// <tuple> -*- C++ -*-

// Copyright (C) 2007-2017 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file include/tuple
 *  This is a Standard C++ Library header.
 */

#ifndef _GLIBCXX_TUPLE
#define _GLIBCXX_TUPLE 1

#pragma GCC system_header

#if __cplusplus < 201103L
# include <bits/c++0x_warning.h>
#else

#include <utility>
#include <array>
#include <bits/uses_allocator.h>
#include <bits/invoke.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  /**
   *  @addtogroup utilities
   *  @{
   */

  template<typename... _Elements>
    class tuple;

  template<typename _Tp>
    struct __is_empty_non_tuple : is_empty<_Tp> { };

  // Using EBO for elements that are tuples causes ambiguous base errors.
  template<typename _El0, typename... _El>
    struct __is_empty_non_tuple<tuple<_El0, _El...>> : false_type { };

  // Use the Empty Base-class Optimization for empty, non-final types.
  template<typename _Tp>
    using __empty_not_final
    = typename conditional<__is_final(_Tp), false_type,
			   __is_empty_non_tuple<_Tp>>::type;

  template<std::size_t _Idx, typename _Head,
	   bool = __empty_not_final<_Head>::value>
    struct _Head_base;

  template<std::size_t _Idx, typename _Head>
    struct _Head_base<_Idx, _Head, true>
    : public _Head
    {
      constexpr _Head_base()
      : _Head() { }

      constexpr _Head_base(const _Head& __h)
      : _Head(__h) { }

      constexpr _Head_base(const _Head_base&) = default;
      constexpr _Head_base(_Head_base&&) = default;

      template<typename _UHead>
        constexpr _Head_base(_UHead&& __h)
	: _Head(std::forward<_UHead>(__h)) { }

      _Head_base(allocator_arg_t, __uses_alloc0)
      : _Head() { }

      template<typename _Alloc>
	_Head_base(allocator_arg_t, __uses_alloc1<_Alloc> __a)
	: _Head(allocator_arg, *__a._M_a) { }

      template<typename _Alloc>
	_Head_base(allocator_arg_t, __uses_alloc2<_Alloc> __a)
	: _Head(*__a._M_a) { }

      template<typename _UHead>
	_Head_base(__uses_alloc0, _UHead&& __uhead)
	: _Head(std::forward<_UHead>(__uhead)) { }

      template<typename _Alloc, typename _UHead>
	_Head_base(__uses_alloc1<_Alloc> __a, _UHead&& __uhead)
	: _Head(allocator_arg, *__a._M_a, std::forward<_UHead>(__uhead)) { }

      template<typename _Alloc, typename _UHead>
	_Head_base(__uses_alloc2<_Alloc> __a, _UHead&& __uhead)
	: _Head(std::forward<_UHead>(__uhead), *__a._M_a) { }

      static constexpr _Head&
      _M_head(_Head_base& __b) noexcept { return __b; }

      static constexpr const _Head&
      _M_head(const _Head_base& __b) noexcept { return __b; }
    };

  template<std::size_t _Idx, typename _Head>
    struct _Head_base<_Idx, _Head, false>
    {
      constexpr _Head_base()
      : _M_head_impl() { }

      constexpr _Head_base(const _Head& __h)
      : _M_head_impl(__h) { }

      constexpr _Head_base(const _Head_base&) = default;
      constexpr _Head_base(_Head_base&&) = default;

      template<typename _UHead>
        constexpr _Head_base(_UHead&& __h)
	: _M_head_impl(std::forward<_UHead>(__h)) { }

      _Head_base(allocator_arg_t, __uses_alloc0)
      : _M_head_impl() { }

      template<typename _Alloc>
	_Head_base(allocator_arg_t, __uses_alloc1<_Alloc> __a)
	: _M_head_impl(allocator_arg, *__a._M_a) { }

      template<typename _Alloc>
	_Head_base(allocator_arg_t, __uses_alloc2<_Alloc> __a)
	: _M_head_impl(*__a._M_a) { }

      template<typename _UHead>
	_Head_base(__uses_alloc0, _UHead&& __uhead)
	: _M_head_impl(std::forward<_UHead>(__uhead)) { }

      template<typename _Alloc, typename _UHead>
	_Head_base(__uses_alloc1<_Alloc> __a, _UHead&& __uhead)
	: _M_head_impl(allocator_arg, *__a._M_a, std::forward<_UHead>(__uhead))
	{ }

      template<typename _Alloc, typename _UHead>
	_Head_base(__uses_alloc2<_Alloc> __a, _UHead&& __uhead)
	: _M_head_impl(std::forward<_UHead>(__uhead), *__a._M_a) { }

      static constexpr _Head&
      _M_head(_Head_base& __b) noexcept { return __b._M_head_impl; }

      static constexpr const _Head&
      _M_head(const _Head_base& __b) noexcept { return __b._M_head_impl; }

      _Head _M_head_impl;
    };

  /**
   * Contains the actual implementation of the @c tuple template, stored
   * as a recursive inheritance hierarchy from the first element (most
   * derived class) to the last (least derived class). The @c Idx
   * parameter gives the 0-based index of the element stored at this
   * point in the hierarchy; we use it to implement a constant-time
   * get() operation.
   */
  template<std::size_t _Idx, typename... _Elements>
    struct _Tuple_impl;

  /**
   * Recursive tuple implementation. Here we store the @c Head element
   * and derive from a @c Tuple_impl containing the remaining elements
   * (which contains the @c Tail).
   */
  template<std::size_t _Idx, typename _Head, typename... _Tail>
    struct _Tuple_impl<_Idx, _Head, _Tail...>
    : public _Tuple_impl<_Idx + 1, _Tail...>,
      private _Head_base<_Idx, _Head>
    {
      template<std::size_t, typename...> friend class _Tuple_impl;

      typedef _Tuple_impl<_Idx + 1, _Tail...> _Inherited;
      typedef _Head_base<_Idx, _Head> _Base;

      static constexpr _Head&
      _M_head(_Tuple_impl& __t) noexcept { return _Base::_M_head(__t); }

      static constexpr const _Head&
      _M_head(const _Tuple_impl& __t) noexcept { return _Base::_M_head(__t); }

      static constexpr _Inherited&
      _M_tail(_Tuple_impl& __t) noexcept { return __t; }

      static constexpr const _Inherited&
      _M_tail(const _Tuple_impl& __t) noexcept { return __t; }

      constexpr _Tuple_impl()
      : _Inherited(), _Base() { }

      explicit
      constexpr _Tuple_impl(const _Head& __head, const _Tail&... __tail)
      : _Inherited(__tail...), _Base(__head) { }

      template<typename _UHead, typename... _UTail, typename = typename
               enable_if<sizeof...(_Tail) == sizeof...(_UTail)>::type>
        explicit
        constexpr _Tuple_impl(_UHead&& __head, _UTail&&... __tail)
	: _Inherited(std::forward<_UTail>(__tail)...),
	  _Base(std::forward<_UHead>(__head)) { }

      constexpr _Tuple_impl(const _Tuple_impl&) = default;

      constexpr
      _Tuple_impl(_Tuple_impl&& __in)
      noexcept(__and_<is_nothrow_move_constructible<_Head>,
	              is_nothrow_move_constructible<_Inherited>>::value)
      : _Inherited(std::move(_M_tail(__in))),
	_Base(std::forward<_Head>(_M_head(__in))) { }

      template<typename... _UElements>
        constexpr _Tuple_impl(const _Tuple_impl<_Idx, _UElements...>& __in)
	: _Inherited(_Tuple_impl<_Idx, _UElements...>::_M_tail(__in)),
	  _Base(_Tuple_impl<_Idx, _UElements...>::_M_head(__in)) { }

