//===----------------------------------------------------------------------===//
//
// 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___UTILITY_PAIR_H
#define _LIBCPP___UTILITY_PAIR_H

#include <__compare/common_comparison_category.h>
#include <__compare/synth_three_way.h>
#include <__concepts/different_from.h>
#include <__config>
#include <__functional/unwrap_ref.h>
#include <__fwd/array.h>
#include <__fwd/get.h>
#include <__fwd/subrange.h>
#include <__fwd/tuple.h>
#include <__tuple/pair_like.h>
#include <__tuple/sfinae_helpers.h>
#include <__tuple/tuple_element.h>
#include <__tuple/tuple_indices.h>
#include <__tuple/tuple_size.h>
#include <__type_traits/common_reference.h>
#include <__type_traits/common_type.h>
#include <__type_traits/conditional.h>
#include <__type_traits/decay.h>
#include <__type_traits/integral_constant.h>
#include <__type_traits/is_assignable.h>
#include <__type_traits/is_constructible.h>
#include <__type_traits/is_convertible.h>
#include <__type_traits/is_copy_assignable.h>
#include <__type_traits/is_default_constructible.h>
#include <__type_traits/is_implicitly_default_constructible.h>
#include <__type_traits/is_move_assignable.h>
#include <__type_traits/is_nothrow_assignable.h>
#include <__type_traits/is_nothrow_constructible.h>
#include <__type_traits/is_nothrow_copy_assignable.h>
#include <__type_traits/is_nothrow_copy_constructible.h>
#include <__type_traits/is_nothrow_default_constructible.h>
#include <__type_traits/is_nothrow_move_assignable.h>
#include <__type_traits/is_same.h>
#include <__type_traits/is_swappable.h>
#include <__type_traits/nat.h>
#include <__type_traits/remove_cvref.h>
#include <__utility/declval.h>
#include <__utility/forward.h>
#include <__utility/move.h>
#include <__utility/piecewise_construct.h>
#include <cstddef>

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

_LIBCPP_BEGIN_NAMESPACE_STD

template <class, class>
struct __non_trivially_copyable_base {
  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI
  __non_trivially_copyable_base() _NOEXCEPT {}
  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI
  __non_trivially_copyable_base(__non_trivially_copyable_base const&) _NOEXCEPT {}
};

#if _LIBCPP_STD_VER >= 23
template <class _Tp>
struct __is_specialization_of_subrange : false_type {};

template <class _Iter, class _Sent, ranges::subrange_kind _Kind>
struct __is_specialization_of_subrange<ranges::subrange<_Iter, _Sent, _Kind>> : true_type {};
#endif

template <class _T1, class _T2>
struct _LIBCPP_TEMPLATE_VIS pair
#if defined(_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR)
: private __non_trivially_copyable_base<_T1, _T2>
#endif
{
    using first_type = _T1;
    using second_type = _T2;

    _T1 first;
    _T2 second;

    _LIBCPP_HIDE_FROM_ABI pair(pair const&) = default;
    _LIBCPP_HIDE_FROM_ABI pair(pair&&) = default;

#ifdef _LIBCPP_CXX03_LANG
    _LIBCPP_HIDE_FROM_ABI
    pair() : first(), second() {}

    _LIBCPP_HIDE_FROM_ABI
    pair(_T1 const& __t1, _T2 const& __t2) : first(__t1), second(__t2) {}

    template <class _U1, class _U2>
    _LIBCPP_HIDE_FROM_ABI
    pair(const pair<_U1, _U2>& __p) : first(__p.first), second(__p.second) {}

