include/tuple

// -*- 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_TUPLE
#define _LIBCPP_TUPLE

/*
    tuple synopsis

namespace std
{

template <class... T>
class tuple {
public:
    explicit(see-below) constexpr tuple();
    explicit(see-below) tuple(const T&...);  // constexpr in C++14
    template <class... U>
        explicit(see-below) tuple(U&&...);  // constexpr in C++14
    tuple(const tuple&) = default;
    tuple(tuple&&) = default;
    template <class... U>
        explicit(see-below) tuple(const tuple<U...>&);  // constexpr in C++14
    template <class... U>
        explicit(see-below) tuple(tuple<U...>&&);  // constexpr in C++14
    template <class U1, class U2>
        explicit(see-below) tuple(const pair<U1, U2>&); // iff sizeof...(T) == 2 // constexpr in C++14
    template <class U1, class U2>
        explicit(see-below) tuple(pair<U1, U2>&&); // iff sizeof...(T) == 2  // constexpr in C++14

    // allocator-extended constructors
    template <class Alloc>
        tuple(allocator_arg_t, const Alloc& a);
    template <class Alloc>
        explicit(see-below) tuple(allocator_arg_t, const Alloc& a, const T&...);          // constexpr in C++20
    template <class Alloc, class... U>
        explicit(see-below) tuple(allocator_arg_t, const Alloc& a, U&&...);               // constexpr in C++20
    template <class Alloc>
        tuple(allocator_arg_t, const Alloc& a, const tuple&);                             // constexpr in C++20
    template <class Alloc>
        tuple(allocator_arg_t, const Alloc& a, tuple&&);                                  // constexpr in C++20
    template <class Alloc, class... U>
        explicit(see-below) tuple(allocator_arg_t, const Alloc& a, const tuple<U...>&);   // constexpr in C++20
    template <class Alloc, class... U>
        explicit(see-below) tuple(allocator_arg_t, const Alloc& a, tuple<U...>&&);        // constexpr in C++20
    template <class Alloc, class U1, class U2>
        explicit(see-below) tuple(allocator_arg_t, const Alloc& a, const pair<U1, U2>&);  // constexpr in C++20
    template <class Alloc, class U1, class U2>
        explicit(see-below) tuple(allocator_arg_t, const Alloc& a, pair<U1, U2>&&);       // constexpr in C++20

    tuple& operator=(const tuple&);                                                       // constexpr in C++20
    tuple& operator=(tuple&&) noexcept(is_nothrow_move_assignable_v<T> && ...);           // constexpr in C++20
    template <class... U>
        tuple& operator=(const tuple<U...>&);                                             // constexpr in C++20
    template <class... U>
        tuple& operator=(tuple<U...>&&);                                                  // constexpr in C++20
    template <class U1, class U2>
        tuple& operator=(const pair<U1, U2>&); // iff sizeof...(T) == 2                   // constexpr in C++20
    template <class U1, class U2>
        tuple& operator=(pair<U1, U2>&&); // iff sizeof...(T) == 2                        // constexpr in C++20

    template<class U, size_t N>
        tuple& operator=(array<U, N> const&) // iff sizeof...(T) == N, EXTENSION
    template<class U, size_t N>
        tuple& operator=(array<U, N>&&) // iff sizeof...(T) == N, EXTENSION

    void swap(tuple&) noexcept(AND(swap(declval<T&>(), declval<T&>())...));               // constexpr in C++20
};


template<class... TTypes, class... UTypes, template<class> class TQual, template<class> class UQual> // since C++23
  requires requires { typename tuple<common_reference_t<TQual<TTypes>, UQual<UTypes>>...>; }
struct basic_common_reference<tuple<TTypes...>, tuple<UTypes...>, TQual, UQual> {
  using type = tuple<common_reference_t<TQual<TTypes>, UQual<UTypes>>...>;
};

template<class... TTypes, class... UTypes>                                // since C++23
  requires requires { typename tuple<common_type_t<TTypes, UTypes>...>; }
struct common_type<tuple<TTypes...>, tuple<UTypes...>> {
  using type = tuple<common_type_t<TTypes, UTypes>...>;
};

template <class ...T>
tuple(T...) -> tuple<T...>;                                         // since C++17
template <class T1, class T2>
tuple(pair<T1, T2>) -> tuple<T1, T2>;                               // since C++17
template <class Alloc, class ...T>
tuple(allocator_arg_t, Alloc, T...) -> tuple<T...>;                 // since C++17
template <class Alloc, class T1, class T2>
tuple(allocator_arg_t, Alloc, pair<T1, T2>) -> tuple<T1, T2>;       // since C++17
template <class Alloc, class ...T>
tuple(allocator_arg_t, Alloc, tuple<T...>) -> tuple<T...>;          // since C++17

inline constexpr unspecified ignore;

template <class... T> tuple<V...>  make_tuple(T&&...); // constexpr in C++14
template <class... T> tuple<ATypes...> forward_as_tuple(T&&...) noexcept; // constexpr in C++14
template <class... T> tuple<T&...> tie(T&...) noexcept; // constexpr in C++14
template <class... Tuples> tuple<CTypes...> tuple_cat(Tuples&&... tpls); // constexpr in C++14

// [tuple.apply], calling a function with a tuple of arguments:
template <class F, class Tuple>
  constexpr decltype(auto) apply(F&& f, Tuple&& t); // C++17
template <class T, class Tuple>
  constexpr T make_from_tuple(Tuple&& t); // C++17

// 20.4.1.4, tuple helper classes:
template <class T> struct tuple_size; // undefined
template <class... T> struct tuple_size<tuple<T...>>;
template <class T>
 inline constexpr size_t tuple_size_v = tuple_size<T>::value; // C++17
template <size_t I, class T> struct tuple_element; // undefined
template <size_t I, class... T> struct tuple_element<I, tuple<T...>>;
template <size_t I, class T>
  using tuple_element_t = typename tuple_element <I, T>::type; // C++14

// 20.4.1.5, element access:
template <size_t I, class... T>
    typename tuple_element<I, tuple<T...>>::type&
    get(tuple<T...>&) noexcept; // constexpr in C++14
template <size_t I, class... T>
    const typename tuple_element<I, tuple<T...>>::type&
    get(const tuple<T...>&) noexcept; // constexpr in C++14
template <size_t I, class... T>
    typename tuple_element<I, tuple<T...>>::type&&
    get(tuple<T...>&&) noexcept; // constexpr in C++14
template <size_t I, class... T>
    const typename tuple_element<I, tuple<T...>>::type&&
    get(const tuple<T...>&&) noexcept; // constexpr in C++14

template <class T1, class... T>
    constexpr T1& get(tuple<T...>&) noexcept;  // C++14
template <class T1, class... T>
    constexpr const T1& get(const tuple<T...>&) noexcept;   // C++14
template <class T1, class... T>
    constexpr T1&& get(tuple<T...>&&) noexcept;   // C++14
template <class T1, class... T>
    constexpr const T1&& get(const tuple<T...>&&) noexcept;   // C++14

// 20.4.1.6, relational operators:
template<class... T, class... U> bool operator==(const tuple<T...>&, const tuple<U...>&); // constexpr in C++14
template<class... T, class... U> bool operator<(const tuple<T...>&, const tuple<U...>&);  // constexpr in C++14, removed in C++20
template<class... T, class... U> bool operator!=(const tuple<T...>&, const tuple<U...>&); // constexpr in C++14, removed in C++20
template<class... T, class... U> bool operator>(const tuple<T...>&, const tuple<U...>&);  // constexpr in C++14, removed in C++20
template<class... T, class... U> bool operator<=(const tuple<T...>&, const tuple<U...>&); // constexpr in C++14, removed in C++20
template<class... T, class... U> bool operator>=(const tuple<T...>&, const tuple<U...>&); // constexpr in C++14, removed in C++20
template<class... T, class... U>
  constexpr common_comparison_category_t<synth-three-way-result<T, U>...>
    operator<=>(const tuple<T...>&, const tuple<U...>&);                                  // since C++20

template <class... Types, class Alloc>
  struct uses_allocator<tuple<Types...>, Alloc>;

template <class... Types>
  void
  swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(noexcept(x.swap(y)));