      template<typename _UHead, typename... _UTails>
        constexpr _Tuple_impl(_Tuple_impl<_Idx, _UHead, _UTails...>&& __in)
	: _Inherited(std::move
		     (_Tuple_impl<_Idx, _UHead, _UTails...>::_M_tail(__in))),
	  _Base(std::forward<_UHead>
		(_Tuple_impl<_Idx, _UHead, _UTails...>::_M_head(__in))) { }

      template<typename _Alloc>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a)
	: _Inherited(__tag, __a),
          _Base(__tag, __use_alloc<_Head>(__a)) { }

      template<typename _Alloc>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
		    const _Head& __head, const _Tail&... __tail)
	: _Inherited(__tag, __a, __tail...),
          _Base(__use_alloc<_Head, _Alloc, _Head>(__a), __head) { }

      template<typename _Alloc, typename _UHead, typename... _UTail,
               typename = typename enable_if<sizeof...(_Tail)
					     == sizeof...(_UTail)>::type>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
	            _UHead&& __head, _UTail&&... __tail)
	: _Inherited(__tag, __a, std::forward<_UTail>(__tail)...),
          _Base(__use_alloc<_Head, _Alloc, _UHead>(__a),
	        std::forward<_UHead>(__head)) { }

      template<typename _Alloc>
        _Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
	            const _Tuple_impl& __in)
	: _Inherited(__tag, __a, _M_tail(__in)),
          _Base(__use_alloc<_Head, _Alloc, _Head>(__a), _M_head(__in)) { }

      template<typename _Alloc>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
	            _Tuple_impl&& __in)
	: _Inherited(__tag, __a, std::move(_M_tail(__in))),
	  _Base(__use_alloc<_Head, _Alloc, _Head>(__a),
	        std::forward<_Head>(_M_head(__in))) { }

      template<typename _Alloc, typename... _UElements>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
	            const _Tuple_impl<_Idx, _UElements...>& __in)
	: _Inherited(__tag, __a,
		     _Tuple_impl<_Idx, _UElements...>::_M_tail(__in)),
	  _Base(__use_alloc<_Head, _Alloc, _Head>(__a),
		_Tuple_impl<_Idx, _UElements...>::_M_head(__in)) { }

      template<typename _Alloc, typename _UHead, typename... _UTails>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
	            _Tuple_impl<_Idx, _UHead, _UTails...>&& __in)
	: _Inherited(__tag, __a, std::move
		     (_Tuple_impl<_Idx, _UHead, _UTails...>::_M_tail(__in))),
	  _Base(__use_alloc<_Head, _Alloc, _UHead>(__a),
                std::forward<_UHead>
		(_Tuple_impl<_Idx, _UHead, _UTails...>::_M_head(__in))) { }

      _Tuple_impl&
      operator=(const _Tuple_impl& __in)
      {
	_M_head(*this) = _M_head(__in);
	_M_tail(*this) = _M_tail(__in);
	return *this;
      }

      _Tuple_impl&
      operator=(_Tuple_impl&& __in)
      noexcept(__and_<is_nothrow_move_assignable<_Head>,
	              is_nothrow_move_assignable<_Inherited>>::value)
      {
	_M_head(*this) = std::forward<_Head>(_M_head(__in));
	_M_tail(*this) = std::move(_M_tail(__in));
	return *this;
      }

      template<typename... _UElements>
        _Tuple_impl&
        operator=(const _Tuple_impl<_Idx, _UElements...>& __in)
        {
	  _M_head(*this) = _Tuple_impl<_Idx, _UElements...>::_M_head(__in);
	  _M_tail(*this) = _Tuple_impl<_Idx, _UElements...>::_M_tail(__in);
	  return *this;
	}

      template<typename _UHead, typename... _UTails>
        _Tuple_impl&
        operator=(_Tuple_impl<_Idx, _UHead, _UTails...>&& __in)
        {
	  _M_head(*this) = std::forward<_UHead>
	    (_Tuple_impl<_Idx, _UHead, _UTails...>::_M_head(__in));
	  _M_tail(*this) = std::move
	    (_Tuple_impl<_Idx, _UHead, _UTails...>::_M_tail(__in));
	  return *this;
	}

    protected:
      void
      _M_swap(_Tuple_impl& __in)
      noexcept(__is_nothrow_swappable<_Head>::value
               && noexcept(_M_tail(__in)._M_swap(_M_tail(__in))))
      {
	using std::swap;
	swap(_M_head(*this), _M_head(__in));
	_Inherited::_M_swap(_M_tail(__in));
      }
    };

  // Basis case of inheritance recursion.
  template<std::size_t _Idx, typename _Head>
    struct _Tuple_impl<_Idx, _Head>
    : private _Head_base<_Idx, _Head>
    {
      template<std::size_t, typename...> friend class _Tuple_impl;

      typedef _Head_base<_Idx, _Head> _Base;

      static constexpr _Head&
      _M_head(_Tuple_impl& __t) noexcept { return _Base::_M_head(__t); }

      static constexpr const _Head&
      _M_head(const _Tuple_impl& __t) noexcept { return _Base::_M_head(__t); }

      constexpr _Tuple_impl()
      : _Base() { }

      explicit
      constexpr _Tuple_impl(const _Head& __head)
      : _Base(__head) { }

      template<typename _UHead>
        explicit
        constexpr _Tuple_impl(_UHead&& __head)
	: _Base(std::forward<_UHead>(__head)) { }

      constexpr _Tuple_impl(const _Tuple_impl&) = default;

      constexpr
      _Tuple_impl(_Tuple_impl&& __in)
      noexcept(is_nothrow_move_constructible<_Head>::value)
      : _Base(std::forward<_Head>(_M_head(__in))) { }

      template<typename _UHead>
        constexpr _Tuple_impl(const _Tuple_impl<_Idx, _UHead>& __in)
	: _Base(_Tuple_impl<_Idx, _UHead>::_M_head(__in)) { }

      template<typename _UHead>
        constexpr _Tuple_impl(_Tuple_impl<_Idx, _UHead>&& __in)
	: _Base(std::forward<_UHead>(_Tuple_impl<_Idx, _UHead>::_M_head(__in)))
	{ }

      template<typename _Alloc>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a)
	: _Base(__tag, __use_alloc<_Head>(__a)) { }

      template<typename _Alloc>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
		    const _Head& __head)
	: _Base(__use_alloc<_Head, _Alloc, _Head>(__a), __head) { }

      template<typename _Alloc, typename _UHead>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
	            _UHead&& __head)
	: _Base(__use_alloc<_Head, _Alloc, _UHead>(__a),
	        std::forward<_UHead>(__head)) { }

      template<typename _Alloc>
        _Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
	            const _Tuple_impl& __in)
	: _Base(__use_alloc<_Head, _Alloc, _Head>(__a), _M_head(__in)) { }

      template<typename _Alloc>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
	            _Tuple_impl&& __in)
	: _Base(__use_alloc<_Head, _Alloc, _Head>(__a),
	        std::forward<_Head>(_M_head(__in))) { }

      template<typename _Alloc, typename _UHead>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
	            const _Tuple_impl<_Idx, _UHead>& __in)
	: _Base(__use_alloc<_Head, _Alloc, _Head>(__a),
		_Tuple_impl<_Idx, _UHead>::_M_head(__in)) { }

      template<typename _Alloc, typename _UHead>
	_Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
	            _Tuple_impl<_Idx, _UHead>&& __in)
	: _Base(__use_alloc<_Head, _Alloc, _UHead>(__a),
                std::forward<_UHead>(_Tuple_impl<_Idx, _UHead>::_M_head(__in)))
	{ }

