, class _Sentinel>
  _LIBCPP_HIDE_FROM_ABI iterator __insert_after_with_sentinel(const_iterator __p, _InputIterator __f, _Sentinel __l);

  _LIBCPP_HIDE_FROM_ABI iterator erase_after(const_iterator __p);
  _LIBCPP_HIDE_FROM_ABI iterator erase_after(const_iterator __f, const_iterator __l);

  _LIBCPP_HIDE_FROM_ABI void swap(forward_list& __x)
#  if _LIBCPP_STD_VER >= 14
      _NOEXCEPT
#  else
      _NOEXCEPT_(!__node_traits::propagate_on_container_swap::value || __is_nothrow_swappable_v<__node_allocator>)
#  endif
  {
    __base::swap(__x);
  }

  _LIBCPP_HIDE_FROM_ABI void resize(size_type __n);
  _LIBCPP_HIDE_FROM_ABI void resize(size_type __n, const value_type& __v);
  _LIBCPP_HIDE_FROM_ABI void clear() _NOEXCEPT { __base::clear(); }

  _LIBCPP_HIDE_FROM_ABI void splice_after(const_iterator __p, forward_list&& __x);
  _LIBCPP_HIDE_FROM_ABI void splice_after(const_iterator __p, forward_list&& __x, const_iterator __i);
  _LIBCPP_HIDE_FROM_ABI void
  splice_after(const_iterator __p, forward_list&& __x, const_iterator __f, const_iterator __l);
  _LIBCPP_HIDE_FROM_ABI void splice_after(const_iterator __p, forward_list& __x);
  _LIBCPP_HIDE_FROM_ABI void splice_after(const_iterator __p, forward_list& __x, const_iterator __i);
  _LIBCPP_HIDE_FROM_ABI void
  splice_after(const_iterator __p, forward_list& __x, const_iterator __f, const_iterator __l);
  _LIBCPP_HIDE_FROM_ABI __remove_return_type remove(const value_type& __v);
  template <class _Predicate>
  _LIBCPP_HIDE_FROM_ABI __remove_return_type remove_if(_Predicate __pred);
  _LIBCPP_HIDE_FROM_ABI __remove_return_type unique() { return unique(__equal_to()); }
  template <class _BinaryPredicate>
  _LIBCPP_HIDE_FROM_ABI __remove_return_type unique(_BinaryPredicate __binary_pred);
#  ifndef _LIBCPP_CXX03_LANG
  _LIBCPP_HIDE_FROM_ABI void merge(forward_list&& __x) { merge(__x, __less<>()); }
  template <class _Compare>
  _LIBCPP_HIDE_FROM_ABI void merge(forward_list&& __x, _Compare __comp) {
    merge(__x, std::move(__comp));
  }
#  endif // _LIBCPP_CXX03_LANG
  _LIBCPP_HIDE_FROM_ABI void merge(forward_list& __x) { merge(__x, __less<>()); }
  template <class _Compare>
  _LIBCPP_HIDE_FROM_ABI void merge(forward_list& __x, _Compare __comp);
  _LIBCPP_HIDE_FROM_ABI void sort() { sort(__less<>()); }
  template <class _Compare>
  _LIBCPP_HIDE_FROM_ABI void sort(_Compare __comp);
  _LIBCPP_HIDE_FROM_ABI void reverse() _NOEXCEPT;

private:
#  ifndef _LIBCPP_CXX03_LANG
  _LIBCPP_HIDE_FROM_ABI void __move_assign(forward_list& __x, true_type)
      _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value);
  _LIBCPP_HIDE_FROM_ABI void __move_assign(forward_list& __x, false_type);
#  endif // _LIBCPP_CXX03_LANG

  template <class _Iter, class _Sent>
  _LIBCPP_HIDE_FROM_ABI void __assign_with_sentinel(_Iter __f, _Sent __l);

  template <class _Compare>
  static _LIBCPP_HIDE_FROM_ABI __node_pointer __merge(__node_pointer __f1, __node_pointer __f2, _Compare& __comp);