}  // std

*/

#include <__assert> // all public C++ headers provide the assertion handler
#include <__compare/common_comparison_category.h>
#include <__compare/synth_three_way.h>
#include <__config>
#include <__functional/unwrap_ref.h>
#include <__memory/allocator_arg_t.h>
#include <__memory/uses_allocator.h>
#include <__tuple>
#include <__utility/forward.h>
#include <__utility/integer_sequence.h>
#include <__utility/move.h>
#include <__utility/pair.h>
#include <__utility/piecewise_construct.h>
#include <__utility/swap.h>
#include <cstddef>
#include <type_traits>
#include <version>

// standard-mandated includes
#include <compare>

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

_LIBCPP_BEGIN_NAMESPACE_STD

#ifndef _LIBCPP_CXX03_LANG


// __tuple_leaf

template <size_t _Ip, class _Hp,
          bool=is_empty<_Hp>::value && !__libcpp_is_final<_Hp>::value
         >
class __tuple_leaf;

template <size_t _Ip, class _Hp, bool _Ep>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
void swap(__tuple_leaf<_Ip, _Hp, _Ep>& __x, __tuple_leaf<_Ip, _Hp, _Ep>& __y)
    _NOEXCEPT_(__is_nothrow_swappable<_Hp>::value)
{
    swap(__x.get(), __y.get());
}

template <size_t _Ip, class _Hp, bool>
class __tuple_leaf
{
    _Hp __value_;

    template <class _Tp>
    static constexpr bool __can_bind_reference() {
#if __has_keyword(__reference_binds_to_temporary)
      return !__reference_binds_to_temporary(_Hp, _Tp);
#else
      return true;
#endif
    }

    _LIBCPP_CONSTEXPR_AFTER_CXX11
    __tuple_leaf& operator=(const __tuple_leaf&);
public:
    _LIBCPP_INLINE_VISIBILITY constexpr __tuple_leaf()
             _NOEXCEPT_(is_nothrow_default_constructible<_Hp>::value) : __value_()
       {static_assert(!is_reference<_Hp>::value,
              "Attempted to default construct a reference element in a tuple");}

    template <class _Alloc>
        _LIBCPP_INLINE_VISIBILITY constexpr
        __tuple_leaf(integral_constant<int, 0>, const _Alloc&)
            : __value_()
        {static_assert(!is_reference<_Hp>::value,
              "Attempted to default construct a reference element in a tuple");}

    template <class _Alloc>
        _LIBCPP_INLINE_VISIBILITY constexpr
        __tuple_leaf(integral_constant<int, 1>, const _Alloc& __a)
            : __value_(allocator_arg_t(), __a)
        {static_assert(!is_reference<_Hp>::value,
              "Attempted to default construct a reference element in a tuple");}

    template <class _Alloc>
        _LIBCPP_INLINE_VISIBILITY constexpr
        __tuple_leaf(integral_constant<int, 2>, const _Alloc& __a)
            : __value_(__a)
        {static_assert(!is_reference<_Hp>::value,
              "Attempted to default construct a reference element in a tuple");}

    template <class _Tp,
              class = __enable_if_t<
                  _And<
                      _IsNotSame<__uncvref_t<_Tp>, __tuple_leaf>,
                      is_constructible<_Hp, _Tp>
                    >::value
                >
            >
        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
        explicit __tuple_leaf(_Tp&& __t) _NOEXCEPT_((is_nothrow_constructible<_Hp, _Tp>::value))
            : __value_(_VSTD::forward<_Tp>(__t))
        {static_assert(__can_bind_reference<_Tp&&>(),
       "Attempted construction of reference element binds to a temporary whose lifetime has ended");}

    template <class _Tp, class _Alloc>
        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
        explicit __tuple_leaf(integral_constant<int, 0>, const _Alloc&, _Tp&& __t)
            : __value_(_VSTD::forward<_Tp>(__t))
        {static_assert(__can_bind_reference<_Tp&&>(),
       "Attempted construction of reference element binds to a temporary whose lifetime has ended");}

    template <class _Tp, class _Alloc>
        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
        explicit __tuple_leaf(integral_constant<int, 1>, const _Alloc& __a, _Tp&& __t)
            : __value_(allocator_arg_t(), __a, _VSTD::forward<_Tp>(__t))
        {static_assert(!is_reference<_Hp>::value,
            "Attempted to uses-allocator construct a reference element in a tuple");}

    template <class _Tp, class _Alloc>
        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
        explicit __tuple_leaf(integral_constant<int, 2>, const _Alloc& __a, _Tp&& __t)
            : __value_(_VSTD::forward<_Tp>(__t), __a)
        {static_assert(!is_reference<_Hp>::value,
           "Attempted to uses-allocator construct a reference element in a tuple");}

    __tuple_leaf(const __tuple_leaf& __t) = default;
    __tuple_leaf(__tuple_leaf&& __t) = default;

    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    int swap(__tuple_leaf& __t) _NOEXCEPT_(__is_nothrow_swappable<__tuple_leaf>::value)
    {
        _VSTD::swap(*this, __t);
        return 0;
    }

    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11       _Hp& get()       _NOEXCEPT {return __value_;}
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11 const _Hp& get() const _NOEXCEPT {return __value_;}
};

template <size_t _Ip, class _Hp>
class __tuple_leaf<_Ip, _Hp, true>
    : private _Hp
{
    _LIBCPP_CONSTEXPR_AFTER_CXX11
    __tuple_leaf& operator=(const __tuple_leaf&);
public:
    _LIBCPP_INLINE_VISIBILITY constexpr __tuple_leaf()
             _NOEXCEPT_(is_nothrow_default_constructible<_Hp>::value) {}

    template <class _Alloc>
        _LIBCPP_INLINE_VISIBILITY constexpr
        __tuple_leaf(integral_constant<int, 0>, const _Alloc&) {}

    template <class _Alloc>
        _LIBCPP_INLINE_VISIBILITY constexpr
        __tuple_leaf(integral_constant<int, 1>, const _Alloc& __a)
            : _Hp(allocator_arg_t(), __a) {}

    template <class _Alloc>
        _LIBCPP_INLINE_VISIBILITY constexpr
        __tuple_leaf(integral_constant<int, 2>, const _Alloc& __a)
            : _Hp(__a) {}

    template <class _Tp,
              class = __enable_if_t<
                  _And<
                    _IsNotSame<__uncvref_t<_Tp>, __tuple_leaf>,
                    is_constructible<_Hp, _Tp>
                  >::value
                >
            >
        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
        explicit __tuple_leaf(_Tp&& __t) _NOEXCEPT_((is_nothrow_constructible<_Hp, _Tp>::value))
            : _Hp(_VSTD::forward<_Tp>(__t)) {}

    template <class _Tp, class _Alloc>
        _LIBCPP_INLINE_VISIBILITY constexpr
        explicit __tuple_leaf(integral_constant<int, 0>, const _Alloc&, _Tp&& __t)
            : _Hp(_VSTD::forward<_Tp>(__t)) {}

    template <class _Tp, class _Alloc>
        _LIBCPP_INLINE_VISIBILITY constexpr
        explicit __tuple_leaf(integral_constant<int, 1>, const _Alloc& __a, _Tp&& __t)
            : _Hp(allocator_arg_t(), __a, _VSTD::forward<_Tp>(__t)) {}

    template <class _Tp, class _Alloc>
        _LIBCPP_INLINE_VISIBILITY constexpr
        explicit __tuple_leaf(integral_constant<int, 2>, const _Alloc& __a, _Tp&& __t)
            : _Hp(_VSTD::forward<_Tp>(__t), __a) {}