    _LIBCPP_HIDE_FROM_ABI
    pair& operator=(pair const& __p) {
        first = __p.first;
        second = __p.second;
        return *this;
    }
#else
    struct _CheckArgs {
      template <int&...>
      static _LIBCPP_HIDE_FROM_ABI constexpr bool __enable_explicit_default() {
          return is_default_constructible<_T1>::value
              && is_default_constructible<_T2>::value
              && !__enable_implicit_default<>();
      }

      template <int&...>
      static _LIBCPP_HIDE_FROM_ABI constexpr bool __enable_implicit_default() {
          return __is_implicitly_default_constructible<_T1>::value
              && __is_implicitly_default_constructible<_T2>::value;
      }

      template <class _U1, class _U2>
      static _LIBCPP_HIDE_FROM_ABI constexpr bool __is_pair_constructible() {
          return is_constructible<first_type, _U1>::value
              && is_constructible<second_type, _U2>::value;
      }

      template <class _U1, class _U2>
      static _LIBCPP_HIDE_FROM_ABI constexpr bool __is_implicit() {
          return is_convertible<_U1, first_type>::value
              && is_convertible<_U2, second_type>::value;
      }

      template <class _U1, class _U2>
      static _LIBCPP_HIDE_FROM_ABI constexpr bool __enable_explicit() {
          return __is_pair_constructible<_U1, _U2>() && !__is_implicit<_U1, _U2>();
      }

      template <class _U1, class _U2>
      static _LIBCPP_HIDE_FROM_ABI constexpr bool __enable_implicit() {
          return __is_pair_constructible<_U1, _U2>() && __is_implicit<_U1, _U2>();
      }
    };

    template <bool _MaybeEnable>
    using _CheckArgsDep _LIBCPP_NODEBUG = typename conditional<
      _MaybeEnable, _CheckArgs, __check_tuple_constructor_fail>::type;

    template<bool _Dummy = true, typename enable_if<
            _CheckArgsDep<_Dummy>::__enable_explicit_default()
    >::type* = nullptr>
    explicit _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
    pair() _NOEXCEPT_(is_nothrow_default_constructible<first_type>::value &&
                      is_nothrow_default_constructible<second_type>::value)
        : first(), second() {}

    template<bool _Dummy = true, typename enable_if<
            _CheckArgsDep<_Dummy>::__enable_implicit_default()
    >::type* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
    pair() _NOEXCEPT_(is_nothrow_default_constructible<first_type>::value &&
                      is_nothrow_default_constructible<second_type>::value)
        : first(), second() {}

    template <bool _Dummy = true, typename enable_if<
             _CheckArgsDep<_Dummy>::template __enable_explicit<_T1 const&, _T2 const&>()
    >::type* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    explicit pair(_T1 const& __t1, _T2 const& __t2)
        _NOEXCEPT_(is_nothrow_copy_constructible<first_type>::value &&
                   is_nothrow_copy_constructible<second_type>::value)
        : first(__t1), second(__t2) {}

    template<bool _Dummy = true, typename enable_if<
            _CheckArgsDep<_Dummy>::template __enable_implicit<_T1 const&, _T2 const&>()
    >::type* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair(_T1 const& __t1, _T2 const& __t2)
        _NOEXCEPT_(is_nothrow_copy_constructible<first_type>::value &&
                   is_nothrow_copy_constructible<second_type>::value)
        : first(__t1), second(__t2) {}

    template <
#if _LIBCPP_STD_VER >= 23 // http://wg21.link/P1951
        class _U1 = _T1, class _U2 = _T2,
#else
        class _U1, class _U2,
#endif
        typename enable_if<_CheckArgs::template __enable_explicit<_U1, _U2>()>::type* = nullptr
    >
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    explicit pair(_U1&& __u1, _U2&& __u2)
        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1>::value &&
                    is_nothrow_constructible<second_type, _U2>::value))
        : first(std::forward<_U1>(__u1)), second(std::forward<_U2>(__u2)) {}

    template <
#if _LIBCPP_STD_VER >= 23 // http://wg21.link/P1951
        class _U1 = _T1, class _U2 = _T2,
#else
        class _U1, class _U2,
#endif
        typename enable_if<_CheckArgs::template __enable_implicit<_U1, _U2>()>::type* = nullptr
    >
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair(_U1&& __u1, _U2&& __u2)
        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1>::value &&
                    is_nothrow_constructible<second_type, _U2>::value))
        : first(std::forward<_U1>(__u1)), second(std::forward<_U2>(__u2)) {}