      _Tuple_impl&
      operator=(const _Tuple_impl& __in)
      {
	_M_head(*this) = _M_head(__in);
	return *this;
      }

      _Tuple_impl&
      operator=(_Tuple_impl&& __in)
      noexcept(is_nothrow_move_assignable<_Head>::value)
      {
	_M_head(*this) = std::forward<_Head>(_M_head(__in));
	return *this;
      }

      template<typename _UHead>
        _Tuple_impl&
        operator=(const _Tuple_impl<_Idx, _UHead>& __in)
        {
	  _M_head(*this) = _Tuple_impl<_Idx, _UHead>::_M_head(__in);
	  return *this;
	}

      template<typename _UHead>
        _Tuple_impl&
        operator=(_Tuple_impl<_Idx, _UHead>&& __in)
        {
	  _M_head(*this)
	    = std::forward<_UHead>(_Tuple_impl<_Idx, _UHead>::_M_head(__in));
	  return *this;
	}

    protected:
      void
      _M_swap(_Tuple_impl& __in)
      noexcept(__is_nothrow_swappable<_Head>::value)
      {
	using std::swap;
	swap(_M_head(*this), _M_head(__in));
      }
    };

  // Concept utility functions, reused in conditionally-explicit
  // constructors.
  template<bool, typename... _Elements>
  struct _TC
  {
    template<typename... _UElements>
    static constexpr bool _ConstructibleTuple()
    {
      return __and_<is_constructible<_Elements, const _UElements&>...>::value;
    }

    template<typename... _UElements>
    static constexpr bool _ImplicitlyConvertibleTuple()
    {
      return __and_<is_convertible<const _UElements&, _Elements>...>::value;
    }

    template<typename... _UElements>
    static constexpr bool _MoveConstructibleTuple()
    {
      return __and_<is_constructible<_Elements, _UElements&&>...>::value;
    }

    template<typename... _UElements>
    static constexpr bool _ImplicitlyMoveConvertibleTuple()
    {
      return __and_<is_convertible<_UElements&&, _Elements>...>::value;
    }

    template<typename _SrcTuple>
    static constexpr bool _NonNestedTuple()
    {
      return  __and_<__not_<is_same<tuple<_Elements...>,
                                   typename remove_cv<
                                     typename remove_reference<_SrcTuple>::type
                                   >::type>>,
                     __not_<is_convertible<_SrcTuple, _Elements...>>,
                     __not_<is_constructible<_Elements..., _SrcTuple>>
              >::value;
    }
    template<typename... _UElements>
    static constexpr bool _NotSameTuple()
    {
      return  __not_<is_same<tuple<_Elements...>,
			     typename remove_const<
			       typename remove_reference<_UElements...>::type
			       >::type>>::value;
    }
  };

  template<typename... _Elements>
  struct _TC<false, _Elements...>
  {
    template<typename... _UElements>
    static constexpr bool _ConstructibleTuple()
    {
      return false;
    }

    template<typename... _UElements>
    static constexpr bool _ImplicitlyConvertibleTuple()
    {
      return false;
    }

    template<typename... _UElements>
    static constexpr bool _MoveConstructibleTuple()
    {
      return false;
    }

    template<typename... _UElements>
    static constexpr bool _ImplicitlyMoveConvertibleTuple()
    {
      return false;
    }

    template<typename... _UElements>
    static constexpr bool _NonNestedTuple()
    {
      return true;
    }
    template<typename... _UElements>
    static constexpr bool _NotSameTuple()
    {
      return  true;
    }
  };

  /// Primary class template, tuple
  template<typename... _Elements>
    class tuple : public _Tuple_impl<0, _Elements...>
    {
      typedef _Tuple_impl<0, _Elements...> _Inherited;

      // Used for constraining the default constructor so
      // that it becomes dependent on the constraints.
      template<typename _Dummy>
      struct _TC2
      {
        static constexpr bool _DefaultConstructibleTuple()
        {
          return __and_<is_default_constructible<_Elements>...>::value;
        }
        static constexpr bool _ImplicitlyDefaultConstructibleTuple()
        {
          return __and_<__is_implicitly_default_constructible<_Elements>...>
            ::value;
        }
      };

    public:
      template<typename _Dummy = void,
               typename enable_if<_TC2<_Dummy>::
                                    _ImplicitlyDefaultConstructibleTuple(),
                                  bool>::type = true>
      constexpr tuple()
      : _Inherited() { }

      template<typename _Dummy = void,
               typename enable_if<_TC2<_Dummy>::
                                    _DefaultConstructibleTuple()
                                  &&
                                  !_TC2<_Dummy>::
                                    _ImplicitlyDefaultConstructibleTuple(),
                                  bool>::type = false>
      explicit constexpr tuple()
      : _Inherited() { }

      // Shortcut for the cases where constructors taking _Elements...
      // need to be constrained.
      template<typename _Dummy> using _TCC =
        _TC<is_same<_Dummy, void>::value,
            _Elements...>;

      template<typename _Dummy = void,
               typename enable_if<
                 _TCC<_Dummy>::template
                   _ConstructibleTuple<_Elements...>()
                 && _TCC<_Dummy>::template
                   _ImplicitlyConvertibleTuple<_Elements...>()
                 && (sizeof...(_Elements) >= 1),
               bool>::type=true>
        constexpr tuple(const _Elements&... __elements)
      : _Inherited(__elements...) { }

      template<typename _Dummy = void,
               typename enable_if<
                 _TCC<_Dummy>::template
                   _ConstructibleTuple<_Elements...>()
                 && !_TCC<_Dummy>::template
                   _ImplicitlyConvertibleTuple<_Elements...>()
                 && (sizeof...(_Elements) >= 1),
               bool>::type=false>
      explicit constexpr tuple(const _Elements&... __elements)
      : _Inherited(__elements...) { }

      // Shortcut for the cases where constructors taking _UElements...
      // need to be constrained.
      template<typename... _UElements> using _TMC =
                  _TC<(sizeof...(_Elements) == sizeof...(_UElements))
		      && (_TC<(sizeof...(_UElements)==1), _Elements...>::
			  template _NotSameTuple<_UElements...>()),
                      _Elements...>;

      // Shortcut for the cases where constructors taking tuple<_UElements...>
      // need to be constrained.
      template<typename... _UElements> using _TMCT =
                  _TC<(sizeof...(_Elements) == sizeof...(_UElements))
		      && !is_same<tuple<_Elements...>,
				  tuple<_UElements...>>::value,
                      _Elements...>;

      template<typename... _UElements, typename
	       enable_if<
		  _TMC<_UElements...>::template
                    _MoveConstructibleTuple<_UElements...>()
                  && _TMC<_UElements...>::template
                    _ImplicitlyMoveConvertibleTuple<_UElements...>()
                  && (sizeof...(_Elements) >= 1),
        bool>::type=true>
        constexpr tuple(_UElements&&... __elements)
        : _Inherited(std::forward<_UElements>(__elements)...) { }

      template<typename... _UElements, typename
        enable_if<
		  _TMC<_UElements...>::template
                    _MoveConstructibleTuple<_UElements...>()
                  && !_TMC<_UElements...>::template
                    _ImplicitlyMoveConvertibleTuple<_UElements...>()
                  && (sizeof...(_Elements) >= 1),
        bool>::type=false>
        explicit constexpr tuple(_UElements&&... __elements)
	: _Inherited(std::forward<_UElements>(__elements)...) {	}

      constexpr tuple(const tuple&) = default;

      constexpr tuple(tuple&&) = default;