    __tuple_leaf(__tuple_leaf const &) = default;
    __tuple_leaf(__tuple_leaf &&) = default;

    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    int
    swap(__tuple_leaf& __t) _NOEXCEPT_(__is_nothrow_swappable<__tuple_leaf>::value)
    {
        _VSTD::swap(*this, __t);
        return 0;
    }

    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11       _Hp& get()       _NOEXCEPT {return static_cast<_Hp&>(*this);}
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11 const _Hp& get() const _NOEXCEPT {return static_cast<const _Hp&>(*this);}
};

template <class ..._Tp>
_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
void __swallow(_Tp&&...) _NOEXCEPT {}

template <class _Tp>
struct __all_default_constructible;

template <class ..._Tp>
struct __all_default_constructible<__tuple_types<_Tp...>>
    : __all<is_default_constructible<_Tp>::value...>
{ };

// __tuple_impl

template<class _Indx, class ..._Tp> struct __tuple_impl;

template<size_t ..._Indx, class ..._Tp>
struct _LIBCPP_DECLSPEC_EMPTY_BASES __tuple_impl<__tuple_indices<_Indx...>, _Tp...>
    : public __tuple_leaf<_Indx, _Tp>...
{
    _LIBCPP_INLINE_VISIBILITY
    constexpr __tuple_impl()
        _NOEXCEPT_(__all<is_nothrow_default_constructible<_Tp>::value...>::value) {}

    template <size_t ..._Uf, class ..._Tf,
              size_t ..._Ul, class ..._Tl, class ..._Up>
        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
        explicit
        __tuple_impl(__tuple_indices<_Uf...>, __tuple_types<_Tf...>,
                     __tuple_indices<_Ul...>, __tuple_types<_Tl...>,
                     _Up&&... __u)
                     _NOEXCEPT_((__all<is_nothrow_constructible<_Tf, _Up>::value...>::value &&
                                 __all<is_nothrow_default_constructible<_Tl>::value...>::value)) :
            __tuple_leaf<_Uf, _Tf>(_VSTD::forward<_Up>(__u))...,
            __tuple_leaf<_Ul, _Tl>()...
            {}

    template <class _Alloc, size_t ..._Uf, class ..._Tf,
              size_t ..._Ul, class ..._Tl, class ..._Up>
        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
        explicit
        __tuple_impl(allocator_arg_t, const _Alloc& __a,
                     __tuple_indices<_Uf...>, __tuple_types<_Tf...>,
                     __tuple_indices<_Ul...>, __tuple_types<_Tl...>,
                     _Up&&... __u) :
            __tuple_leaf<_Uf, _Tf>(__uses_alloc_ctor<_Tf, _Alloc, _Up>(), __a,
            _VSTD::forward<_Up>(__u))...,
            __tuple_leaf<_Ul, _Tl>(__uses_alloc_ctor<_Tl, _Alloc>(), __a)...
            {}

    template <class _Tuple,
              class = typename enable_if
                      <
                         __tuple_constructible<_Tuple, tuple<_Tp...> >::value
                      >::type
             >
        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
        __tuple_impl(_Tuple&& __t) _NOEXCEPT_((__all<is_nothrow_constructible<_Tp, typename tuple_element<_Indx,
                                       typename __make_tuple_types<_Tuple>::type>::type>::value...>::value))
            : __tuple_leaf<_Indx, _Tp>(_VSTD::forward<typename tuple_element<_Indx,
                                       typename __make_tuple_types<_Tuple>::type>::type>(_VSTD::get<_Indx>(__t)))...
            {}

    template <class _Alloc, class _Tuple,
              class = typename enable_if
                      <
                         __tuple_constructible<_Tuple, tuple<_Tp...> >::value
                      >::type
             >
        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
        __tuple_impl(allocator_arg_t, const _Alloc& __a, _Tuple&& __t)
            : __tuple_leaf<_Indx, _Tp>(__uses_alloc_ctor<_Tp, _Alloc, typename tuple_element<_Indx,
                                       typename __make_tuple_types<_Tuple>::type>::type>(), __a,
                                       _VSTD::forward<typename tuple_element<_Indx,
                                       typename __make_tuple_types<_Tuple>::type>::type>(_VSTD::get<_Indx>(__t)))...
            {}

    __tuple_impl(const __tuple_impl&) = default;
    __tuple_impl(__tuple_impl&&) = default;

    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    void swap(__tuple_impl& __t)
        _NOEXCEPT_(__all<__is_nothrow_swappable<_Tp>::value...>::value)
    {
        _VSTD::__swallow(__tuple_leaf<_Indx, _Tp>::swap(static_cast<__tuple_leaf<_Indx, _Tp>&>(__t))...);
    }
};

template<class _Dest, class _Source, size_t ..._Np>
_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
void __memberwise_copy_assign(_Dest& __dest, _Source const& __source, __tuple_indices<_Np...>) {
    _VSTD::__swallow(((_VSTD::get<_Np>(__dest) = _VSTD::get<_Np>(__source)), void(), 0)...);
}

template<class _Dest, class _Source, class ..._Up, size_t ..._Np>
_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
void __memberwise_forward_assign(_Dest& __dest, _Source&& __source, __tuple_types<_Up...>, __tuple_indices<_Np...>) {
    _VSTD::__swallow(((
        _VSTD::get<_Np>(__dest) = _VSTD::forward<_Up>(_VSTD::get<_Np>(__source))
    ), void(), 0)...);
}

template <class ..._Tp>
class _LIBCPP_TEMPLATE_VIS tuple
{
    typedef __tuple_impl<typename __make_tuple_indices<sizeof...(_Tp)>::type, _Tp...> _BaseT;

    _BaseT __base_;

    template <size_t _Jp, class ..._Up> friend _LIBCPP_CONSTEXPR_AFTER_CXX11
        typename tuple_element<_Jp, tuple<_Up...> >::type& get(tuple<_Up...>&) _NOEXCEPT;
    template <size_t _Jp, class ..._Up> friend _LIBCPP_CONSTEXPR_AFTER_CXX11
        const typename tuple_element<_Jp, tuple<_Up...> >::type& get(const tuple<_Up...>&) _NOEXCEPT;
    template <size_t _Jp, class ..._Up> friend _LIBCPP_CONSTEXPR_AFTER_CXX11
        typename tuple_element<_Jp, tuple<_Up...> >::type&& get(tuple<_Up...>&&) _NOEXCEPT;
    template <size_t _Jp, class ..._Up> friend _LIBCPP_CONSTEXPR_AFTER_CXX11
        const typename tuple_element<_Jp, tuple<_Up...> >::type&& get(const tuple<_Up...>&&) _NOEXCEPT;
public:
    // [tuple.cnstr]

    // tuple() constructors (including allocator_arg_t variants)
    template <template<class...> class _IsImpDefault = __is_implicitly_default_constructible, __enable_if_t<
        _And<
            _IsImpDefault<_Tp>... // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
    tuple()
        _NOEXCEPT_(_And<is_nothrow_default_constructible<_Tp>...>::value)
    { }

    template <template<class...> class _IsImpDefault = __is_implicitly_default_constructible,
              template<class...> class _IsDefault = is_default_constructible, __enable_if_t<
        _And<
            _IsDefault<_Tp>...,
            _Not<_Lazy<_And, _IsImpDefault<_Tp>...> > // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
    explicit tuple()
        _NOEXCEPT_(_And<is_nothrow_default_constructible<_Tp>...>::value)
    { }

    template <class _Alloc, template<class...> class _IsImpDefault = __is_implicitly_default_constructible, __enable_if_t<
        _And<
            _IsImpDefault<_Tp>... // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple(allocator_arg_t, _Alloc const& __a)
      : __base_(allocator_arg_t(), __a,
                    __tuple_indices<>(), __tuple_types<>(),
                    typename __make_tuple_indices<sizeof...(_Tp), 0>::type(),
                    __tuple_types<_Tp...>()) {}

    template <class _Alloc,
              template<class...> class _IsImpDefault = __is_implicitly_default_constructible,
              template<class...> class _IsDefault = is_default_constructible, __enable_if_t<
        _And<
            _IsDefault<_Tp>...,
            _Not<_Lazy<_And, _IsImpDefault<_Tp>...> > // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    explicit tuple(allocator_arg_t, _Alloc const& __a)
      : __base_(allocator_arg_t(), __a,
                    __tuple_indices<>(), __tuple_types<>(),
                    typename __make_tuple_indices<sizeof...(_Tp), 0>::type(),
                    __tuple_types<_Tp...>()) {}