#if _LIBCPP_STD_VER >= 23
    template<class _U1, class _U2, __enable_if_t<
            _CheckArgs::template __is_pair_constructible<_U1&, _U2&>()
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI constexpr
    explicit(!_CheckArgs::template __is_implicit<_U1&, _U2&>()) pair(pair<_U1, _U2>& __p)
        noexcept((is_nothrow_constructible<first_type, _U1&>::value &&
                  is_nothrow_constructible<second_type, _U2&>::value))
        : first(__p.first), second(__p.second) {}
#endif

    template<class _U1, class _U2, typename enable_if<
            _CheckArgs::template __enable_explicit<_U1 const&, _U2 const&>()
    >::type* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    explicit pair(pair<_U1, _U2> const& __p)
        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1 const&>::value &&
                    is_nothrow_constructible<second_type, _U2 const&>::value))
        : first(__p.first), second(__p.second) {}

    template<class _U1, class _U2, typename enable_if<
            _CheckArgs::template __enable_implicit<_U1 const&, _U2 const&>()
    >::type* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair(pair<_U1, _U2> const& __p)
        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1 const&>::value &&
                    is_nothrow_constructible<second_type, _U2 const&>::value))
        : first(__p.first), second(__p.second) {}

    template<class _U1, class _U2, typename enable_if<
            _CheckArgs::template __enable_explicit<_U1, _U2>()
    >::type* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    explicit pair(pair<_U1, _U2>&&__p)
        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1&&>::value &&
                    is_nothrow_constructible<second_type, _U2&&>::value))
        : first(std::forward<_U1>(__p.first)), second(std::forward<_U2>(__p.second)) {}

    template<class _U1, class _U2, typename enable_if<
            _CheckArgs::template __enable_implicit<_U1, _U2>()
    >::type* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair(pair<_U1, _U2>&& __p)
        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1&&>::value &&
                    is_nothrow_constructible<second_type, _U2&&>::value))
        : first(std::forward<_U1>(__p.first)), second(std::forward<_U2>(__p.second)) {}

#if _LIBCPP_STD_VER >= 23
    template<class _U1, class _U2, __enable_if_t<
            _CheckArgs::template __is_pair_constructible<const _U1&&, const _U2&&>()
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI constexpr
    explicit(!_CheckArgs::template __is_implicit<const _U1&&, const _U2&&>())
    pair(const pair<_U1, _U2>&& __p)
        noexcept(is_nothrow_constructible<first_type, const _U1&&>::value &&
                 is_nothrow_constructible<second_type, const _U2&&>::value)
        : first(std::move(__p.first)), second(std::move(__p.second)) {}
#endif

#  if _LIBCPP_STD_VER >= 23
    // This is a workaround for http://llvm.org/PR60710. We should be able to remove it once Clang is fixed.
    template <class _PairLike, bool _Enable = tuple_size<remove_cvref_t<_PairLike>>::value == 2>
    _LIBCPP_HIDE_FROM_ABI static constexpr bool __pair_like_explicit_wknd() {
        if constexpr (tuple_size<remove_cvref_t<_PairLike>>::value == 2) {
            return !is_convertible_v<decltype(std::get<0>(std::declval<_PairLike&&>())), first_type> ||
                   !is_convertible_v<decltype(std::get<1>(std::declval<_PairLike&&>())), second_type>;
        }
        return false;
    }

    template <__pair_like _PairLike>
      requires(is_constructible_v<first_type, decltype(std::get<0>(std::declval<_PairLike&&>()))> &&
               is_constructible_v<second_type, decltype(std::get<1>(std::declval<_PairLike&&>()))>)
    _LIBCPP_HIDE_FROM_ABI constexpr explicit(__pair_like_explicit_wknd<_PairLike>())
        pair(_PairLike&& __p)
        : first(std::get<0>(std::forward<_PairLike>(__p))), second(std::get<1>(std::forward<_PairLike>(__p))) {}
#  endif

    template <class... _Args1, class... _Args2>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
    pair(piecewise_construct_t __pc,
         tuple<_Args1...> __first_args, tuple<_Args2...> __second_args)
        _NOEXCEPT_((is_nothrow_constructible<first_type, _Args1...>::value &&
                    is_nothrow_constructible<second_type, _Args2...>::value))
        : pair(__pc, __first_args, __second_args,
                typename __make_tuple_indices<sizeof...(_Args1)>::type(),
                typename __make_tuple_indices<sizeof...(_Args2) >::type()) {}