      // Shortcut for the cases where constructors taking tuples
      // must avoid creating temporaries.
      template<typename _Dummy> using _TNTC =
        _TC<is_same<_Dummy, void>::value && sizeof...(_Elements) == 1,
            _Elements...>;

      template<typename... _UElements, typename _Dummy = void, typename
        enable_if<_TMCT<_UElements...>::template
                    _ConstructibleTuple<_UElements...>()
                  && _TMCT<_UElements...>::template
                    _ImplicitlyConvertibleTuple<_UElements...>()
                  && _TNTC<_Dummy>::template
                    _NonNestedTuple<const tuple<_UElements...>&>(),
        bool>::type=true>
        constexpr tuple(const tuple<_UElements...>& __in)
        : _Inherited(static_cast<const _Tuple_impl<0, _UElements...>&>(__in))
        { }

      template<typename... _UElements, typename _Dummy = void, typename
        enable_if<_TMCT<_UElements...>::template
                    _ConstructibleTuple<_UElements...>()
                  && !_TMCT<_UElements...>::template
                    _ImplicitlyConvertibleTuple<_UElements...>()
                  && _TNTC<_Dummy>::template
                    _NonNestedTuple<const tuple<_UElements...>&>(),
        bool>::type=false>
        explicit constexpr tuple(const tuple<_UElements...>& __in)
        : _Inherited(static_cast<const _Tuple_impl<0, _UElements...>&>(__in))
        { }

      template<typename... _UElements, typename _Dummy = void, typename
        enable_if<_TMCT<_UElements...>::template
                    _MoveConstructibleTuple<_UElements...>()
                  && _TMCT<_UElements...>::template
                    _ImplicitlyMoveConvertibleTuple<_UElements...>()
                  && _TNTC<_Dummy>::template
                    _NonNestedTuple<tuple<_UElements...>&&>(),
        bool>::type=true>
        constexpr tuple(tuple<_UElements...>&& __in)
        : _Inherited(static_cast<_Tuple_impl<0, _UElements...>&&>(__in)) { }

      template<typename... _UElements, typename _Dummy = void, typename
        enable_if<_TMCT<_UElements...>::template
                    _MoveConstructibleTuple<_UElements...>()
                  && !_TMCT<_UElements...>::template
                    _ImplicitlyMoveConvertibleTuple<_UElements...>()
                  && _TNTC<_Dummy>::template
                    _NonNestedTuple<tuple<_UElements...>&&>(),
        bool>::type=false>
        explicit constexpr tuple(tuple<_UElements...>&& __in)
        : _Inherited(static_cast<_Tuple_impl<0, _UElements...>&&>(__in)) { }

      // Allocator-extended constructors.

      template<typename _Alloc>
	tuple(allocator_arg_t __tag, const _Alloc& __a)
	: _Inherited(__tag, __a) { }

      template<typename _Alloc, typename _Dummy = void,
               typename enable_if<
                 _TCC<_Dummy>::template
                   _ConstructibleTuple<_Elements...>()
                 && _TCC<_Dummy>::template
                   _ImplicitlyConvertibleTuple<_Elements...>(),
               bool>::type=true>
	tuple(allocator_arg_t __tag, const _Alloc& __a,
	      const _Elements&... __elements)
	: _Inherited(__tag, __a, __elements...) { }

      template<typename _Alloc, typename _Dummy = void,
               typename enable_if<
                 _TCC<_Dummy>::template
                   _ConstructibleTuple<_Elements...>()
                 && !_TCC<_Dummy>::template
                   _ImplicitlyConvertibleTuple<_Elements...>(),
               bool>::type=false>
	explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
                       const _Elements&... __elements)
	: _Inherited(__tag, __a, __elements...) { }

      template<typename _Alloc, typename... _UElements, typename
        enable_if<_TMC<_UElements...>::template
                    _MoveConstructibleTuple<_UElements...>()
                  && _TMC<_UElements...>::template
                    _ImplicitlyMoveConvertibleTuple<_UElements...>(),
        bool>::type=true>
	tuple(allocator_arg_t __tag, const _Alloc& __a,
	      _UElements&&... __elements)
	: _Inherited(__tag, __a, std::forward<_UElements>(__elements)...)
       	{ }

      template<typename _Alloc, typename... _UElements, typename
        enable_if<_TMC<_UElements...>::template
                    _MoveConstructibleTuple<_UElements...>()
                  && !_TMC<_UElements...>::template
                    _ImplicitlyMoveConvertibleTuple<_UElements...>(),
        bool>::type=false>
	explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
	      _UElements&&... __elements)
	: _Inherited(__tag, __a, std::forward<_UElements>(__elements)...)
        { }

      template<typename _Alloc>
	tuple(allocator_arg_t __tag, const _Alloc& __a, const tuple& __in)
	: _Inherited(__tag, __a, static_cast<const _Inherited&>(__in)) { }

      template<typename _Alloc>
	tuple(allocator_arg_t __tag, const _Alloc& __a, tuple&& __in)
	: _Inherited(__tag, __a, static_cast<_Inherited&&>(__in)) { }

      template<typename _Alloc, typename _Dummy = void,
	       typename... _UElements, typename
        enable_if<_TMCT<_UElements...>::template
                    _ConstructibleTuple<_UElements...>()
                  && _TMCT<_UElements...>::template
                    _ImplicitlyConvertibleTuple<_UElements...>()
                  && _TNTC<_Dummy>::template
                    _NonNestedTuple<tuple<_UElements...>&&>(),
        bool>::type=true>
	tuple(allocator_arg_t __tag, const _Alloc& __a,
	      const tuple<_UElements...>& __in)
	: _Inherited(__tag, __a,
	             static_cast<const _Tuple_impl<0, _UElements...>&>(__in))
	{ }

      template<typename _Alloc, typename _Dummy = void,
	       typename... _UElements, typename
        enable_if<_TMCT<_UElements...>::template
                    _ConstructibleTuple<_UElements...>()
                  && !_TMCT<_UElements...>::template
                    _ImplicitlyConvertibleTuple<_UElements...>()
                  && _TNTC<_Dummy>::template
                    _NonNestedTuple<tuple<_UElements...>&&>(),
        bool>::type=false>
	explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
	      const tuple<_UElements...>& __in)
	: _Inherited(__tag, __a,
	             static_cast<const _Tuple_impl<0, _UElements...>&>(__in))
	{ }

      template<typename _Alloc, typename _Dummy = void,
	       typename... _UElements, typename
        enable_if<_TMCT<_UElements...>::template
                    _MoveConstructibleTuple<_UElements...>()
                  && _TMCT<_UElements...>::template
                    _ImplicitlyMoveConvertibleTuple<_UElements...>()
                  && _TNTC<_Dummy>::template
                    _NonNestedTuple<tuple<_UElements...>&&>(),
        bool>::type=true>
	tuple(allocator_arg_t __tag, const _Alloc& __a,
	      tuple<_UElements...>&& __in)
	: _Inherited(__tag, __a,
	             static_cast<_Tuple_impl<0, _UElements...>&&>(__in))
	{ }

      template<typename _Alloc, typename _Dummy = void,
	       typename... _UElements, typename
        enable_if<_TMCT<_UElements...>::template
                    _MoveConstructibleTuple<_UElements...>()
                  && !_TMCT<_UElements...>::template
                    _ImplicitlyMoveConvertibleTuple<_UElements...>()
                  && _TNTC<_Dummy>::template
                    _NonNestedTuple<tuple<_UElements...>&&>(),
        bool>::type=false>
	explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
	      tuple<_UElements...>&& __in)
	: _Inherited(__tag, __a,
	             static_cast<_Tuple_impl<0, _UElements...>&&>(__in))
	{ }

      tuple&
      operator=(const tuple& __in)
      {
	static_cast<_Inherited&>(*this) = __in;
	return *this;
      }

      tuple&
      operator=(tuple&& __in)
      noexcept(is_nothrow_move_assignable<_Inherited>::value)
      {
	static_cast<_Inherited&>(*this) = std::move(__in);
	return *this;
      }

      template<typename... _UElements>
	typename
	       enable_if<sizeof...(_UElements)
			 == sizeof...(_Elements), tuple&>::type
        operator=(const tuple<_UElements...>& __in)
        {
	  static_cast<_Inherited&>(*this) = __in;
	  return *this;
	}

      template<typename... _UElements>
	typename
	       enable_if<sizeof...(_UElements)
			 == sizeof...(_Elements), tuple&>::type
        operator=(tuple<_UElements...>&& __in)
        {
	  static_cast<_Inherited&>(*this) = std::move(__in);
	  return *this;
	}

      void
      swap(tuple& __in)
      noexcept(noexcept(__in._M_swap(__in)))
      { _Inherited::_M_swap(__in); }
    };