    // tuple(const T&...) constructors (including allocator_arg_t variants)
    template <template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) >= 1>,
            is_copy_constructible<_Tp>...,
            is_convertible<const _Tp&, _Tp>... // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    tuple(const _Tp& ... __t)
        _NOEXCEPT_(_And<is_nothrow_copy_constructible<_Tp>...>::value)
        : __base_(typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
                typename __make_tuple_indices<0>::type(),
                typename __make_tuple_types<tuple, 0>::type(),
                __t...
               ) {}

    template <template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) >= 1>,
            is_copy_constructible<_Tp>...,
            _Not<_Lazy<_And, is_convertible<const _Tp&, _Tp>...> > // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    explicit tuple(const _Tp& ... __t)
        _NOEXCEPT_(_And<is_nothrow_copy_constructible<_Tp>...>::value)
        : __base_(typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
                typename __make_tuple_indices<0>::type(),
                typename __make_tuple_types<tuple, 0>::type(),
                __t...
               ) {}

    template <class _Alloc, template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) >= 1>,
            is_copy_constructible<_Tp>...,
            is_convertible<const _Tp&, _Tp>... // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
        : __base_(allocator_arg_t(), __a,
                typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
                typename __make_tuple_indices<0>::type(),
                typename __make_tuple_types<tuple, 0>::type(),
                __t...
               ) {}

    template <class _Alloc, template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) >= 1>,
            is_copy_constructible<_Tp>...,
            _Not<_Lazy<_And, is_convertible<const _Tp&, _Tp>...> > // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    explicit tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
        : __base_(allocator_arg_t(), __a,
                typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
                typename __make_tuple_indices<0>::type(),
                typename __make_tuple_types<tuple, 0>::type(),
                __t...
               ) {}

    // tuple(U&& ...) constructors (including allocator_arg_t variants)
    template <class ..._Up> struct _IsThisTuple : false_type { };
    template <class _Up> struct _IsThisTuple<_Up> : is_same<__uncvref_t<_Up>, tuple> { };

    template <class ..._Up>
    struct _EnableUTypesCtor : _And<
        _BoolConstant<sizeof...(_Tp) >= 1>,
        _Not<_IsThisTuple<_Up...> >, // extension to allow mis-behaved user constructors
        is_constructible<_Tp, _Up>...
    > { };

    template <class ..._Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableUTypesCtor<_Up...>,
            is_convertible<_Up, _Tp>... // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    tuple(_Up&&... __u)
        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
        : __base_(typename __make_tuple_indices<sizeof...(_Up)>::type(),
                    typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
                    typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),
                    typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                    _VSTD::forward<_Up>(__u)...) {}

    template <class ..._Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableUTypesCtor<_Up...>,
            _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    explicit tuple(_Up&&... __u)
        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
        : __base_(typename __make_tuple_indices<sizeof...(_Up)>::type(),
                    typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
                    typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),
                    typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                    _VSTD::forward<_Up>(__u)...) {}

    template <class _Alloc, class ..._Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableUTypesCtor<_Up...>,
            is_convertible<_Up, _Tp>... // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
        : __base_(allocator_arg_t(), __a,
                    typename __make_tuple_indices<sizeof...(_Up)>::type(),
                    typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
                    typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),
                    typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                    _VSTD::forward<_Up>(__u)...) {}

    template <class _Alloc, class ..._Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableUTypesCtor<_Up...>,
            _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    explicit tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
        : __base_(allocator_arg_t(), __a,
                    typename __make_tuple_indices<sizeof...(_Up)>::type(),
                    typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
                    typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),
                    typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                    _VSTD::forward<_Up>(__u)...) {}

    // Copy and move constructors (including the allocator_arg_t variants)
    tuple(const tuple&) = default;
    tuple(tuple&&) = default;

    template <class _Alloc, template<class...> class _And = _And, __enable_if_t<
        _And<is_copy_constructible<_Tp>...>::value
    , int> = 0>
    tuple(allocator_arg_t, const _Alloc& __alloc, const tuple& __t)
        : __base_(allocator_arg_t(), __alloc, __t)
    { }

    template <class _Alloc, template<class...> class _And = _And, __enable_if_t<
        _And<is_move_constructible<_Tp>...>::value
    , int> = 0>
    tuple(allocator_arg_t, const _Alloc& __alloc, tuple&& __t)
        : __base_(allocator_arg_t(), __alloc, _VSTD::move(__t))
    { }

    // tuple(const tuple<U...>&) constructors (including allocator_arg_t variants)
    template <class ..._Up>
    struct _EnableCopyFromOtherTuple : _And<
        _Not<is_same<tuple<_Tp...>, tuple<_Up...> > >,
        _Lazy<_Or,
            _BoolConstant<sizeof...(_Tp) != 1>,
            // _Tp and _Up are 1-element packs - the pack expansions look
            // weird to avoid tripping up the type traits in degenerate cases
            _Lazy<_And,
                _Not<is_convertible<const tuple<_Up>&, _Tp> >...,
                _Not<is_constructible<_Tp, const tuple<_Up>&> >...
            >
        >,
        is_constructible<_Tp, const _Up&>...
    > { };

    template <class ..._Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableCopyFromOtherTuple<_Up...>,
            is_convertible<const _Up&, _Tp>... // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    tuple(const tuple<_Up...>& __t)
        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, const _Up&>...>::value))
        : __base_(__t)
    { }

    template <class ..._Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableCopyFromOtherTuple<_Up...>,
            _Not<_Lazy<_And, is_convertible<const _Up&, _Tp>...> > // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    explicit tuple(const tuple<_Up...>& __t)
        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, const _Up&>...>::value))
        : __base_(__t)
    { }

    template <class ..._Up, class _Alloc, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableCopyFromOtherTuple<_Up...>,
            is_convertible<const _Up&, _Tp>... // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple(allocator_arg_t, const _Alloc& __a, const tuple<_Up...>& __t)
        : __base_(allocator_arg_t(), __a, __t)
    { }

    template <class ..._Up, class _Alloc, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableCopyFromOtherTuple<_Up...>,
            _Not<_Lazy<_And, is_convertible<const _Up&, _Tp>...> > // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    explicit tuple(allocator_arg_t, const _Alloc& __a, const tuple<_Up...>& __t)
        : __base_(allocator_arg_t(), __a, __t)
    { }

    // tuple(tuple<U...>&&) constructors (including allocator_arg_t variants)
    template <class ..._Up>
    struct _EnableMoveFromOtherTuple : _And<
        _Not<is_same<tuple<_Tp...>, tuple<_Up...> > >,
        _Lazy<_Or,
            _BoolConstant<sizeof...(_Tp) != 1>,
            // _Tp and _Up are 1-element packs - the pack expansions look
            // weird to avoid tripping up the type traits in degenerate cases
            _Lazy<_And,
                _Not<is_convertible<tuple<_Up>, _Tp> >...,
                _Not<is_constructible<_Tp, tuple<_Up> > >...
            >
        >,
        is_constructible<_Tp, _Up>...
    > { };

    template <class ..._Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableMoveFromOtherTuple<_Up...>,
            is_convertible<_Up, _Tp>... // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    tuple(tuple<_Up...>&& __t)
        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
        : __base_(_VSTD::move(__t))
    { }

    template <class ..._Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableMoveFromOtherTuple<_Up...>,
            _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    explicit tuple(tuple<_Up...>&& __t)
        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
        : __base_(_VSTD::move(__t))
    { }

    template <class _Alloc, class ..._Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableMoveFromOtherTuple<_Up...>,
            is_convertible<_Up, _Tp>... // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple(allocator_arg_t, const _Alloc& __a, tuple<_Up...>&& __t)
        : __base_(allocator_arg_t(), __a, _VSTD::move(__t))
    { }

    template <class _Alloc, class ..._Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
            _EnableMoveFromOtherTuple<_Up...>,
            _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    explicit tuple(allocator_arg_t, const _Alloc& __a, tuple<_Up...>&& __t)
        : __base_(allocator_arg_t(), __a, _VSTD::move(__t))
    { }