  // TODO: Make this _LIBCPP_HIDE_FROM_ABI
  template <class _Compare>
  static _LIBCPP_HIDDEN __node_pointer __sort(__node_pointer __f, difference_type __sz, _Compare& __comp);
};

#  if _LIBCPP_STD_VER >= 17
template <class _InputIterator,
          class _Alloc = allocator<__iter_value_type<_InputIterator>>,
          class        = enable_if_t<__has_input_iterator_category<_InputIterator>::value>,
          class        = enable_if_t<__is_allocator<_Alloc>::value> >
forward_list(_InputIterator, _InputIterator) -> forward_list<__iter_value_type<_InputIterator>, _Alloc>;

template <class _InputIterator,
          class _Alloc,
          class = enable_if_t<__has_input_iterator_category<_InputIterator>::value>,
          class = enable_if_t<__is_allocator<_Alloc>::value> >
forward_list(_InputIterator, _InputIterator, _Alloc) -> forward_list<__iter_value_type<_InputIterator>, _Alloc>;
#  endif

#  if _LIBCPP_STD_VER >= 23
template <ranges::input_range _Range,
          class _Alloc = allocator<ranges::range_value_t<_Range>>,
          class        = enable_if_t<__is_allocator<_Alloc>::value> >
forward_list(from_range_t, _Range&&, _Alloc = _Alloc()) -> forward_list<ranges::range_value_t<_Range>, _Alloc>;
#  endif

template <class _Tp, class _Alloc>
inline forward_list<_Tp, _Alloc>::forward_list(const allocator_type& __a) : __base(__a) {}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(size_type __n) {
  if (__n > 0) {
    for (__begin_node_pointer __p = __base::__before_begin(); __n > 0; --__n, __p = __p->__next_as_begin()) {
      __p->__next_ = this->__create_node(/* next = */ nullptr);
    }
  }
}

#  if _LIBCPP_STD_VER >= 14
template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(size_type __n, const allocator_type& __base_alloc) : __base(__base_alloc) {
  if (__n > 0) {
    for (__begin_node_pointer __p = __base::__before_begin(); __n > 0; --__n, __p = __p->__next_as_begin()) {
      __p->__next_ = this->__create_node(/* next = */ nullptr);
    }
  }
}
#  endif

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(size_type __n, const value_type& __v) {
  insert_after(cbefore_begin(), __n, __v);
}

template <class _Tp, class _Alloc>
template <class _InputIterator, __enable_if_t<__has_input_iterator_category<_InputIterator>::value, int> >
forward_list<_Tp, _Alloc>::forward_list(_InputIterator __f, _InputIterator __l) {
  insert_after(cbefore_begin(), __f, __l);
}

template <class _Tp, class _Alloc>
template <class _InputIterator, __enable_if_t<__has_input_iterator_category<_InputIterator>::value, int> >
forward_list<_Tp, _Alloc>::forward_list(_InputIterator __f, _InputIterator __l, const allocator_type& __a)
    : __base(__a) {
  insert_after(cbefore_begin(), __f, __l);
}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(const forward_list& __x)
    : __base(__node_traits::select_on_container_copy_construction(__x.__alloc_)) {
  insert_after(cbefore_begin(), __x.begin(), __x.end());
}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(const forward_list& __x, const __type_identity_t<allocator_type>& __a)
    : __base(__a) {
  insert_after(cbefore_begin(), __x.begin(), __x.end());
}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>& forward_list<_Tp, _Alloc>::operator=(const forward_list& __x) {
  if (this != std::addressof(__x)) {
    __base::__copy_assign_alloc(__x);
    assign(__x.begin(), __x.end());
  }
  return *this;
}