#if __cpp_deduction_guides >= 201606
  template<typename... _UTypes>
    tuple(_UTypes...) -> tuple<_UTypes...>;
  template<typename _T1, typename _T2>
    tuple(pair<_T1, _T2>) -> tuple<_T1, _T2>;
  template<typename _Alloc, typename... _UTypes>
    tuple(allocator_arg_t, _Alloc, _UTypes...) -> tuple<_UTypes...>;
  template<typename _Alloc, typename _T1, typename _T2>
    tuple(allocator_arg_t, _Alloc, pair<_T1, _T2>) -> tuple<_T1, _T2>;
  template<typename _Alloc, typename... _UTypes>
    tuple(allocator_arg_t, _Alloc, tuple<_UTypes...>) -> tuple<_UTypes...>;
#endif

  // Explicit specialization, zero-element tuple.
  template<>
    class tuple<>
    {
    public:
      void swap(tuple&) noexcept { /* no-op */ }
      // We need the default since we're going to define no-op
      // allocator constructors.
      tuple() = default;
      // No-op allocator constructors.
      template<typename _Alloc>
	tuple(allocator_arg_t, const _Alloc&) { }
      template<typename _Alloc>
	tuple(allocator_arg_t, const _Alloc&, const tuple&) { }
    };

  /// Partial specialization, 2-element tuple.
  /// Includes construction and assignment from a pair.
  template<typename _T1, typename _T2>
    class tuple<_T1, _T2> : public _Tuple_impl<0, _T1, _T2>
    {
      typedef _Tuple_impl<0, _T1, _T2> _Inherited;

    public:
      template <typename _U1 = _T1,
                typename _U2 = _T2,
                typename enable_if<__and_<
                                     __is_implicitly_default_constructible<_U1>,
                                     __is_implicitly_default_constructible<_U2>>
                                   ::value, bool>::type = true>

      constexpr tuple()
      : _Inherited() { }

      template <typename _U1 = _T1,
                typename _U2 = _T2,
                typename enable_if<
                  __and_<
                    is_default_constructible<_U1>,
                    is_default_constructible<_U2>,
                    __not_<
                      __and_<__is_implicitly_default_constructible<_U1>,
                             __is_implicitly_default_constructible<_U2>>>>
                  ::value, bool>::type = false>

      explicit constexpr tuple()
      : _Inherited() { }

      // Shortcut for the cases where constructors taking _T1, _T2
      // need to be constrained.
      template<typename _Dummy> using _TCC =
        _TC<is_same<_Dummy, void>::value, _T1, _T2>;

      template<typename _Dummy = void, typename
               enable_if<_TCC<_Dummy>::template
                           _ConstructibleTuple<_T1, _T2>()
                         && _TCC<_Dummy>::template
                           _ImplicitlyConvertibleTuple<_T1, _T2>(),
	bool>::type = true>
        constexpr tuple(const _T1& __a1, const _T2& __a2)
        : _Inherited(__a1, __a2) { }

      template<typename _Dummy = void, typename
               enable_if<_TCC<_Dummy>::template
                           _ConstructibleTuple<_T1, _T2>()
                         && !_TCC<_Dummy>::template
                           _ImplicitlyConvertibleTuple<_T1, _T2>(),
	bool>::type = false>
        explicit constexpr tuple(const _T1& __a1, const _T2& __a2)
        : _Inherited(__a1, __a2) { }

      // Shortcut for the cases where constructors taking _U1, _U2
      // need to be constrained.
      using _TMC = _TC<true, _T1, _T2>;

      template<typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && _TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>()
	          && !is_same<typename decay<_U1>::type,
			      allocator_arg_t>::value,
	bool>::type = true>
        constexpr tuple(_U1&& __a1, _U2&& __a2)
	: _Inherited(std::forward<_U1>(__a1), std::forward<_U2>(__a2)) { }

      template<typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && !_TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>()
	          && !is_same<typename decay<_U1>::type,
			      allocator_arg_t>::value,
	bool>::type = false>
        explicit constexpr tuple(_U1&& __a1, _U2&& __a2)
	: _Inherited(std::forward<_U1>(__a1), std::forward<_U2>(__a2)) { }

      constexpr tuple(const tuple&) = default;

      constexpr tuple(tuple&&) = default;

      template<typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _ConstructibleTuple<_U1, _U2>()
                  && _TMC::template
                    _ImplicitlyConvertibleTuple<_U1, _U2>(),
	bool>::type = true>
        constexpr tuple(const tuple<_U1, _U2>& __in)
	: _Inherited(static_cast<const _Tuple_impl<0, _U1, _U2>&>(__in)) { }

      template<typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _ConstructibleTuple<_U1, _U2>()
                  && !_TMC::template
                    _ImplicitlyConvertibleTuple<_U1, _U2>(),
	bool>::type = false>
        explicit constexpr tuple(const tuple<_U1, _U2>& __in)
	: _Inherited(static_cast<const _Tuple_impl<0, _U1, _U2>&>(__in)) { }

      template<typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && _TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>(),
	bool>::type = true>
        constexpr tuple(tuple<_U1, _U2>&& __in)
	: _Inherited(static_cast<_Tuple_impl<0, _U1, _U2>&&>(__in)) { }

      template<typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && !_TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>(),
	bool>::type = false>
        explicit constexpr tuple(tuple<_U1, _U2>&& __in)
	: _Inherited(static_cast<_Tuple_impl<0, _U1, _U2>&&>(__in)) { }

      template<typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _ConstructibleTuple<_U1, _U2>()
                  && _TMC::template
                    _ImplicitlyConvertibleTuple<_U1, _U2>(),
	bool>::type = true>
        constexpr tuple(const pair<_U1, _U2>& __in)
	: _Inherited(__in.first, __in.second) { }

      template<typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _ConstructibleTuple<_U1, _U2>()
                  && !_TMC::template
                    _ImplicitlyConvertibleTuple<_U1, _U2>(),
	bool>::type = false>
        explicit constexpr tuple(const pair<_U1, _U2>& __in)
	: _Inherited(__in.first, __in.second) { }

      template<typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && _TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>(),
	bool>::type = true>
        constexpr tuple(pair<_U1, _U2>&& __in)
	: _Inherited(std::forward<_U1>(__in.first),
		     std::forward<_U2>(__in.second)) { }

      template<typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && !_TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>(),
	bool>::type = false>
        explicit constexpr tuple(pair<_U1, _U2>&& __in)
	: _Inherited(std::forward<_U1>(__in.first),
		     std::forward<_U2>(__in.second)) { }

      // Allocator-extended constructors.