    // tuple(const pair<U1, U2>&) constructors (including allocator_arg_t variants)
    template <class _Up1, class _Up2, class ..._DependentTp>
    struct _EnableImplicitCopyFromPair : _And<
        is_constructible<_FirstType<_DependentTp...>, const _Up1&>,
        is_constructible<_SecondType<_DependentTp...>, const _Up2&>,
        is_convertible<const _Up1&, _FirstType<_DependentTp...> >, // explicit check
        is_convertible<const _Up2&, _SecondType<_DependentTp...> >
    > { };

    template <class _Up1, class _Up2, class ..._DependentTp>
    struct _EnableExplicitCopyFromPair : _And<
        is_constructible<_FirstType<_DependentTp...>, const _Up1&>,
        is_constructible<_SecondType<_DependentTp...>, const _Up2&>,
        _Not<is_convertible<const _Up1&, _FirstType<_DependentTp...> > >, // explicit check
        _Not<is_convertible<const _Up2&, _SecondType<_DependentTp...> > >
    > { };

    template <class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) == 2>,
            _EnableImplicitCopyFromPair<_Up1, _Up2, _Tp...>
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    tuple(const pair<_Up1, _Up2>& __p)
        _NOEXCEPT_((_And<
            is_nothrow_constructible<_FirstType<_Tp...>, const _Up1&>,
            is_nothrow_constructible<_SecondType<_Tp...>, const _Up2&>
        >::value))
        : __base_(__p)
    { }

    template <class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) == 2>,
            _EnableExplicitCopyFromPair<_Up1, _Up2, _Tp...>
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    explicit tuple(const pair<_Up1, _Up2>& __p)
        _NOEXCEPT_((_And<
            is_nothrow_constructible<_FirstType<_Tp...>, const _Up1&>,
            is_nothrow_constructible<_SecondType<_Tp...>, const _Up2&>
        >::value))
        : __base_(__p)
    { }

    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) == 2>,
            _EnableImplicitCopyFromPair<_Up1, _Up2, _Tp...>
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple(allocator_arg_t, const _Alloc& __a, const pair<_Up1, _Up2>& __p)
        : __base_(allocator_arg_t(), __a, __p)
    { }

    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) == 2>,
            _EnableExplicitCopyFromPair<_Up1, _Up2, _Tp...>
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    explicit tuple(allocator_arg_t, const _Alloc& __a, const pair<_Up1, _Up2>& __p)
        : __base_(allocator_arg_t(), __a, __p)
    { }

    // tuple(pair<U1, U2>&&) constructors (including allocator_arg_t variants)
    template <class _Up1, class _Up2, class ..._DependentTp>
    struct _EnableImplicitMoveFromPair : _And<
        is_constructible<_FirstType<_DependentTp...>, _Up1>,
        is_constructible<_SecondType<_DependentTp...>, _Up2>,
        is_convertible<_Up1, _FirstType<_DependentTp...> >, // explicit check
        is_convertible<_Up2, _SecondType<_DependentTp...> >
    > { };

    template <class _Up1, class _Up2, class ..._DependentTp>
    struct _EnableExplicitMoveFromPair : _And<
        is_constructible<_FirstType<_DependentTp...>, _Up1>,
        is_constructible<_SecondType<_DependentTp...>, _Up2>,
        _Not<is_convertible<_Up1, _FirstType<_DependentTp...> > >, // explicit check
        _Not<is_convertible<_Up2, _SecondType<_DependentTp...> > >
    > { };

    template <class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) == 2>,
            _EnableImplicitMoveFromPair<_Up1, _Up2, _Tp...>
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    tuple(pair<_Up1, _Up2>&& __p)
        _NOEXCEPT_((_And<
            is_nothrow_constructible<_FirstType<_Tp...>, _Up1>,
            is_nothrow_constructible<_SecondType<_Tp...>, _Up2>
        >::value))
        : __base_(_VSTD::move(__p))
    { }

    template <class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) == 2>,
            _EnableExplicitMoveFromPair<_Up1, _Up2, _Tp...>
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    explicit tuple(pair<_Up1, _Up2>&& __p)
        _NOEXCEPT_((_And<
            is_nothrow_constructible<_FirstType<_Tp...>, _Up1>,
            is_nothrow_constructible<_SecondType<_Tp...>, _Up2>
        >::value))
        : __base_(_VSTD::move(__p))
    { }

    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) == 2>,
            _EnableImplicitMoveFromPair<_Up1, _Up2, _Tp...>
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple(allocator_arg_t, const _Alloc& __a, pair<_Up1, _Up2>&& __p)
        : __base_(allocator_arg_t(), __a, _VSTD::move(__p))
    { }

    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) == 2>,
            _EnableExplicitMoveFromPair<_Up1, _Up2, _Tp...>
        >::value
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    explicit tuple(allocator_arg_t, const _Alloc& __a, pair<_Up1, _Up2>&& __p)
        : __base_(allocator_arg_t(), __a, _VSTD::move(__p))
    { }

    // [tuple.assign]
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple& operator=(_If<_And<is_copy_assignable<_Tp>...>::value, tuple, __nat> const& __tuple)
        _NOEXCEPT_((_And<is_nothrow_copy_assignable<_Tp>...>::value))
    {
        _VSTD::__memberwise_copy_assign(*this, __tuple,
            typename __make_tuple_indices<sizeof...(_Tp)>::type());
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple& operator=(_If<_And<is_move_assignable<_Tp>...>::value, tuple, __nat>&& __tuple)
        _NOEXCEPT_((_And<is_nothrow_move_assignable<_Tp>...>::value))
    {
        _VSTD::__memberwise_forward_assign(*this, _VSTD::move(__tuple),
            __tuple_types<_Tp...>(),
            typename __make_tuple_indices<sizeof...(_Tp)>::type());
        return *this;
    }

    template<class... _Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) == sizeof...(_Up)>,
            is_assignable<_Tp&, _Up const&>...
        >::value
    ,int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple& operator=(tuple<_Up...> const& __tuple)
        _NOEXCEPT_((_And<is_nothrow_assignable<_Tp&, _Up const&>...>::value))
    {
        _VSTD::__memberwise_copy_assign(*this, __tuple,
            typename __make_tuple_indices<sizeof...(_Tp)>::type());
        return *this;
    }

    template<class... _Up, __enable_if_t<
        _And<
            _BoolConstant<sizeof...(_Tp) == sizeof...(_Up)>,
            is_assignable<_Tp&, _Up>...
        >::value
    ,int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple& operator=(tuple<_Up...>&& __tuple)
        _NOEXCEPT_((_And<is_nothrow_assignable<_Tp&, _Up>...>::value))
    {
        _VSTD::__memberwise_forward_assign(*this, _VSTD::move(__tuple),
            __tuple_types<_Up...>(),
            typename __make_tuple_indices<sizeof...(_Tp)>::type());
        return *this;
    }