#  ifndef _LIBCPP_CXX03_LANG
template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(forward_list&& __x, const __type_identity_t<allocator_type>& __a)
    : __base(std::move(__x), __a) {
  if (this->__alloc_ != __x.__alloc_) {
    typedef move_iterator<iterator> _Ip;
    insert_after(cbefore_begin(), _Ip(__x.begin()), _Ip(__x.end()));
  }
}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(initializer_list<value_type> __il) {
  insert_after(cbefore_begin(), __il.begin(), __il.end());
}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(initializer_list<value_type> __il, const allocator_type& __a) : __base(__a) {
  insert_after(cbefore_begin(), __il.begin(), __il.end());
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::__move_assign(forward_list& __x, true_type)
    _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value) {
  clear();
  __base::__move_assign_alloc(__x);
  __base::__before_begin()->__next_ = __x.__before_begin()->__next_;
  __x.__before_begin()->__next_     = nullptr;
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::__move_assign(forward_list& __x, false_type) {
  if (this->__alloc_ == __x.__alloc_)
    __move_assign(__x, true_type());
  else {
    typedef move_iterator<iterator> _Ip;
    assign(_Ip(__x.begin()), _Ip(__x.end()));
  }
}

template <class _Tp, class _Alloc>
inline forward_list<_Tp, _Alloc>& forward_list<_Tp, _Alloc>::operator=(forward_list&& __x) noexcept(
    (__node_traits::propagate_on_container_move_assignment::value &&
     is_nothrow_move_assignable<allocator_type>::value) ||
    allocator_traits<allocator_type>::is_always_equal::value) {
  __move_assign(__x, integral_constant<bool, __node_traits::propagate_on_container_move_assignment::value>());
  return *this;
}

template <class _Tp, class _Alloc>
inline forward_list<_Tp, _Alloc>& forward_list<_Tp, _Alloc>::operator=(initializer_list<value_type> __il) {
  assign(__il.begin(), __il.end());
  return *this;
}

#  endif // _LIBCPP_CXX03_LANG

template <class _Tp, class _Alloc>
template <class _InputIterator, __enable_if_t<__has_input_iterator_category<_InputIterator>::value, int> >
void forward_list<_Tp, _Alloc>::assign(_InputIterator __f, _InputIterator __l) {
  __assign_with_sentinel(__f, __l);
}

template <class _Tp, class _Alloc>
template <class _Iter, class _Sent>
_LIBCPP_HIDE_FROM_ABI void forward_list<_Tp, _Alloc>::__assign_with_sentinel(_Iter __f, _Sent __l) {
  iterator __i = before_begin();
  iterator __j = std::next(__i);
  iterator __e = end();
  for (; __j != __e && __f != __l; ++__i, (void)++__j, ++__f)
    *__j = *__f;
  if (__j == __e)
    __insert_after_with_sentinel(__i, std::move(__f), std::move(__l));
  else
    erase_after(__i, __e);
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::assign(size_type __n, const value_type& __v) {
  iterator __i = before_begin();
  iterator __j = std::next(__i);
  iterator __e = end();
  for (; __j != __e && __n > 0; --__n, ++__i, ++__j)
    *__j = __v;
  if (__j == __e)
    insert_after(__i, __n, __v);
  else
    erase_after(__i, __e);
}

#  ifndef _LIBCPP_CXX03_LANG

template <class _Tp, class _Alloc>
inline void forward_list<_Tp, _Alloc>::assign(initializer_list<value_type> __il) {
  assign(__il.begin(), __il.end());
}

template <class _Tp, class _Alloc>
template <class... _Args>
#    if _LIBCPP_STD_VER >= 17
typename forward_list<_Tp, _Alloc>::reference
#    else
void
#    endif
forward_list<_Tp, _Alloc>::emplace_front(_Args&&... __args) {
  __base::__before_begin()->__next_ =
      this->__create_node(/* next = */ __base::__before_begin()->__next_, std::forward<_Args>(__args)...);
#    if _LIBCPP_STD_VER >= 17
  return __base::__before_begin()->__next_->__get_value();
#    endif
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::push_front(value_type&& __v) {
  __base::__before_begin()->__next_ =
      this->__create_node(/* next = */ __base::__before_begin()->__next_, std::move(__v));
}