      template<typename _Alloc>
	tuple(allocator_arg_t __tag, const _Alloc& __a)
	: _Inherited(__tag, __a) { }

      template<typename _Alloc, typename _Dummy = void,
               typename enable_if<
                 _TCC<_Dummy>::template
                   _ConstructibleTuple<_T1, _T2>()
                 && _TCC<_Dummy>::template
                   _ImplicitlyConvertibleTuple<_T1, _T2>(),
               bool>::type=true>

	tuple(allocator_arg_t __tag, const _Alloc& __a,
	      const _T1& __a1, const _T2& __a2)
	: _Inherited(__tag, __a, __a1, __a2) { }

      template<typename _Alloc, typename _Dummy = void,
               typename enable_if<
                 _TCC<_Dummy>::template
                   _ConstructibleTuple<_T1, _T2>()
                 && !_TCC<_Dummy>::template
                   _ImplicitlyConvertibleTuple<_T1, _T2>(),
               bool>::type=false>

	explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
	      const _T1& __a1, const _T2& __a2)
	: _Inherited(__tag, __a, __a1, __a2) { }

      template<typename _Alloc, typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && _TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>(),
	bool>::type = true>
	tuple(allocator_arg_t __tag, const _Alloc& __a, _U1&& __a1, _U2&& __a2)
	: _Inherited(__tag, __a, std::forward<_U1>(__a1),
	             std::forward<_U2>(__a2)) { }

      template<typename _Alloc, typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && !_TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>(),
	bool>::type = false>
	explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
                       _U1&& __a1, _U2&& __a2)
	: _Inherited(__tag, __a, std::forward<_U1>(__a1),
	             std::forward<_U2>(__a2)) { }

      template<typename _Alloc>
	tuple(allocator_arg_t __tag, const _Alloc& __a, const tuple& __in)
	: _Inherited(__tag, __a, static_cast<const _Inherited&>(__in)) { }

      template<typename _Alloc>
	tuple(allocator_arg_t __tag, const _Alloc& __a, tuple&& __in)
	: _Inherited(__tag, __a, static_cast<_Inherited&&>(__in)) { }

      template<typename _Alloc, typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _ConstructibleTuple<_U1, _U2>()
                  && _TMC::template
                    _ImplicitlyConvertibleTuple<_U1, _U2>(),
	bool>::type = true>
	tuple(allocator_arg_t __tag, const _Alloc& __a,
	      const tuple<_U1, _U2>& __in)
	: _Inherited(__tag, __a,
	             static_cast<const _Tuple_impl<0, _U1, _U2>&>(__in))
	{ }

      template<typename _Alloc, typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _ConstructibleTuple<_U1, _U2>()
                  && !_TMC::template
                    _ImplicitlyConvertibleTuple<_U1, _U2>(),
	bool>::type = false>
	explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
	      const tuple<_U1, _U2>& __in)
	: _Inherited(__tag, __a,
	             static_cast<const _Tuple_impl<0, _U1, _U2>&>(__in))
	{ }

      template<typename _Alloc, typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && _TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>(),
	bool>::type = true>
	tuple(allocator_arg_t __tag, const _Alloc& __a, tuple<_U1, _U2>&& __in)
	: _Inherited(__tag, __a, static_cast<_Tuple_impl<0, _U1, _U2>&&>(__in))
	{ }

      template<typename _Alloc, typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && !_TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>(),
	bool>::type = false>
	explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
                       tuple<_U1, _U2>&& __in)
	: _Inherited(__tag, __a, static_cast<_Tuple_impl<0, _U1, _U2>&&>(__in))
	{ }

      template<typename _Alloc, typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _ConstructibleTuple<_U1, _U2>()
                  && _TMC::template
                    _ImplicitlyConvertibleTuple<_U1, _U2>(),
	bool>::type = true>
        tuple(allocator_arg_t __tag, const _Alloc& __a,
	      const pair<_U1, _U2>& __in)
	: _Inherited(__tag, __a, __in.first, __in.second) { }

      template<typename _Alloc, typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _ConstructibleTuple<_U1, _U2>()
                  && !_TMC::template
                    _ImplicitlyConvertibleTuple<_U1, _U2>(),
	bool>::type = false>
        explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
	      const pair<_U1, _U2>& __in)
	: _Inherited(__tag, __a, __in.first, __in.second) { }

      template<typename _Alloc, typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && _TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>(),
	bool>::type = true>
        tuple(allocator_arg_t __tag, const _Alloc& __a, pair<_U1, _U2>&& __in)
	: _Inherited(__tag, __a, std::forward<_U1>(__in.first),
		     std::forward<_U2>(__in.second)) { }

      template<typename _Alloc, typename _U1, typename _U2, typename
        enable_if<_TMC::template
                    _MoveConstructibleTuple<_U1, _U2>()
                  && !_TMC::template
                    _ImplicitlyMoveConvertibleTuple<_U1, _U2>(),
	bool>::type = false>
        explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
                       pair<_U1, _U2>&& __in)
	: _Inherited(__tag, __a, std::forward<_U1>(__in.first),
		     std::forward<_U2>(__in.second)) { }

      tuple&
      operator=(const tuple& __in)
      {
	static_cast<_Inherited&>(*this) = __in;
	return *this;
      }

      tuple&
      operator=(tuple&& __in)
      noexcept(is_nothrow_move_assignable<_Inherited>::value)
      {
	static_cast<_Inherited&>(*this) = std::move(__in);
	return *this;
      }

      template<typename _U1, typename _U2>
        tuple&
        operator=(const tuple<_U1, _U2>& __in)
        {
	  static_cast<_Inherited&>(*this) = __in;
	  return *this;
	}

      template<typename _U1, typename _U2>
        tuple&
        operator=(tuple<_U1, _U2>&& __in)
        {
	  static_cast<_Inherited&>(*this) = std::move(__in);
	  return *this;
	}

      template<typename _U1, typename _U2>
        tuple&
        operator=(const pair<_U1, _U2>& __in)
        {
	  this->_M_head(*this) = __in.first;
	  this->_M_tail(*this)._M_head(*this) = __in.second;
	  return *this;
	}

      template<typename _U1, typename _U2>
        tuple&
        operator=(pair<_U1, _U2>&& __in)
        {
	  this->_M_head(*this) = std::forward<_U1>(__in.first);
	  this->_M_tail(*this)._M_head(*this) = std::forward<_U2>(__in.second);
	  return *this;
	}

      void
      swap(tuple& __in)
      noexcept(noexcept(__in._M_swap(__in)))
      { _Inherited::_M_swap(__in); }
    };


  /// class tuple_size
  template<typename... _Elements>
    struct tuple_size<tuple<_Elements...>>
    : public integral_constant<std::size_t, sizeof...(_Elements)> { };

#if __cplusplus > 201402L
  template <typename _Tp>
    inline constexpr size_t tuple_size_v = tuple_size<_Tp>::value;
#endif

  /**
   * Recursive case for tuple_element: strip off the first element in
   * the tuple and retrieve the (i-1)th element of the remaining tuple.
   */
  template<std::size_t __i, typename _Head, typename... _Tail>
    struct tuple_element<__i, tuple<_Head, _Tail...> >
    : tuple_element<__i - 1, tuple<_Tail...> > { };

  /**
   * Basis case for tuple_element: The first element is the one we're seeking.
   */
  template<typename _Head, typename... _Tail>
    struct tuple_element<0, tuple<_Head, _Tail...> >
    {
      typedef _Head type;
    };