    template<class _Up1, class _Up2, class _Dep = true_type, __enable_if_t<
        _And<_Dep,
            _BoolConstant<sizeof...(_Tp) == 2>,
            is_assignable<_FirstType<_Tp..., _Dep>&, _Up1 const&>,
            is_assignable<_SecondType<_Tp..., _Dep>&, _Up2 const&>
        >::value
    ,int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple& operator=(pair<_Up1, _Up2> const& __pair)
        _NOEXCEPT_((_And<
            is_nothrow_assignable<_FirstType<_Tp...>&, _Up1 const&>,
            is_nothrow_assignable<_SecondType<_Tp...>&, _Up2 const&>
        >::value))
    {
        _VSTD::get<0>(*this) = __pair.first;
        _VSTD::get<1>(*this) = __pair.second;
        return *this;
    }

    template<class _Up1, class _Up2, class _Dep = true_type, __enable_if_t<
        _And<_Dep,
            _BoolConstant<sizeof...(_Tp) == 2>,
            is_assignable<_FirstType<_Tp..., _Dep>&, _Up1>,
            is_assignable<_SecondType<_Tp..., _Dep>&, _Up2>
        >::value
    ,int> = 0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple& operator=(pair<_Up1, _Up2>&& __pair)
        _NOEXCEPT_((_And<
            is_nothrow_assignable<_FirstType<_Tp...>&, _Up1>,
            is_nothrow_assignable<_SecondType<_Tp...>&, _Up2>
        >::value))
    {
        _VSTD::get<0>(*this) = _VSTD::forward<_Up1>(__pair.first);
        _VSTD::get<1>(*this) = _VSTD::forward<_Up2>(__pair.second);
        return *this;
    }

    // EXTENSION
    template<class _Up, size_t _Np, class = __enable_if_t<
        _And<
            _BoolConstant<_Np == sizeof...(_Tp)>,
            is_assignable<_Tp&, _Up const&>...
        >::value
    > >
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple& operator=(array<_Up, _Np> const& __array)
        _NOEXCEPT_((_And<is_nothrow_assignable<_Tp&, _Up const&>...>::value))
    {
        _VSTD::__memberwise_copy_assign(*this, __array,
            typename __make_tuple_indices<sizeof...(_Tp)>::type());
        return *this;
    }

    // EXTENSION
    template<class _Up, size_t _Np, class = void, class = __enable_if_t<
        _And<
            _BoolConstant<_Np == sizeof...(_Tp)>,
            is_assignable<_Tp&, _Up>...
        >::value
    > >
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    tuple& operator=(array<_Up, _Np>&& __array)
        _NOEXCEPT_((_And<is_nothrow_assignable<_Tp&, _Up>...>::value))
    {
        _VSTD::__memberwise_forward_assign(*this, _VSTD::move(__array),
            __tuple_types<_If<true, _Up, _Tp>...>(),
            typename __make_tuple_indices<sizeof...(_Tp)>::type());
        return *this;
    }

    // [tuple.swap]
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    void swap(tuple& __t) _NOEXCEPT_(__all<__is_nothrow_swappable<_Tp>::value...>::value)
        {__base_.swap(__t.__base_);}
};

template <>
class _LIBCPP_TEMPLATE_VIS tuple<>
{
public:
    _LIBCPP_INLINE_VISIBILITY constexpr
        tuple() _NOEXCEPT = default;
    template <class _Alloc>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
        tuple(allocator_arg_t, const _Alloc&) _NOEXCEPT {}
    template <class _Alloc>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
        tuple(allocator_arg_t, const _Alloc&, const tuple&) _NOEXCEPT {}
    template <class _Up>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
        tuple(array<_Up, 0>) _NOEXCEPT {}
    template <class _Alloc, class _Up>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
        tuple(allocator_arg_t, const _Alloc&, array<_Up, 0>) _NOEXCEPT {}
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    void swap(tuple&) _NOEXCEPT {}
};

#if _LIBCPP_STD_VER > 20
template <class... _TTypes, class... _UTypes, template<class> class _TQual, template<class> class _UQual>
    requires requires { typename tuple<common_reference_t<_TQual<_TTypes>, _UQual<_UTypes>>...>; }
struct basic_common_reference<tuple<_TTypes...>, tuple<_UTypes...>, _TQual, _UQual> {
    using type = tuple<common_reference_t<_TQual<_TTypes>, _UQual<_UTypes>>...>;
};

template <class... _TTypes, class... _UTypes>
    requires requires { typename tuple<common_type_t<_TTypes, _UTypes>...>; }
struct common_type<tuple<_TTypes...>, tuple<_UTypes...>> {
    using type = tuple<common_type_t<_TTypes, _UTypes>...>;
};
#endif // _LIBCPP_STD_VER > 20

#if _LIBCPP_STD_VER > 14
template <class ..._Tp>
tuple(_Tp...) -> tuple<_Tp...>;
template <class _Tp1, class _Tp2>
tuple(pair<_Tp1, _Tp2>) -> tuple<_Tp1, _Tp2>;
template <class _Alloc, class ..._Tp>
tuple(allocator_arg_t, _Alloc, _Tp...) -> tuple<_Tp...>;
template <class _Alloc, class _Tp1, class _Tp2>
tuple(allocator_arg_t, _Alloc, pair<_Tp1, _Tp2>) -> tuple<_Tp1, _Tp2>;
template <class _Alloc, class ..._Tp>
tuple(allocator_arg_t, _Alloc, tuple<_Tp...>) -> tuple<_Tp...>;
#endif

template <class ..._Tp>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
typename enable_if
<
    __all<__is_swappable<_Tp>::value...>::value,
    void
>::type
swap(tuple<_Tp...>& __t, tuple<_Tp...>& __u)
                 _NOEXCEPT_(__all<__is_nothrow_swappable<_Tp>::value...>::value)
    {__t.swap(__u);}

// get

template <size_t _Ip, class ..._Tp>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
typename tuple_element<_Ip, tuple<_Tp...> >::type&
get(tuple<_Tp...>& __t) _NOEXCEPT
{
    typedef _LIBCPP_NODEBUG typename tuple_element<_Ip, tuple<_Tp...> >::type type;
    return static_cast<__tuple_leaf<_Ip, type>&>(__t.__base_).get();
}

template <size_t _Ip, class ..._Tp>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
const typename tuple_element<_Ip, tuple<_Tp...> >::type&
get(const tuple<_Tp...>& __t) _NOEXCEPT
{
    typedef _LIBCPP_NODEBUG typename tuple_element<_Ip, tuple<_Tp...> >::type type;
    return static_cast<const __tuple_leaf<_Ip, type>&>(__t.__base_).get();
}

template <size_t _Ip, class ..._Tp>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
typename tuple_element<_Ip, tuple<_Tp...> >::type&&
get(tuple<_Tp...>&& __t) _NOEXCEPT
{
    typedef _LIBCPP_NODEBUG typename tuple_element<_Ip, tuple<_Tp...> >::type type;
    return static_cast<type&&>(
             static_cast<__tuple_leaf<_Ip, type>&&>(__t.__base_).get());
}

template <size_t _Ip, class ..._Tp>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
const typename tuple_element<_Ip, tuple<_Tp...> >::type&&
get(const tuple<_Tp...>&& __t) _NOEXCEPT
{
    typedef _LIBCPP_NODEBUG typename tuple_element<_Ip, tuple<_Tp...> >::type type;
    return static_cast<const type&&>(
             static_cast<const __tuple_leaf<_Ip, type>&&>(__t.__base_).get());
}

#if _LIBCPP_STD_VER > 11

namespace __find_detail {

static constexpr size_t __not_found = static_cast<size_t>(-1);
static constexpr size_t __ambiguous = __not_found - 1;

inline _LIBCPP_INLINE_VISIBILITY
constexpr size_t __find_idx_return(size_t __curr_i, size_t __res, bool __matches) {
    return !__matches ? __res :
        (__res == __not_found ? __curr_i : __ambiguous);
}

template <size_t _Nx>
inline _LIBCPP_INLINE_VISIBILITY
constexpr size_t __find_idx(size_t __i, const bool (&__matches)[_Nx]) {
  return __i == _Nx ? __not_found :
      __find_idx_return(__i, __find_idx(__i + 1, __matches), __matches[__i]);
}

template <class _T1, class ..._Args>
struct __find_exactly_one_checked {
    static constexpr bool __matches[sizeof...(_Args)] = {is_same<_T1, _Args>::value...};
    static constexpr size_t value = __find_detail::__find_idx(0, __matches);
    static_assert(value != __not_found, "type not found in type list" );
    static_assert(value != __ambiguous, "type occurs more than once in type list");
};

template <class _T1>
struct __find_exactly_one_checked<_T1> {
    static_assert(!is_same<_T1, _T1>::value, "type not in empty type list");
};