    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
    pair& operator=(__conditional_t<
                        is_copy_assignable<first_type>::value &&
                        is_copy_assignable<second_type>::value,
                    pair, __nat> const& __p)
        _NOEXCEPT_(is_nothrow_copy_assignable<first_type>::value &&
                   is_nothrow_copy_assignable<second_type>::value)
    {
        first = __p.first;
        second = __p.second;
        return *this;
    }

    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
    pair& operator=(__conditional_t<
                        is_move_assignable<first_type>::value &&
                        is_move_assignable<second_type>::value,
                    pair, __nat>&& __p)
        _NOEXCEPT_(is_nothrow_move_assignable<first_type>::value &&
                   is_nothrow_move_assignable<second_type>::value)
    {
        first = std::forward<first_type>(__p.first);
        second = std::forward<second_type>(__p.second);
        return *this;
    }

    template <class _U1, class _U2, __enable_if_t<
        is_assignable<first_type&, _U1 const&>::value &&
        is_assignable<second_type&, _U2 const&>::value
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
    pair& operator=(pair<_U1, _U2> const& __p) {
        first = __p.first;
        second = __p.second;
        return *this;
    }

    template <class _U1, class _U2, __enable_if_t<
        is_assignable<first_type&, _U1>::value &&
        is_assignable<second_type&, _U2>::value
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
    pair& operator=(pair<_U1, _U2>&& __p) {
        first = std::forward<_U1>(__p.first);
        second = std::forward<_U2>(__p.second);
        return *this;
    }

#  if _LIBCPP_STD_VER >= 23
    _LIBCPP_HIDE_FROM_ABI constexpr
    const pair& operator=(pair const& __p) const
      noexcept(is_nothrow_copy_assignable_v<const first_type> &&
               is_nothrow_copy_assignable_v<const second_type>)
      requires(is_copy_assignable_v<const first_type> &&
               is_copy_assignable_v<const second_type>) {
        first = __p.first;
        second = __p.second;
        return *this;
    }

    _LIBCPP_HIDE_FROM_ABI constexpr
    const pair& operator=(pair&& __p) const
      noexcept(is_nothrow_assignable_v<const first_type&, first_type> &&
               is_nothrow_assignable_v<const second_type&, second_type>)
      requires(is_assignable_v<const first_type&, first_type> &&
               is_assignable_v<const second_type&, second_type>) {
        first = std::forward<first_type>(__p.first);
        second = std::forward<second_type>(__p.second);
        return *this;
    }

    template<class _U1, class _U2>
    _LIBCPP_HIDE_FROM_ABI constexpr
    const pair& operator=(const pair<_U1, _U2>& __p) const
      requires(is_assignable_v<const first_type&, const _U1&> &&
               is_assignable_v<const second_type&, const _U2&>) {
        first = __p.first;
        second = __p.second;
        return *this;
    }

    template<class _U1, class _U2>
    _LIBCPP_HIDE_FROM_ABI constexpr
    const pair& operator=(pair<_U1, _U2>&& __p) const
      requires(is_assignable_v<const first_type&, _U1> &&
               is_assignable_v<const second_type&, _U2>) {
        first = std::forward<_U1>(__p.first);
        second = std::forward<_U2>(__p.second);
        return *this;
    }

