// <variant> -*- C++ -*-

// Copyright (C) 2016 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 variant
 *  This is the <variant> C++ Library header.
 */

#ifndef _GLIBCXX_VARIANT
#define _GLIBCXX_VARIANT 1

#pragma GCC system_header

#if __cplusplus <= 201402L
# include <bits/c++17_warning.h>
#else

#include <type_traits>
#include <utility>
#include <bits/enable_special_members.h>
#include <bits/move.h>
#include <bits/uses_allocator.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  template<typename... _Types> class tuple;
  template<typename... _Types> class variant;
  template <typename> struct hash;

  template<typename _Variant>
    struct variant_size;

  template<typename _Variant>
    struct variant_size<const _Variant> : variant_size<_Variant> {};

  template<typename _Variant>
    struct variant_size<volatile _Variant> : variant_size<_Variant> {};

  template<typename _Variant>
    struct variant_size<const volatile _Variant> : variant_size<_Variant> {};

  template<typename... _Types>
    struct variant_size<variant<_Types...>>
    : std::integral_constant<size_t, sizeof...(_Types)> {};

  template<typename _Variant>
    constexpr size_t variant_size_v = variant_size<_Variant>::value;

  template<size_t _Np, typename _Variant>
    struct variant_alternative;

  template<size_t _Np, typename _First, typename... _Rest>
    struct variant_alternative<_Np, variant<_First, _Rest...>>
    : variant_alternative<_Np-1, variant<_Rest...>> {};

  template<typename _First, typename... _Rest>
    struct variant_alternative<0, variant<_First, _Rest...>>
    { using type = _First; };

  template<size_t _Np, typename _Variant>
    using variant_alternative_t =
      typename variant_alternative<_Np, _Variant>::type;

  constexpr size_t variant_npos = -1;

namespace __detail
{
namespace __variant
{
  // Returns the first apparence of _Tp in _Types.
  // Returns sizeof...(_Types) if _Tp is not in _Types.
  template<typename _Tp, typename... _Types>
    struct __index_of : std::integral_constant<size_t, 0> {};

  template<typename _Tp, typename... _Types>
    constexpr size_t __index_of_v = __index_of<_Tp, _Types...>::value;

  template<typename _Tp, typename _First, typename... _Rest>
    struct __index_of<_Tp, _First, _Rest...> :
      std::integral_constant<size_t, is_same_v<_Tp, _First>
	? 0 : __index_of_v<_Tp, _Rest...> + 1> {};

  // Extract _From's qualifiers and references and apply it to _To.
  // __reserved_type_map<const int&, char> is const char&.
  template<typename _From, typename _To>
    struct __reserved_type_map_impl
    { using type = _To; };

  template<typename _From, typename _To>
    using __reserved_type_map =
      typename __reserved_type_map_impl<_From, _To>::type;

  template<typename _From, typename _To>
    struct __reserved_type_map_impl<_From&, _To>
    { using type = add_lvalue_reference_t<__reserved_type_map<_From, _To>>; };

  template<typename _From, typename _To>
    struct __reserved_type_map_impl<_From&&, _To>
    { using type = add_rvalue_reference_t<__reserved_type_map<_From, _To>>; };

  template<typename _From, typename _To>
    struct __reserved_type_map_impl<const _From, _To>
    { using type = add_const_t<__reserved_type_map<_From, _To>>; };

  template<typename _From, typename _To>
    struct __reserved_type_map_impl<volatile _From, _To>
    { using type = add_volatile_t<__reserved_type_map<_From, _To>>; };

  template<typename _From, typename _To>
    struct __reserved_type_map_impl<const volatile _From, _To>
    { using type = add_cv_t<__reserved_type_map<_From, _To>>; };

  // Stores a reference alternative as a... well, reference.
  template<typename _Reference>
    struct _Reference_storage
    {
      static_assert(is_reference_v<_Reference>,
		    "BUG: _Reference should be a reference");

      _Reference_storage(_Reference __ref) noexcept : _M_storage(__ref) { }

      operator _Reference() noexcept
      { return static_cast<_Reference>(_M_storage); }

      _Reference _M_storage;
    };

  // Stores a void alternative, because it is not a regular type.
  template<typename _Void>
    struct _Void_storage { };

  // Map from the alternative type to a non-qualified storage type.
  template<typename _Alternative, typename = void>
    struct __storage_type
    { using type = _Alternative; };

  template<typename _Alternative>
    struct __storage_type<_Alternative,
			  enable_if_t<is_reference_v<_Alternative>>>
    { using type = _Reference_storage<_Alternative>; };

  template<typename _Alternative>
    struct __storage_type<_Alternative, enable_if_t<is_void_v<_Alternative>>>
    { using type = _Void_storage<_Alternative>; };

  template<typename _Type>
    using __storage = typename __storage_type<_Type>::type;

  template<typename _Type, bool __is_literal = std::is_literal_type_v<_Type>>
    struct _Uninitialized;

  template<typename _Type>
    struct _Uninitialized<_Type, true>
    {
      constexpr _Uninitialized() = default;

      template<typename... _Args>
      constexpr _Uninitialized(in_place_index_t<0>, _Args&&... __args)
      : _M_storage(std::forward<_Args>(__args)...)
      { }