} // namespace __find_detail

template <typename _T1, typename... _Args>
struct __find_exactly_one_t
    : public __find_detail::__find_exactly_one_checked<_T1, _Args...> {
};

template <class _T1, class... _Args>
inline _LIBCPP_INLINE_VISIBILITY
constexpr _T1& get(tuple<_Args...>& __tup) noexcept
{
    return _VSTD::get<__find_exactly_one_t<_T1, _Args...>::value>(__tup);
}

template <class _T1, class... _Args>
inline _LIBCPP_INLINE_VISIBILITY
constexpr _T1 const& get(tuple<_Args...> const& __tup) noexcept
{
    return _VSTD::get<__find_exactly_one_t<_T1, _Args...>::value>(__tup);
}

template <class _T1, class... _Args>
inline _LIBCPP_INLINE_VISIBILITY
constexpr _T1&& get(tuple<_Args...>&& __tup) noexcept
{
    return _VSTD::get<__find_exactly_one_t<_T1, _Args...>::value>(_VSTD::move(__tup));
}

template <class _T1, class... _Args>
inline _LIBCPP_INLINE_VISIBILITY
constexpr _T1 const&& get(tuple<_Args...> const&& __tup) noexcept
{
    return _VSTD::get<__find_exactly_one_t<_T1, _Args...>::value>(_VSTD::move(__tup));
}

#endif

// tie

template <class ..._Tp>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
tuple<_Tp&...>
tie(_Tp&... __t) _NOEXCEPT
{
    return tuple<_Tp&...>(__t...);
}

template <class _Up>
struct __ignore_t
{
    template <class _Tp>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    const __ignore_t& operator=(_Tp&&) const {return *this;}
};

namespace {
  constexpr __ignore_t<unsigned char> ignore = __ignore_t<unsigned char>();
} // namespace

template <class... _Tp>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
tuple<typename __unwrap_ref_decay<_Tp>::type...>
make_tuple(_Tp&&... __t)
{
    return tuple<typename __unwrap_ref_decay<_Tp>::type...>(_VSTD::forward<_Tp>(__t)...);
}

template <class... _Tp>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
tuple<_Tp&&...>
forward_as_tuple(_Tp&&... __t) _NOEXCEPT
{
    return tuple<_Tp&&...>(_VSTD::forward<_Tp>(__t)...);
}

template <size_t _Ip>
struct __tuple_equal
{
    template <class _Tp, class _Up>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    bool operator()(const _Tp& __x, const _Up& __y)
    {
        return __tuple_equal<_Ip - 1>()(__x, __y) && _VSTD::get<_Ip-1>(__x) == _VSTD::get<_Ip-1>(__y);
    }
};

template <>
struct __tuple_equal<0>
{
    template <class _Tp, class _Up>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    bool operator()(const _Tp&, const _Up&)
    {
        return true;
    }
};

template <class ..._Tp, class ..._Up>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
bool
operator==(const tuple<_Tp...>& __x, const tuple<_Up...>& __y)
{
    static_assert (sizeof...(_Tp) == sizeof...(_Up), "Can't compare tuples of different sizes");
    return __tuple_equal<sizeof...(_Tp)>()(__x, __y);
}

#if _LIBCPP_STD_VER > 17

// operator<=>

template <class ..._Tp, class ..._Up, size_t ..._Is>
_LIBCPP_HIDE_FROM_ABI constexpr
auto
__tuple_compare_three_way(const tuple<_Tp...>& __x, const tuple<_Up...>& __y, index_sequence<_Is...>) {
    common_comparison_category_t<__synth_three_way_result<_Tp, _Up>...> __result = strong_ordering::equal;
    static_cast<void>(((__result = _VSTD::__synth_three_way(_VSTD::get<_Is>(__x), _VSTD::get<_Is>(__y)), __result != 0) || ...));
    return __result;
}

template <class ..._Tp, class ..._Up>
requires (sizeof...(_Tp) == sizeof...(_Up))
_LIBCPP_HIDE_FROM_ABI constexpr
common_comparison_category_t<__synth_three_way_result<_Tp, _Up>...>
operator<=>(const tuple<_Tp...>& __x, const tuple<_Up...>& __y)
{
    return _VSTD::__tuple_compare_three_way(__x, __y, index_sequence_for<_Tp...>{});
}

#else // _LIBCPP_STD_VER > 17

template <class ..._Tp, class ..._Up>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
bool
operator!=(const tuple<_Tp...>& __x, const tuple<_Up...>& __y)
{
    return !(__x == __y);
}

template <size_t _Ip>
struct __tuple_less
{
    template <class _Tp, class _Up>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    bool operator()(const _Tp& __x, const _Up& __y)
    {
        const size_t __idx = tuple_size<_Tp>::value - _Ip;
        if (_VSTD::get<__idx>(__x) < _VSTD::get<__idx>(__y))
            return true;
        if (_VSTD::get<__idx>(__y) < _VSTD::get<__idx>(__x))
            return false;
        return __tuple_less<_Ip-1>()(__x, __y);
    }
};

template <>
struct __tuple_less<0>
{
    template <class _Tp, class _Up>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    bool operator()(const _Tp&, const _Up&)
    {
        return false;
    }
};

template <class ..._Tp, class ..._Up>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
bool
operator<(const tuple<_Tp...>& __x, const tuple<_Up...>& __y)
{
    static_assert (sizeof...(_Tp) == sizeof...(_Up), "Can't compare tuples of different sizes");
    return __tuple_less<sizeof...(_Tp)>()(__x, __y);
}

template <class ..._Tp, class ..._Up>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
bool
operator>(const tuple<_Tp...>& __x, const tuple<_Up...>& __y)
{
    return __y < __x;
}

template <class ..._Tp, class ..._Up>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
bool
operator>=(const tuple<_Tp...>& __x, const tuple<_Up...>& __y)
{
    return !(__x < __y);
}

template <class ..._Tp, class ..._Up>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
bool
operator<=(const tuple<_Tp...>& __x, const tuple<_Up...>& __y)
{
    return !(__y < __x);
}

#endif // _LIBCPP_STD_VER > 17

// tuple_cat

template <class _Tp, class _Up> struct __tuple_cat_type;

template <class ..._Ttypes, class ..._Utypes>
struct __tuple_cat_type<tuple<_Ttypes...>, __tuple_types<_Utypes...> >
{
    typedef _LIBCPP_NODEBUG tuple<_Ttypes..., _Utypes...> type;
};

template <class _ResultTuple, bool _Is_Tuple0TupleLike, class ..._Tuples>
struct __tuple_cat_return_1
{
};

template <class ..._Types, class _Tuple0>
struct __tuple_cat_return_1<tuple<_Types...>, true, _Tuple0>
{
  using type _LIBCPP_NODEBUG = typename __tuple_cat_type<
      tuple<_Types...>,
      typename __make_tuple_types<__uncvref_t<_Tuple0> >::type
    >::type;
};

template <class ..._Types, class _Tuple0, class _Tuple1, class ..._Tuples>
struct __tuple_cat_return_1<tuple<_Types...>, true, _Tuple0, _Tuple1, _Tuples...>
    : public __tuple_cat_return_1<
                 typename __tuple_cat_type<
                     tuple<_Types...>,
                     typename __make_tuple_types<__uncvref_t<_Tuple0> >::type
                 >::type,
                 __tuple_like<typename remove_reference<_Tuple1>::type>::value,
                 _Tuple1, _Tuples...>
{
};