    template <__pair_like _PairLike>
      requires(__different_from<_PairLike, pair> &&
               !__is_specialization_of_subrange<remove_cvref_t<_PairLike>>::value &&
               is_assignable_v<first_type&, decltype(std::get<0>(std::declval<_PairLike>()))> &&
               is_assignable_v<second_type&, decltype(std::get<1>(std::declval<_PairLike>()))>)
    _LIBCPP_HIDE_FROM_ABI constexpr pair& operator=(_PairLike&& __p) {
        first  = std::get<0>(std::forward<_PairLike>(__p));
        second = std::get<1>(std::forward<_PairLike>(__p));
        return *this;
    }

    template <__pair_like _PairLike>
      requires(__different_from<_PairLike, pair> &&
               !__is_specialization_of_subrange<remove_cvref_t<_PairLike>>::value &&
               is_assignable_v<first_type const&, decltype(std::get<0>(std::declval<_PairLike>()))> &&
               is_assignable_v<second_type const&, decltype(std::get<1>(std::declval<_PairLike>()))>)
    _LIBCPP_HIDE_FROM_ABI constexpr pair const& operator=(_PairLike&& __p) const {
        first  = std::get<0>(std::forward<_PairLike>(__p));
        second = std::get<1>(std::forward<_PairLike>(__p));
        return *this;
    }
#  endif // _LIBCPP_STD_VER >= 23

    // Prior to C++23, we provide an approximation of constructors and assignment operators from
    // pair-like types. This was historically provided as an extension.
#if _LIBCPP_STD_VER < 23
    // from std::tuple
    template<class _U1, class _U2, __enable_if_t<
        is_convertible<_U1 const&, _T1>::value &&
        is_convertible<_U2 const&, _T2>::value
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair(tuple<_U1, _U2> const& __p)
        : first(std::get<0>(__p)),
          second(std::get<1>(__p)) {}

    template<class _U1, class _U2, __enable_if_t<
        is_constructible<_T1, _U1 const&>::value &&
        is_constructible<_T2, _U2 const&>::value &&
        !(is_convertible<_U1 const&, _T1>::value &&
          is_convertible<_U2 const&, _T2>::value)
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    explicit
    pair(tuple<_U1, _U2> const& __p)
        : first(std::get<0>(__p)),
          second(std::get<1>(__p)) {}

    template<class _U1, class _U2, __enable_if_t<
        is_convertible<_U1, _T1>::value &&
        is_convertible<_U2, _T2>::value
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair(tuple<_U1, _U2>&& __p)
        : first(std::get<0>(std::move(__p))),
          second(std::get<1>(std::move(__p))) {}

    template<class _U1, class _U2, __enable_if_t<
        is_constructible<_T1, _U1>::value &&
        is_constructible<_T2, _U2>::value &&
        !(is_convertible<_U1, _T1>::value &&
          is_convertible<_U2, _T2>::value)
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    explicit
    pair(tuple<_U1, _U2>&& __p)
        : first(std::get<0>(std::move(__p))),
          second(std::get<1>(std::move(__p))) {}


    template<class _U1, class _U2, __enable_if_t<
        is_assignable<_T1&, _U1 const&>::value &&
        is_assignable<_T2&, _U2 const&>::value
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair& operator=(tuple<_U1, _U2> const& __p) {
        first = std::get<0>(__p);
        second = std::get<1>(__p);
        return *this;
    }

    template<class _U1, class _U2, __enable_if_t<
        is_assignable<_T1&, _U1&&>::value &&
        is_assignable<_T2&, _U2&&>::value
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair& operator=(tuple<_U1, _U2>&& __p) {
        first = std::get<0>(std::move(__p));
        second = std::get<1>(std::move(__p));
        return *this;
    }