#  endif // _LIBCPP_CXX03_LANG

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::push_front(const value_type& __v) {
  __base::__before_begin()->__next_ = this->__create_node(/* next = */ __base::__before_begin()->__next_, __v);
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::pop_front() {
  _LIBCPP_ASSERT_NON_NULL(!empty(), "forward_list::pop_front called on an empty list");
  __node_pointer __p                = __base::__before_begin()->__next_;
  __base::__before_begin()->__next_ = __p->__next_;
  this->__delete_node(__p);
}

#  ifndef _LIBCPP_CXX03_LANG

template <class _Tp, class _Alloc>
template <class... _Args>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::emplace_after(const_iterator __p, _Args&&... __args) {
  __begin_node_pointer const __r = __p.__get_begin();
  __r->__next_                   = this->__create_node(/* next = */ __r->__next_, std::forward<_Args>(__args)...);
  return iterator(__r->__next_);
}

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::insert_after(const_iterator __p, value_type&& __v) {
  __begin_node_pointer const __r = __p.__get_begin();
  __r->__next_                   = this->__create_node(/* next = */ __r->__next_, std::move(__v));
  return iterator(__r->__next_);
}

#  endif // _LIBCPP_CXX03_LANG

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::insert_after(const_iterator __p, const value_type& __v) {
  __begin_node_pointer const __r = __p.__get_begin();
  __r->__next_                   = this->__create_node(/* next = */ __r->__next_, __v);
  return iterator(__r->__next_);
}

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::insert_after(const_iterator __p, size_type __n, const value_type& __v) {
  __begin_node_pointer __r = __p.__get_begin();
  if (__n > 0) {
    __node_pointer __first = this->__create_node(/* next = */ nullptr, __v);
    __node_pointer __last  = __first;
#  if _LIBCPP_HAS_EXCEPTIONS
    try {
#  endif // _LIBCPP_HAS_EXCEPTIONS
      for (--__n; __n != 0; --__n, __last = __last->__next_) {
        __last->__next_ = this->__create_node(/* next = */ nullptr, __v);
      }
#  if _LIBCPP_HAS_EXCEPTIONS
    } catch (...) {
      while (__first != nullptr) {
        __node_pointer __next = __first->__next_;
        this->__delete_node(__first);
        __first = __next;
      }
      throw;
    }
#  endif // _LIBCPP_HAS_EXCEPTIONS
    __last->__next_ = __r->__next_;
    __r->__next_    = __first;
    __r             = static_cast<__begin_node_pointer>(__last);
  }
  return iterator(__r);
}

template <class _Tp, class _Alloc>
template <class _InputIterator, __enable_if_t<__has_input_iterator_category<_InputIterator>::value, int> >
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::insert_after(const_iterator __p, _InputIterator __f, _InputIterator __l) {
  return __insert_after_with_sentinel(__p, std::move(__f), std::move(__l));
}

template <class _Tp, class _Alloc>
template <class _InputIterator, class _Sentinel>
_LIBCPP_HIDE_FROM_ABI typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::__insert_after_with_sentinel(const_iterator __p, _InputIterator __f, _Sentinel __l) {
  __begin_node_pointer __r = __p.__get_begin();

  if (__f != __l) {
    __node_pointer __first = this->__create_node(/* next = */ nullptr, *__f);
    __node_pointer __last  = __first;

#  if _LIBCPP_HAS_EXCEPTIONS
    try {
#  endif // _LIBCPP_HAS_EXCEPTIONS
      for (++__f; __f != __l; ++__f, ((void)(__last = __last->__next_))) {
        __last->__next_ = this->__create_node(/* next = */ nullptr, *__f);
      }
#  if _LIBCPP_HAS_EXCEPTIONS
    } catch (...) {
      while (__first != nullptr) {
        __node_pointer __next = __first->__next_;
        this->__delete_node(__first);
        __first = __next;
      }
      throw;
    }
#  endif // _LIBCPP_HAS_EXCEPTIONS