  /**
   * Error case for tuple_element: invalid index.
   */
  template<size_t __i>
    struct tuple_element<__i, tuple<>>
    {
      static_assert(__i < tuple_size<tuple<>>::value,
	  "tuple index is in range");
    };

  template<std::size_t __i, typename _Head, typename... _Tail>
    constexpr _Head&
    __get_helper(_Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
    { return _Tuple_impl<__i, _Head, _Tail...>::_M_head(__t); }

  template<std::size_t __i, typename _Head, typename... _Tail>
    constexpr const _Head&
    __get_helper(const _Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
    { return _Tuple_impl<__i, _Head, _Tail...>::_M_head(__t); }

  /// Return a reference to the ith element of a tuple.
  template<std::size_t __i, typename... _Elements>
    constexpr __tuple_element_t<__i, tuple<_Elements...>>&
    get(tuple<_Elements...>& __t) noexcept
    { return std::__get_helper<__i>(__t); }

  /// Return a const reference to the ith element of a const tuple.
  template<std::size_t __i, typename... _Elements>
    constexpr const __tuple_element_t<__i, tuple<_Elements...>>&
    get(const tuple<_Elements...>& __t) noexcept
    { return std::__get_helper<__i>(__t); }

  /// Return an rvalue reference to the ith element of a tuple rvalue.
  template<std::size_t __i, typename... _Elements>
    constexpr __tuple_element_t<__i, tuple<_Elements...>>&&
    get(tuple<_Elements...>&& __t) noexcept
    {
      typedef __tuple_element_t<__i, tuple<_Elements...>> __element_type;
      return std::forward<__element_type&&>(std::get<__i>(__t));
    }

  /// Return a const rvalue reference to the ith element of a const tuple rvalue.
  template<std::size_t __i, typename... _Elements>
    constexpr const __tuple_element_t<__i, tuple<_Elements...>>&&
    get(const tuple<_Elements...>&& __t) noexcept
    {
      typedef __tuple_element_t<__i, tuple<_Elements...>> __element_type;
      return std::forward<const __element_type&&>(std::get<__i>(__t));
    }

#if __cplusplus > 201103L

#define __cpp_lib_tuples_by_type 201304

  template<typename _Head, size_t __i, typename... _Tail>
    constexpr _Head&
    __get_helper2(_Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
    { return _Tuple_impl<__i, _Head, _Tail...>::_M_head(__t); }

  template<typename _Head, size_t __i, typename... _Tail>
    constexpr const _Head&
    __get_helper2(const _Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
    { return _Tuple_impl<__i, _Head, _Tail...>::_M_head(__t); }

  /// Return a reference to the unique element of type _Tp of a tuple.
  template <typename _Tp, typename... _Types>
    constexpr _Tp&
    get(tuple<_Types...>& __t) noexcept
    { return std::__get_helper2<_Tp>(__t); }

  /// Return a reference to the unique element of type _Tp of a tuple rvalue.
  template <typename _Tp, typename... _Types>
    constexpr _Tp&&
    get(tuple<_Types...>&& __t) noexcept
    { return std::forward<_Tp&&>(std::__get_helper2<_Tp>(__t)); }

  /// Return a const reference to the unique element of type _Tp of a tuple.
  template <typename _Tp, typename... _Types>
    constexpr const _Tp&
    get(const tuple<_Types...>& __t) noexcept
    { return std::__get_helper2<_Tp>(__t); }

  /// Return a const reference to the unique element of type _Tp of
  /// a const tuple rvalue.
  template <typename _Tp, typename... _Types>
    constexpr const _Tp&&
    get(const tuple<_Types...>&& __t) noexcept
    { return std::forward<const _Tp&&>(std::__get_helper2<_Tp>(__t)); }
#endif

  // This class performs the comparison operations on tuples
  template<typename _Tp, typename _Up, size_t __i, size_t __size>
    struct __tuple_compare
    {
      static constexpr bool
      __eq(const _Tp& __t, const _Up& __u)
      {
	return bool(std::get<__i>(__t) == std::get<__i>(__u))
	  && __tuple_compare<_Tp, _Up, __i + 1, __size>::__eq(__t, __u);
      }

      static constexpr bool
      __less(const _Tp& __t, const _Up& __u)
      {
	return bool(std::get<__i>(__t) < std::get<__i>(__u))
	  || (!bool(std::get<__i>(__u) < std::get<__i>(__t))
	      && __tuple_compare<_Tp, _Up, __i + 1, __size>::__less(__t, __u));
      }
    };

  template<typename _Tp, typename _Up, size_t __size>
    struct __tuple_compare<_Tp, _Up, __size, __size>
    {
      static constexpr bool
      __eq(const _Tp&, const _Up&) { return true; }

      static constexpr bool
      __less(const _Tp&, const _Up&) { return false; }
    };

  template<typename... _TElements, typename... _UElements>
    constexpr bool
    operator==(const tuple<_TElements...>& __t,
	       const tuple<_UElements...>& __u)
    {
      static_assert(sizeof...(_TElements) == sizeof...(_UElements),
	  "tuple objects can only be compared if they have equal sizes.");
      using __compare = __tuple_compare<tuple<_TElements...>,
					tuple<_UElements...>,
					0, sizeof...(_TElements)>;
      return __compare::__eq(__t, __u);
    }

  template<typename... _TElements, typename... _UElements>
    constexpr bool
    operator<(const tuple<_TElements...>& __t,
	      const tuple<_UElements...>& __u)
    {
      static_assert(sizeof...(_TElements) == sizeof...(_UElements),
	  "tuple objects can only be compared if they have equal sizes.");
      using __compare = __tuple_compare<tuple<_TElements...>,
					tuple<_UElements...>,
					0, sizeof...(_TElements)>;
      return __compare::__less(__t, __u);
    }

  template<typename... _TElements, typename... _UElements>
    constexpr bool
    operator!=(const tuple<_TElements...>& __t,
	       const tuple<_UElements...>& __u)
    { return !(__t == __u); }

  template<typename... _TElements, typename... _UElements>
    constexpr bool
    operator>(const tuple<_TElements...>& __t,
	      const tuple<_UElements...>& __u)
    { return __u < __t; }

  template<typename... _TElements, typename... _UElements>
    constexpr bool
    operator<=(const tuple<_TElements...>& __t,
	       const tuple<_UElements...>& __u)
    { return !(__u < __t); }

  template<typename... _TElements, typename... _UElements>
    constexpr bool
    operator>=(const tuple<_TElements...>& __t,
	       const tuple<_UElements...>& __u)
    { return !(__t < __u); }

  // NB: DR 705.
  template<typename... _Elements>
    constexpr tuple<typename __decay_and_strip<_Elements>::__type...>
    make_tuple(_Elements&&... __args)
    {
      typedef tuple<typename __decay_and_strip<_Elements>::__type...>
	__result_type;
      return __result_type(std::forward<_Elements>(__args)...);
    }

  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2275. Why is forward_as_tuple not constexpr?
  template<typename... _Elements>
    constexpr tuple<_Elements&&...>
    forward_as_tuple(_Elements&&... __args) noexcept
    { return tuple<_Elements&&...>(std::forward<_Elements>(__args)...); }

  template<size_t, typename, typename, size_t>
    struct __make_tuple_impl;

  template<size_t _Idx, typename _Tuple, typename... _Tp, size_t _Nm>
    struct __make_tuple_impl<_Idx, tuple<_Tp...>, _Tuple, _Nm>
    : __make_tuple_impl<_Idx + 1,
			tuple<_Tp..., __tuple_element_t<_Idx, _Tuple>>,
			_Tuple, _Nm>
    { };

  template<std::size_t _Nm, typename _Tuple, typename... _Tp>
    struct __make_tuple_impl<_Nm, tuple<_Tp...>, _Tuple, _Nm>
    {
      typedef tuple<_Tp...> __type;
    };

  template<typename _Tuple>
    struct __do_make_tuple
    : __make_tuple_impl<0, tuple<>, _Tuple, std::tuple_size<_Tuple>::value>
    { };