    // from std::array
    template<class _Up, __enable_if_t<
        is_convertible<_Up const&, _T1>::value &&
        is_convertible<_Up const&, _T2>::value
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair(array<_Up, 2> const& __p)
        : first(__p[0]),
          second(__p[1]) {}

    template<class _Up, __enable_if_t<
        is_constructible<_T1, _Up const&>::value &&
        is_constructible<_T2, _Up const&>::value &&
        !(is_convertible<_Up const&, _T1>::value &&
          is_convertible<_Up const&, _T2>::value)
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    explicit
    pair(array<_Up, 2> const& __p)
        : first(__p[0]),
          second(__p[1]) {}

    template<class _Up, __enable_if_t<
        is_convertible<_Up, _T1>::value &&
        is_convertible<_Up, _T2>::value
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair(array<_Up, 2>&& __p)
        : first(std::move(__p)[0]),
          second(std::move(__p)[1]) {}

    template<class _Up, __enable_if_t<
        is_constructible<_T1, _Up>::value &&
        is_constructible<_T2, _Up>::value &&
        !(is_convertible<_Up, _T1>::value &&
          is_convertible<_Up, _T2>::value)
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    explicit
    pair(array<_Up, 2>&& __p)
        : first(std::move(__p)[0]),
          second(std::move(__p)[1]) {}


    template<class _Up, __enable_if_t<
        is_assignable<_T1&, _Up const&>::value &&
        is_assignable<_T2&, _Up const&>::value
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair& operator=(array<_Up, 2> const& __p) {
        first = std::get<0>(__p);
        second = std::get<1>(__p);
        return *this;
    }

    template<class _Up, __enable_if_t<
        is_assignable<_T1&, _Up>::value &&
        is_assignable<_T2&, _Up>::value
    >* = nullptr>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    pair& operator=(array<_Up, 2>&& __p) {
        first = std::get<0>(std::move(__p));
        second = std::get<1>(std::move(__p));
        return *this;
    }
#endif // _LIBCPP_STD_VER < 23
#endif // _LIBCPP_CXX03_LANG

    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
    void
    swap(pair& __p) _NOEXCEPT_(__is_nothrow_swappable<first_type>::value &&
                               __is_nothrow_swappable<second_type>::value)
    {
        using std::swap;
        swap(first,  __p.first);
        swap(second, __p.second);
    }

#if _LIBCPP_STD_VER >= 23
    _LIBCPP_HIDE_FROM_ABI constexpr
    void swap(const pair& __p) const
        noexcept(__is_nothrow_swappable<const first_type>::value &&
                 __is_nothrow_swappable<const second_type>::value)
    {
        using std::swap;
        swap(first,  __p.first);
        swap(second, __p.second);
    }
#endif
private:

#ifndef _LIBCPP_CXX03_LANG
    template <class... _Args1, class... _Args2, size_t... _I1, size_t... _I2>
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
    pair(piecewise_construct_t,
         tuple<_Args1...>& __first_args, tuple<_Args2...>& __second_args,
         __tuple_indices<_I1...>, __tuple_indices<_I2...>);
#endif
};

#if _LIBCPP_STD_VER >= 17
template<class _T1, class _T2>
pair(_T1, _T2) -> pair<_T1, _T2>;
#endif

// [pairs.spec], specialized algorithms

template <class _T1, class _T2, class _U1, class _U2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
bool
operator==(const pair<_T1,_T2>& __x, const pair<_U1,_U2>& __y)
{
    return __x.first == __y.first && __x.second == __y.second;
}

#if _LIBCPP_STD_VER >= 20

template <class _T1, class _T2, class _U1, class _U2>
_LIBCPP_HIDE_FROM_ABI constexpr
common_comparison_category_t<
        __synth_three_way_result<_T1, _U1>,
        __synth_three_way_result<_T2, _U2> >
operator<=>(const pair<_T1,_T2>& __x, const pair<_U1,_U2>& __y)
{
    if (auto __c = std::__synth_three_way(__x.first, __y.first); __c != 0) {
      return __c;
    }
    return std::__synth_three_way(__x.second, __y.second);
}