template <class ..._Tuples> struct __tuple_cat_return;

template <class _Tuple0, class ..._Tuples>
struct __tuple_cat_return<_Tuple0, _Tuples...>
    : public __tuple_cat_return_1<tuple<>,
         __tuple_like<typename remove_reference<_Tuple0>::type>::value, _Tuple0,
                                                                     _Tuples...>
{
};

template <>
struct __tuple_cat_return<>
{
    typedef _LIBCPP_NODEBUG tuple<> type;
};

inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
tuple<>
tuple_cat()
{
    return tuple<>();
}

template <class _Rp, class _Indices, class _Tuple0, class ..._Tuples>
struct __tuple_cat_return_ref_imp;

template <class ..._Types, size_t ..._I0, class _Tuple0>
struct __tuple_cat_return_ref_imp<tuple<_Types...>, __tuple_indices<_I0...>, _Tuple0>
{
    typedef _LIBCPP_NODEBUG typename remove_reference<_Tuple0>::type _T0;
    typedef tuple<_Types..., typename __apply_cv<_Tuple0,
                          typename tuple_element<_I0, _T0>::type>::type&&...> type;
};

template <class ..._Types, size_t ..._I0, class _Tuple0, class _Tuple1, class ..._Tuples>
struct __tuple_cat_return_ref_imp<tuple<_Types...>, __tuple_indices<_I0...>,
                                  _Tuple0, _Tuple1, _Tuples...>
    : public __tuple_cat_return_ref_imp<
         tuple<_Types..., typename __apply_cv<_Tuple0,
               typename tuple_element<_I0,
                  typename remove_reference<_Tuple0>::type>::type>::type&&...>,
         typename __make_tuple_indices<tuple_size<typename
                                 remove_reference<_Tuple1>::type>::value>::type,
         _Tuple1, _Tuples...>
{
};

template <class _Tuple0, class ..._Tuples>
struct __tuple_cat_return_ref
    : public __tuple_cat_return_ref_imp<tuple<>,
               typename __make_tuple_indices<
                        tuple_size<typename remove_reference<_Tuple0>::type>::value
               >::type, _Tuple0, _Tuples...>
{
};

template <class _Types, class _I0, class _J0>
struct __tuple_cat;

template <class ..._Types, size_t ..._I0, size_t ..._J0>
struct __tuple_cat<tuple<_Types...>, __tuple_indices<_I0...>, __tuple_indices<_J0...> >
{
    template <class _Tuple0>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    typename __tuple_cat_return_ref<tuple<_Types...>&&, _Tuple0&&>::type
    operator()(tuple<_Types...> __t, _Tuple0&& __t0)
    {
        (void)__t; // avoid unused parameter warning on GCC when _I0 is empty
        return _VSTD::forward_as_tuple(
            _VSTD::forward<_Types>(_VSTD::get<_I0>(__t))...,
            _VSTD::get<_J0>(_VSTD::forward<_Tuple0>(__t0))...);
    }

    template <class _Tuple0, class _Tuple1, class ..._Tuples>
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
    typename __tuple_cat_return_ref<tuple<_Types...>&&, _Tuple0&&, _Tuple1&&, _Tuples&&...>::type
    operator()(tuple<_Types...> __t, _Tuple0&& __t0, _Tuple1&& __t1, _Tuples&& ...__tpls)
    {
        (void)__t; // avoid unused parameter warning on GCC when _I0 is empty
        typedef _LIBCPP_NODEBUG typename remove_reference<_Tuple0>::type _T0;
        typedef _LIBCPP_NODEBUG typename remove_reference<_Tuple1>::type _T1;
        return __tuple_cat<
            tuple<_Types...,
                  typename __apply_cv<_Tuple0, typename tuple_element<
                                                   _J0, _T0>::type>::type&&...>,
            typename __make_tuple_indices<sizeof...(_Types) +
                                          tuple_size<_T0>::value>::type,
            typename __make_tuple_indices<tuple_size<_T1>::value>::type>()(
            _VSTD::forward_as_tuple(
                _VSTD::forward<_Types>(_VSTD::get<_I0>(__t))...,
                _VSTD::get<_J0>(_VSTD::forward<_Tuple0>(__t0))...),
            _VSTD::forward<_Tuple1>(__t1), _VSTD::forward<_Tuples>(__tpls)...);
    }
};

template <class _Tuple0, class... _Tuples>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
typename __tuple_cat_return<_Tuple0, _Tuples...>::type
tuple_cat(_Tuple0&& __t0, _Tuples&&... __tpls)
{
    typedef _LIBCPP_NODEBUG typename remove_reference<_Tuple0>::type _T0;
    return __tuple_cat<tuple<>, __tuple_indices<>,
                  typename __make_tuple_indices<tuple_size<_T0>::value>::type>()
                  (tuple<>(), _VSTD::forward<_Tuple0>(__t0),
                                            _VSTD::forward<_Tuples>(__tpls)...);
}

template <class ..._Tp, class _Alloc>
struct _LIBCPP_TEMPLATE_VIS uses_allocator<tuple<_Tp...>, _Alloc>
    : true_type {};

template <class _T1, class _T2>
template <class... _Args1, class... _Args2, size_t ..._I1, size_t ..._I2>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
pair<_T1, _T2>::pair(piecewise_construct_t,
                     tuple<_Args1...>& __first_args, tuple<_Args2...>& __second_args,
                     __tuple_indices<_I1...>, __tuple_indices<_I2...>)
    :  first(_VSTD::forward<_Args1>(_VSTD::get<_I1>( __first_args))...),
      second(_VSTD::forward<_Args2>(_VSTD::get<_I2>(__second_args))...)
{
}

#if _LIBCPP_STD_VER > 14
template <class _Tp>
inline constexpr size_t tuple_size_v = tuple_size<_Tp>::value;

#define _LIBCPP_NOEXCEPT_RETURN(...) noexcept(noexcept(__VA_ARGS__)) { return __VA_ARGS__; }

template <class _Fn, class _Tuple, size_t ..._Id>
inline _LIBCPP_INLINE_VISIBILITY
constexpr decltype(auto) __apply_tuple_impl(_Fn && __f, _Tuple && __t,
                                            __tuple_indices<_Id...>)
_LIBCPP_NOEXCEPT_RETURN(
    _VSTD::__invoke(
        _VSTD::forward<_Fn>(__f),
        _VSTD::get<_Id>(_VSTD::forward<_Tuple>(__t))...)
)

template <class _Fn, class _Tuple>
inline _LIBCPP_INLINE_VISIBILITY
constexpr decltype(auto) apply(_Fn && __f, _Tuple && __t)
_LIBCPP_NOEXCEPT_RETURN(
    _VSTD::__apply_tuple_impl(
        _VSTD::forward<_Fn>(__f), _VSTD::forward<_Tuple>(__t),
        typename __make_tuple_indices<tuple_size_v<remove_reference_t<_Tuple>>>::type{})
)

template <class _Tp, class _Tuple, size_t... _Idx>
inline _LIBCPP_INLINE_VISIBILITY
constexpr _Tp __make_from_tuple_impl(_Tuple&& __t, __tuple_indices<_Idx...>)
_LIBCPP_NOEXCEPT_RETURN(
    _Tp(_VSTD::get<_Idx>(_VSTD::forward<_Tuple>(__t))...)
)

template <class _Tp, class _Tuple>
inline _LIBCPP_INLINE_VISIBILITY
constexpr _Tp make_from_tuple(_Tuple&& __t)
_LIBCPP_NOEXCEPT_RETURN(
    _VSTD::__make_from_tuple_impl<_Tp>(_VSTD::forward<_Tuple>(__t),
        typename __make_tuple_indices<tuple_size_v<remove_reference_t<_Tuple>>>::type{})
)

#undef _LIBCPP_NOEXCEPT_RETURN

#endif // _LIBCPP_STD_VER > 14

#endif // !defined(_LIBCPP_CXX03_LANG)

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP_TUPLE