    __last->__next_ = __r->__next_;
    __r->__next_    = __first;
    __r             = static_cast<__begin_node_pointer>(__last);
  }

  return iterator(__r);
}

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::iterator forward_list<_Tp, _Alloc>::erase_after(const_iterator __f) {
  __begin_node_pointer __p = __f.__get_begin();
  __node_pointer __n       = __p->__next_;
  __p->__next_             = __n->__next_;
  this->__delete_node(__n);
  return iterator(__p->__next_);
}

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::erase_after(const_iterator __f, const_iterator __l) {
  __node_pointer __e = __l.__get_unsafe_node_pointer();
  if (__f != __l) {
    __begin_node_pointer __bp = __f.__get_begin();

    __node_pointer __n = __bp->__next_;
    if (__n != __e) {
      __bp->__next_ = __e;
      do {
        __node_pointer __tmp = __n->__next_;
        this->__delete_node(__n);
        __n = __tmp;
      } while (__n != __e);
    }
  }
  return iterator(__e);
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::resize(size_type __n) {
  size_type __sz = 0;
  iterator __p   = before_begin();
  iterator __i   = begin();
  iterator __e   = end();
  for (; __i != __e && __sz < __n; ++__p, ++__i, ++__sz)
    ;
  if (__i != __e)
    erase_after(__p, __e);
  else {
    __n -= __sz;
    if (__n > 0) {
      for (__begin_node_pointer __ptr = __p.__get_begin(); __n > 0; --__n, __ptr = __ptr->__next_as_begin()) {
        __ptr->__next_ = this->__create_node(/* next = */ nullptr);
      }
    }
  }
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::resize(size_type __n, const value_type& __v) {
  size_type __sz = 0;
  iterator __p   = before_begin();
  iterator __i   = begin();
  iterator __e   = end();
  for (; __i != __e && __sz < __n; ++__p, ++__i, ++__sz)
    ;
  if (__i != __e)
    erase_after(__p, __e);
  else {
    __n -= __sz;
    if (__n > 0) {
      for (__begin_node_pointer __ptr = __p.__get_begin(); __n > 0; --__n, __ptr = __ptr->__next_as_begin()) {
        __ptr->__next_ = this->__create_node(/* next = */ nullptr, __v);
      }
    }
  }
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::splice_after(const_iterator __p, forward_list& __x) {
  if (!__x.empty()) {
    if (__p.__get_begin()->__next_ != nullptr) {
      const_iterator __lm1 = __x.before_begin();
      while (__lm1.__get_begin()->__next_ != nullptr)
        ++__lm1;
      __lm1.__get_begin()->__next_ = __p.__get_begin()->__next_;
    }
    __p.__get_begin()->__next_    = __x.__before_begin()->__next_;
    __x.__before_begin()->__next_ = nullptr;
  }
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::splice_after(const_iterator __p, forward_list& /*__other*/, const_iterator __i) {
  const_iterator __lm1 = std::next(__i);
  if (__p != __i && __p != __lm1) {
    __i.__get_begin()->__next_   = __lm1.__get_begin()->__next_;
    __lm1.__get_begin()->__next_ = __p.__get_begin()->__next_;
    __p.__get_begin()->__next_   = __lm1.__get_unsafe_node_pointer();
  }
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::splice_after(
    const_iterator __p, forward_list& /*__other*/, const_iterator __f, const_iterator __l) {
  if (__f != __l && __p != __f) {
    const_iterator __lm1 = __f;
    while (__lm1.__get_begin()->__next_ != __l.__get_begin())
      ++__lm1;
    if (__f != __lm1) {
      __lm1.__get_begin()->__next_ = __p.__get_begin()->__next_;
      __p.__get_begin()->__next_   = __f.__get_begin()->__next_;
      __f.__get_begin()->__next_   = __l.__get_unsafe_node_pointer();
    }
  }
}

template <class _Tp, class _Alloc>
inline _LIBCPP_HIDE_FROM_ABI void forward_list<_Tp, _Alloc>::splice_after(const_iterator __p, forward_list&& __x) {
  splice_after(__p, __x);
}

template <class _Tp, class _Alloc>
inline _LIBCPP_HIDE_FROM_ABI void
forward_list<_Tp, _Alloc>::splice_after(const_iterator __p, forward_list&& __x, const_iterator __i) {
  splice_after(__p, __x, __i);
}

template <class _Tp, class _Alloc>
inline _LIBCPP_HIDE_FROM_ABI void forward_list<_Tp, _Alloc>::splice_after(
    const_iterator __p, forward_list&& __x, const_iterator __f, const_iterator __l) {
  splice_after(__p, __x, __f, __l);
}