#else // _LIBCPP_STD_VER >= 20

template <class _T1, class _T2, class _U1, class _U2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
bool
operator!=(const pair<_T1,_T2>& __x, const pair<_U1,_U2>& __y)
{
    return !(__x == __y);
}

template <class _T1, class _T2, class _U1, class _U2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
bool
operator< (const pair<_T1,_T2>& __x, const pair<_U1,_U2>& __y)
{
    return __x.first < __y.first || (!(__y.first < __x.first) && __x.second < __y.second);
}

template <class _T1, class _T2, class _U1, class _U2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
bool
operator> (const pair<_T1,_T2>& __x, const pair<_U1,_U2>& __y)
{
    return __y < __x;
}

template <class _T1, class _T2, class _U1, class _U2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
bool
operator>=(const pair<_T1,_T2>& __x, const pair<_U1,_U2>& __y)
{
    return !(__x < __y);
}

template <class _T1, class _T2, class _U1, class _U2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
bool
operator<=(const pair<_T1,_T2>& __x, const pair<_U1,_U2>& __y)
{
    return !(__y < __x);
}

#endif // _LIBCPP_STD_VER >= 20

#if _LIBCPP_STD_VER >= 23
template <class _T1, class _T2, class _U1, class _U2, template<class> class _TQual, template<class> class _UQual>
    requires requires { typename pair<common_reference_t<_TQual<_T1>, _UQual<_U1>>,
                                      common_reference_t<_TQual<_T2>, _UQual<_U2>>>; }
struct basic_common_reference<pair<_T1, _T2>, pair<_U1, _U2>, _TQual, _UQual> {
    using type = pair<common_reference_t<_TQual<_T1>, _UQual<_U1>>,
                      common_reference_t<_TQual<_T2>, _UQual<_U2>>>;
};

template <class _T1, class _T2, class _U1, class _U2>
    requires requires { typename pair<common_type_t<_T1, _U1>, common_type_t<_T2, _U2>>; }
struct common_type<pair<_T1, _T2>, pair<_U1, _U2>> {
    using type = pair<common_type_t<_T1, _U1>, common_type_t<_T2, _U2>>;
};
#endif // _LIBCPP_STD_VER >= 23

template <class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
typename enable_if
<
    __is_swappable<_T1>::value &&
    __is_swappable<_T2>::value,
    void
>::type
swap(pair<_T1, _T2>& __x, pair<_T1, _T2>& __y)
                     _NOEXCEPT_((__is_nothrow_swappable<_T1>::value &&
                                 __is_nothrow_swappable<_T2>::value))
{
    __x.swap(__y);
}

#if _LIBCPP_STD_VER >= 23
template <class _T1, class _T2>
  requires (__is_swappable<const _T1>::value &&
            __is_swappable<const _T2>::value)
_LIBCPP_HIDE_FROM_ABI constexpr
void swap(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
    noexcept(noexcept(__x.swap(__y)))
{
    __x.swap(__y);
}
#endif

template <class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
pair<typename __unwrap_ref_decay<_T1>::type, typename __unwrap_ref_decay<_T2>::type>
make_pair(_T1&& __t1, _T2&& __t2)
{
    return pair<typename __unwrap_ref_decay<_T1>::type, typename __unwrap_ref_decay<_T2>::type>
               (std::forward<_T1>(__t1), std::forward<_T2>(__t2));
}

template <class _T1, class _T2>
  struct _LIBCPP_TEMPLATE_VIS tuple_size<pair<_T1, _T2> >
    : public integral_constant<size_t, 2> {};

template <size_t _Ip, class _T1, class _T2>
struct _LIBCPP_TEMPLATE_VIS tuple_element<_Ip, pair<_T1, _T2> >
{
    static_assert(_Ip < 2, "Index out of bounds in std::tuple_element<std::pair<T1, T2>>");
};

template <class _T1, class _T2>
struct _LIBCPP_TEMPLATE_VIS tuple_element<0, pair<_T1, _T2> >
{
    using type _LIBCPP_NODEBUG = _T1;
};

template <class _T1, class _T2>
struct _LIBCPP_TEMPLATE_VIS tuple_element<1, pair<_T1, _T2> >
{
    using type _LIBCPP_NODEBUG = _T2;
};

template <size_t _Ip> struct __get_pair;