  // Returns the std::tuple equivalent of a tuple-like type.
  template<typename _Tuple>
    struct __make_tuple
    : public __do_make_tuple<typename std::remove_cv
            <typename std::remove_reference<_Tuple>::type>::type>
    { };

  // Combines several std::tuple's into a single one.
  template<typename...>
    struct __combine_tuples;

  template<>
    struct __combine_tuples<>
    {
      typedef tuple<> __type;
    };

  template<typename... _Ts>
    struct __combine_tuples<tuple<_Ts...>>
    {
      typedef tuple<_Ts...> __type;
    };

  template<typename... _T1s, typename... _T2s, typename... _Rem>
    struct __combine_tuples<tuple<_T1s...>, tuple<_T2s...>, _Rem...>
    {
      typedef typename __combine_tuples<tuple<_T1s..., _T2s...>,
					_Rem...>::__type __type;
    };

  // Computes the result type of tuple_cat given a set of tuple-like types.
  template<typename... _Tpls>
    struct __tuple_cat_result
    {
      typedef typename __combine_tuples
        <typename __make_tuple<_Tpls>::__type...>::__type __type;
    };

  // Helper to determine the index set for the first tuple-like
  // type of a given set.
  template<typename...>
    struct __make_1st_indices;

  template<>
    struct __make_1st_indices<>
    {
      typedef std::_Index_tuple<> __type;
    };

  template<typename _Tp, typename... _Tpls>
    struct __make_1st_indices<_Tp, _Tpls...>
    {
      typedef typename std::_Build_index_tuple<std::tuple_size<
	typename std::remove_reference<_Tp>::type>::value>::__type __type;
    };

  // Performs the actual concatenation by step-wise expanding tuple-like
  // objects into the elements,  which are finally forwarded into the
  // result tuple.
  template<typename _Ret, typename _Indices, typename... _Tpls>
    struct __tuple_concater;

  template<typename _Ret, std::size_t... _Is, typename _Tp, typename... _Tpls>
    struct __tuple_concater<_Ret, std::_Index_tuple<_Is...>, _Tp, _Tpls...>
    {
      template<typename... _Us>
        static constexpr _Ret
        _S_do(_Tp&& __tp, _Tpls&&... __tps, _Us&&... __us)
        {
	  typedef typename __make_1st_indices<_Tpls...>::__type __idx;
	  typedef __tuple_concater<_Ret, __idx, _Tpls...>      __next;
	  return __next::_S_do(std::forward<_Tpls>(__tps)...,
			       std::forward<_Us>(__us)...,
			       std::get<_Is>(std::forward<_Tp>(__tp))...);
	}
    };

  template<typename _Ret>
    struct __tuple_concater<_Ret, std::_Index_tuple<>>
    {
      template<typename... _Us>
	static constexpr _Ret
	_S_do(_Us&&... __us)
        {
	  return _Ret(std::forward<_Us>(__us)...);
	}
    };

  /// tuple_cat
  template<typename... _Tpls, typename = typename
           enable_if<__and_<__is_tuple_like<_Tpls>...>::value>::type>
    constexpr auto
    tuple_cat(_Tpls&&... __tpls)
    -> typename __tuple_cat_result<_Tpls...>::__type
    {
      typedef typename __tuple_cat_result<_Tpls...>::__type __ret;
      typedef typename __make_1st_indices<_Tpls...>::__type __idx;
      typedef __tuple_concater<__ret, __idx, _Tpls...> __concater;
      return __concater::_S_do(std::forward<_Tpls>(__tpls)...);
    }

  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2301. Why is tie not constexpr?
  /// tie
  template<typename... _Elements>
    constexpr tuple<_Elements&...>
    tie(_Elements&... __args) noexcept
    { return tuple<_Elements&...>(__args...); }

  /// swap
  template<typename... _Elements>
    inline
#if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
    // Constrained free swap overload, see p0185r1
    typename enable_if<__and_<__is_swappable<_Elements>...>::value
      >::type
#else
    void
#endif
    swap(tuple<_Elements...>& __x, tuple<_Elements...>& __y)
    noexcept(noexcept(__x.swap(__y)))
    { __x.swap(__y); }

#if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
  template<typename... _Elements>
    typename enable_if<!__and_<__is_swappable<_Elements>...>::value>::type
    swap(tuple<_Elements...>&, tuple<_Elements...>&) = delete;
#endif

  // A class (and instance) which can be used in 'tie' when an element
  // of a tuple is not required.
  // _GLIBCXX14_CONSTEXPR
  // 2933. PR for LWG 2773 could be clearer
  struct _Swallow_assign
  {
    template<class _Tp>
      _GLIBCXX14_CONSTEXPR const _Swallow_assign&
      operator=(const _Tp&) const
      { return *this; }
  };

  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2773. Making std::ignore constexpr
  _GLIBCXX17_INLINE constexpr _Swallow_assign ignore{};

  /// Partial specialization for tuples
  template<typename... _Types, typename _Alloc>
    struct uses_allocator<tuple<_Types...>, _Alloc> : true_type { };

  // See stl_pair.h...
  template<class _T1, class _T2>
    template<typename... _Args1, typename... _Args2>
      inline
      pair<_T1, _T2>::
      pair(piecewise_construct_t,
	   tuple<_Args1...> __first, tuple<_Args2...> __second)
      : pair(__first, __second,
	     typename _Build_index_tuple<sizeof...(_Args1)>::__type(),
	     typename _Build_index_tuple<sizeof...(_Args2)>::__type())
      { }

  template<class _T1, class _T2>
    template<typename... _Args1, std::size_t... _Indexes1,
             typename... _Args2, std::size_t... _Indexes2>
      inline
      pair<_T1, _T2>::
      pair(tuple<_Args1...>& __tuple1, tuple<_Args2...>& __tuple2,
	   _Index_tuple<_Indexes1...>, _Index_tuple<_Indexes2...>)
      : first(std::forward<_Args1>(std::get<_Indexes1>(__tuple1))...),
        second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...)
      { }

#if __cplusplus > 201402L
# define __cpp_lib_apply 201603

  template <typename _Fn, typename _Tuple, size_t... _Idx>
    constexpr decltype(auto)
    __apply_impl(_Fn&& __f, _Tuple&& __t, index_sequence<_Idx...>)
    {
      return std::__invoke(std::forward<_Fn>(__f),
			   std::get<_Idx>(std::forward<_Tuple>(__t))...);
    }

  template <typename _Fn, typename _Tuple>
    constexpr decltype(auto)
    apply(_Fn&& __f, _Tuple&& __t)
    {
      using _Indices = make_index_sequence<tuple_size_v<decay_t<_Tuple>>>;
      return std::__apply_impl(std::forward<_Fn>(__f),
			       std::forward<_Tuple>(__t),
			       _Indices{});
    }

#define __cpp_lib_make_from_tuple  201606

  template <typename _Tp, typename _Tuple, size_t... _Idx>
    constexpr _Tp
    __make_from_tuple_impl(_Tuple&& __t, index_sequence<_Idx...>)
    { return _Tp(std::get<_Idx>(std::forward<_Tuple>(__t))...); }

  template <typename _Tp, typename _Tuple>
    constexpr _Tp
    make_from_tuple(_Tuple&& __t)
    {
      return __make_from_tuple_impl<_Tp>(
        std::forward<_Tuple>(__t),
	make_index_sequence<tuple_size_v<decay_t<_Tuple>>>{});
    }
#endif // C++17

  /// @}

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++11

#endif // _GLIBCXX_TUPLE