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::__remove_return_type forward_list<_Tp, _Alloc>::remove(const value_type& __v) {
  forward_list<_Tp, _Alloc> __deleted_nodes(get_allocator()); // collect the nodes we're removing
  typename forward_list<_Tp, _Alloc>::size_type __count_removed = 0;
  const iterator __e                                            = end();
  for (iterator __i = before_begin(); __i.__get_begin()->__next_ != nullptr;) {
    if (__i.__get_begin()->__next_->__get_value() == __v) {
      ++__count_removed;
      iterator __j = std::next(__i, 2);
      for (; __j != __e && *__j == __v; ++__j)
        ++__count_removed;
      __deleted_nodes.splice_after(__deleted_nodes.before_begin(), *this, __i, __j);
      if (__j == __e)
        break;
      __i = __j;
    } else
      ++__i;
  }

  return (__remove_return_type)__count_removed;
}

template <class _Tp, class _Alloc>
template <class _Predicate>
typename forward_list<_Tp, _Alloc>::__remove_return_type forward_list<_Tp, _Alloc>::remove_if(_Predicate __pred) {
  forward_list<_Tp, _Alloc> __deleted_nodes(get_allocator()); // collect the nodes we're removing
  typename forward_list<_Tp, _Alloc>::size_type __count_removed = 0;
  const iterator __e                                            = end();
  for (iterator __i = before_begin(); __i.__get_begin()->__next_ != nullptr;) {
    if (__pred(__i.__get_begin()->__next_->__get_value())) {
      ++__count_removed;
      iterator __j = std::next(__i, 2);
      for (; __j != __e && __pred(*__j); ++__j)
        ++__count_removed;
      __deleted_nodes.splice_after(__deleted_nodes.before_begin(), *this, __i, __j);
      if (__j == __e)
        break;
      __i = __j;
    } else
      ++__i;
  }

  return (__remove_return_type)__count_removed;
}

template <class _Tp, class _Alloc>
template <class _BinaryPredicate>
typename forward_list<_Tp, _Alloc>::__remove_return_type
forward_list<_Tp, _Alloc>::unique(_BinaryPredicate __binary_pred) {
  forward_list<_Tp, _Alloc> __deleted_nodes(get_allocator()); // collect the nodes we're removing
  typename forward_list<_Tp, _Alloc>::size_type __count_removed = 0;
  for (iterator __i = begin(), __e = end(); __i != __e;) {
    iterator __j = std::next(__i);
    for (; __j != __e && __binary_pred(*__i, *__j); ++__j)
      ++__count_removed;
    if (__i.__get_begin()->__next_ != __j.__get_unsafe_node_pointer())
      __deleted_nodes.splice_after(__deleted_nodes.before_begin(), *this, __i, __j);
    __i = __j;
  }

  return (__remove_return_type)__count_removed;
}

template <class _Tp, class _Alloc>
template <class _Compare>
void forward_list<_Tp, _Alloc>::merge(forward_list& __x, _Compare __comp) {
  if (this != std::addressof(__x)) {
    __base::__before_begin()->__next_ =
        __merge(__base::__before_begin()->__next_, __x.__before_begin()->__next_, __comp);
    __x.__before_begin()->__next_ = nullptr;
  }
}