template <>
struct __get_pair<0>
{
    template <class _T1, class _T2>
    static
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    _T1&
    get(pair<_T1, _T2>& __p) _NOEXCEPT {return __p.first;}

    template <class _T1, class _T2>
    static
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    const _T1&
    get(const pair<_T1, _T2>& __p) _NOEXCEPT {return __p.first;}

    template <class _T1, class _T2>
    static
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    _T1&&
    get(pair<_T1, _T2>&& __p) _NOEXCEPT {return std::forward<_T1>(__p.first);}

    template <class _T1, class _T2>
    static
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    const _T1&&
    get(const pair<_T1, _T2>&& __p) _NOEXCEPT {return std::forward<const _T1>(__p.first);}
};

template <>
struct __get_pair<1>
{
    template <class _T1, class _T2>
    static
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    _T2&
    get(pair<_T1, _T2>& __p) _NOEXCEPT {return __p.second;}

    template <class _T1, class _T2>
    static
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    const _T2&
    get(const pair<_T1, _T2>& __p) _NOEXCEPT {return __p.second;}

    template <class _T1, class _T2>
    static
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    _T2&&
    get(pair<_T1, _T2>&& __p) _NOEXCEPT {return std::forward<_T2>(__p.second);}

    template <class _T1, class _T2>
    static
    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
    const _T2&&
    get(const pair<_T1, _T2>&& __p) _NOEXCEPT {return std::forward<const _T2>(__p.second);}
};

template <size_t _Ip, class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
typename tuple_element<_Ip, pair<_T1, _T2> >::type&
get(pair<_T1, _T2>& __p) _NOEXCEPT
{
    return __get_pair<_Ip>::get(__p);
}

template <size_t _Ip, class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
const typename tuple_element<_Ip, pair<_T1, _T2> >::type&
get(const pair<_T1, _T2>& __p) _NOEXCEPT
{
    return __get_pair<_Ip>::get(__p);
}

template <size_t _Ip, class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
typename tuple_element<_Ip, pair<_T1, _T2> >::type&&
get(pair<_T1, _T2>&& __p) _NOEXCEPT
{
    return __get_pair<_Ip>::get(std::move(__p));
}

template <size_t _Ip, class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
const typename tuple_element<_Ip, pair<_T1, _T2> >::type&&
get(const pair<_T1, _T2>&& __p) _NOEXCEPT
{
    return __get_pair<_Ip>::get(std::move(__p));
}

#if _LIBCPP_STD_VER >= 14
template <class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI
constexpr _T1 & get(pair<_T1, _T2>& __p) _NOEXCEPT
{
    return __get_pair<0>::get(__p);
}

template <class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI
constexpr _T1 const & get(pair<_T1, _T2> const& __p) _NOEXCEPT
{
    return __get_pair<0>::get(__p);
}

template <class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI
constexpr _T1 && get(pair<_T1, _T2>&& __p) _NOEXCEPT
{
    return __get_pair<0>::get(std::move(__p));
}

template <class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI
constexpr _T1 const && get(pair<_T1, _T2> const&& __p) _NOEXCEPT
{
    return __get_pair<0>::get(std::move(__p));
}

template <class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI
constexpr _T1 & get(pair<_T2, _T1>& __p) _NOEXCEPT
{
    return __get_pair<1>::get(__p);
}

template <class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI
constexpr _T1 const & get(pair<_T2, _T1> const& __p) _NOEXCEPT
{
    return __get_pair<1>::get(__p);
}

template <class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI
constexpr _T1 && get(pair<_T2, _T1>&& __p) _NOEXCEPT
{
    return __get_pair<1>::get(std::move(__p));
}

template <class _T1, class _T2>
inline _LIBCPP_HIDE_FROM_ABI
constexpr _T1 const && get(pair<_T2, _T1> const&& __p) _NOEXCEPT
{
    return __get_pair<1>::get(std::move(__p));
}

#endif // _LIBCPP_STD_VER >= 14

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___UTILITY_PAIR_H