template <class _Tp, class _Alloc>
template <class _Compare>
typename forward_list<_Tp, _Alloc>::__node_pointer
forward_list<_Tp, _Alloc>::__merge(__node_pointer __f1, __node_pointer __f2, _Compare& __comp) {
  if (__f1 == nullptr)
    return __f2;
  if (__f2 == nullptr)
    return __f1;
  __node_pointer __r;
  if (__comp(__f2->__get_value(), __f1->__get_value())) {
    __node_pointer __t = __f2;
    while (__t->__next_ != nullptr && __comp(__t->__next_->__get_value(), __f1->__get_value()))
      __t = __t->__next_;
    __r          = __f2;
    __f2         = __t->__next_;
    __t->__next_ = __f1;
  } else
    __r = __f1;
  __node_pointer __p = __f1;
  __f1               = __f1->__next_;
  while (__f1 != nullptr && __f2 != nullptr) {
    if (__comp(__f2->__get_value(), __f1->__get_value())) {
      __node_pointer __t = __f2;
      while (__t->__next_ != nullptr && __comp(__t->__next_->__get_value(), __f1->__get_value()))
        __t = __t->__next_;
      __p->__next_ = __f2;
      __f2         = __t->__next_;
      __t->__next_ = __f1;
    }
    __p  = __f1;
    __f1 = __f1->__next_;
  }
  if (__f2 != nullptr)
    __p->__next_ = __f2;
  return __r;
}

template <class _Tp, class _Alloc>
template <class _Compare>
inline void forward_list<_Tp, _Alloc>::sort(_Compare __comp) {
  __base::__before_begin()->__next_ = __sort(__base::__before_begin()->__next_, std::distance(begin(), end()), __comp);
}

template <class _Tp, class _Alloc>
template <class _Compare>
typename forward_list<_Tp, _Alloc>::__node_pointer
forward_list<_Tp, _Alloc>::__sort(__node_pointer __f1, difference_type __sz, _Compare& __comp) {
  switch (__sz) {
  case 0:
  case 1:
    return __f1;
  case 2:
    if (__comp(__f1->__next_->__get_value(), __f1->__get_value())) {
      __node_pointer __t = __f1->__next_;
      __t->__next_       = __f1;
      __f1->__next_      = nullptr;
      __f1               = __t;
    }
    return __f1;
  }
  difference_type __sz1 = __sz / 2;
  difference_type __sz2 = __sz - __sz1;
  __node_pointer __t    = std::next(iterator(__f1), __sz1 - 1).__get_unsafe_node_pointer();
  __node_pointer __f2   = __t->__next_;
  __t->__next_          = nullptr;
  return __merge(__sort(__f1, __sz1, __comp), __sort(__f2, __sz2, __comp), __comp);
}

template <class _Tp, class _Alloc>
void forward_list<_Tp, _Alloc>::reverse() _NOEXCEPT {
  __node_pointer __p = __base::__before_begin()->__next_;
  if (__p != nullptr) {
    __node_pointer __f = __p->__next_;
    __p->__next_       = nullptr;
    while (__f != nullptr) {
      __node_pointer __t = __f->__next_;
      __f->__next_       = __p;
      __p                = __f;
      __f                = __t;
    }
    __base::__before_begin()->__next_ = __p;
  }
}

template <class _Tp, class _Alloc>
_LIBCPP_HIDE_FROM_ABI bool operator==(const forward_list<_Tp, _Alloc>& __x, const forward_list<_Tp, _Alloc>& __y) {
  typedef forward_list<_Tp, _Alloc> _Cp;
  typedef typename _Cp::const_iterator _Ip;
  _Ip __ix = __x.begin();
  _Ip __ex = __x.end();
  _Ip __iy = __y.begin();
  _Ip __ey = __y.end();
  for (; __ix != __ex && __iy != __ey; ++__ix, ++__iy)
    if (!(*__ix == *__iy))
      return false;
  return (__ix == __ex) == (__iy == __ey);
}

#  if _LIBCPP_STD_VER <= 17

template <class _Tp, class _Alloc>
inline _LIBCPP_HIDE_FROM_ABI bool
operator!=(const forward_list<_Tp, _Alloc>& __x, const forward_list<_Tp, _Alloc>& __y) {
  return !(__x == __y);
}

template <class _Tp, class _Alloc>
inline _LIBCPP_HIDE_FROM_ABI bool
operator<(const forward_list<_Tp, _Alloc>& __x, const forward_list<_Tp, _Alloc>& __y) {
  return std::lexicographical_compare(__x.begin(), __x.end(), __y.begin(), __y.end());
}

template <class _Tp, class _Alloc>
inline _LIBCPP_HIDE_FROM_ABI bool
operator>(const forward_list<_Tp, _Alloc>& __x, const forward_list<_Tp, _Alloc>& __y) {
  return __y < __x;
}

template <class _Tp, class _Alloc>
inline _LIBCPP_HIDE_FROM_ABI bool
operator>=(const forward_list<_Tp, _Alloc>& __x, const forward_list<_Tp, _Alloc>& __y) {
  return !(__x < __y);
}

template <class _Tp, class _Alloc>
inline _LIBCPP_HIDE_FROM_ABI bool
operator<=(const forward_list<_Tp, _Alloc>& __x, const forward_list<_Tp, _Alloc>& __y) {
  return !(__y < __x);
}

#  else // #if _LIBCPP_STD_VER <= 17

template <class _Tp, class _Allocator>
_LIBCPP_HIDE_FROM_ABI __synth_three_way_result<_Tp>
operator<=>(const forward_list<_Tp, _Allocator>& __x, const forward_list<_Tp, _Allocator>& __y) {
  return std::lexicographical_compare_three_way(__x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
}

#  endif // #if _LIBCPP_STD_VER <= 17

template <class _Tp, class _Alloc>
inline _LIBCPP_HIDE_FROM_ABI void swap(forward_list<_Tp, _Alloc>& __x, forward_list<_Tp, _Alloc>& __y)
    _NOEXCEPT_(_NOEXCEPT_(__x.swap(__y))) {
  __x.swap(__y);
}

#  if _LIBCPP_STD_VER >= 20
template <class _Tp, class _Allocator, class _Predicate>
inline _LIBCPP_HIDE_FROM_ABI typename forward_list<_Tp, _Allocator>::size_type
erase_if(forward_list<_Tp, _Allocator>& __c, _Predicate __pred) {
  return __c.remove_if(__pred);
}

template <class _Tp, class _Allocator, class _Up>
inline _LIBCPP_HIDE_FROM_ABI typename forward_list<_Tp, _Allocator>::size_type
erase(forward_list<_Tp, _Allocator>& __c, const _Up& __v) {
  return std::erase_if(__c, [&](const auto& __elem) -> bool { return __elem == __v; });
}
#  endif

template <class _Tp, class _Allocator>
struct __container_traits<forward_list<_Tp, _Allocator> > {
  // http://eel.is/c++draft/container.reqmts
  // Unless otherwise specified (see [associative.reqmts.except], [unord.req.except], [deque.modifiers],
  // [inplace.vector.modifiers], and [vector.modifiers]) all container types defined in this Clause meet the following
  // additional requirements:
  // - If an exception is thrown by an insert() or emplace() function while inserting a single element, that
  // function has no effects.
  static _LIBCPP_CONSTEXPR const bool __emplacement_has_strong_exception_safety_guarantee = true;
};

_LIBCPP_END_NAMESPACE_STD

#  if _LIBCPP_STD_VER >= 17
_LIBCPP_BEGIN_NAMESPACE_STD
namespace pmr {
template <class _ValueT>
using forward_list _LIBCPP_AVAILABILITY_PMR = std::forward_list<_ValueT, polymorphic_allocator<_ValueT>>;
} // namespace pmr
_LIBCPP_END_NAMESPACE_STD
#  endif

_LIBCPP_POP_MACROS

#  if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
#    include <algorithm>
#    include <atomic>
#    include <concepts>
#    include <cstdint>
#    include <cstdlib>
#    include <cstring>
#    include <functional>
#    include <iosfwd>
#    include <iterator>
#    include <stdexcept>
#    include <type_traits>
#    include <typeinfo>
#  endif
#endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)

#endif // _LIBCPP_FORWARD_LIST