      _Type _M_storage;
    };

  template<typename _Type>
    struct _Uninitialized<_Type, false>
    {
      constexpr _Uninitialized() = default;

      template<typename... _Args>
      constexpr _Uninitialized(in_place_index_t<0>, _Args&&... __args)
      { ::new (&_M_storage) _Type(std::forward<_Args>(__args)...); }

      typename std::aligned_storage<sizeof(_Type), alignof(_Type)>::type
	  _M_storage;
    };

  // Reverse mapping of __storage_type.
  template<typename _Storage_type>
    struct __alternative_type
    {
      static_assert(!is_reference_v<_Storage_type>,
		    "BUG: _Storage_type should not be reference");
      using type = _Storage_type;
    };

  template<typename _Reference>
    struct __alternative_type<_Reference_storage<_Reference>>
    { using type = _Reference; };

  template<typename _Void>
    struct __alternative_type<_Void_storage<_Void>>
    { using type = _Void; };

  // Given a qualified storage type, return the desired reference.
  // The qualified storage type is supposed to carry the variant object's
  // qualifications and reference information, and the designated alternative's
  // storage type.
  // Returns the qualification-collapsed alternative references.
  //
  // For example, __get_alternative<_Reference_storage<int&&>&> returns int&.
  template<typename _Qualified_storage>
    decltype(auto)
    __get_alternative(void* __ptr)
    {
      using _Storage = decay_t<_Qualified_storage>;
      using _Alternative = typename __alternative_type<_Storage>::type;
      return __reserved_type_map<_Qualified_storage, _Alternative>(
	*static_cast<_Storage*>(__ptr));
    }

  // Various functions as "vtable" entries, where those vtables are used by
  // polymorphic operations.
  template<typename _Lhs, typename _Rhs>
    constexpr void
    __erased_ctor(void* __lhs, void* __rhs)
    { ::new (__lhs) decay_t<_Lhs>(__get_alternative<_Rhs>(__rhs)); }

  template<typename _Alloc, typename _Lhs, typename _Rhs>
    constexpr void
    __erased_use_alloc_ctor(const _Alloc& __a, void* __lhs, void* __rhs)
    {
      __uses_allocator_construct(__a, static_cast<decay_t<_Lhs>*>(__lhs),
				 __get_alternative<_Rhs>(__rhs));
    }

  // TODO: Find a potential chance to reuse this accross the project.
  template<typename _Tp>
    constexpr void
    __erased_dtor(void* __ptr)
    {
      using _Storage = decay_t<_Tp>;
      static_cast<_Storage*>(__ptr)->~_Storage();
    }

  template<typename _Lhs, typename _Rhs>
    constexpr void
    __erased_assign(void* __lhs, void* __rhs)
    { __get_alternative<_Lhs>(__lhs) = __get_alternative<_Rhs>(__rhs); }

  template<typename _Lhs, typename _Rhs>
    constexpr void
    __erased_swap(void* __lhs, void* __rhs)
    {
      using std::swap;
      swap(__get_alternative<_Lhs>(__lhs), __get_alternative<_Rhs>(__rhs));
    }

  template<typename _Lhs, typename _Rhs>
    constexpr bool
    __erased_equal_to(void* __lhs, void* __rhs)
    { return __get_alternative<_Lhs>(__lhs) == __get_alternative<_Rhs>(__rhs); }

  template<typename _Lhs, typename _Rhs>
    constexpr bool
    __erased_less_than(void* __lhs, void* __rhs)
    { return __get_alternative<_Lhs>(__lhs) < __get_alternative<_Rhs>(__rhs); }

  template<typename _Tp>
    constexpr size_t
    __erased_hash(void* __t)
    { return std::hash<decay_t<_Tp>>{}(__get_alternative<_Tp>(__t)); }

  template<typename... _Types>
    struct _Variant_base;

  template<typename... _Types>
    struct _Variant_storage
    { constexpr _Variant_storage() = default; };

  // Use recursive unions to implement a trivially destructible variant.
  template<typename _First, typename... _Rest>
    struct _Variant_storage<_First, _Rest...>
    {
      constexpr _Variant_storage() = default;

      template<typename... _Args>
	constexpr _Variant_storage(in_place_index_t<0>, _Args&&... __args)
	: _M_first(in_place<0>, forward<_Args>(__args)...)
	{ }

      template<size_t _Np, typename... _Args,
	       typename = enable_if_t<0 < _Np && _Np < sizeof...(_Rest) + 1>>
	constexpr _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
	: _M_rest(in_place<_Np - 1>, forward<_Args>(__args)...)
	{ }

      ~_Variant_storage() = default;

      constexpr void*
      _M_storage() const
      {
	return const_cast<void*>(
	  static_cast<const void*>(std::addressof(_M_first._M_storage)));
      }

      union
      {
	_Uninitialized<__storage<_First>> _M_first;
	_Variant_storage<_Rest...> _M_rest;
      };
    };

  template<typename _Derived, bool __is_trivially_destructible>
    struct _Dtor_mixin
    {
      ~_Dtor_mixin()
      { static_cast<_Derived*>(this)->_M_destroy(); }
    };

  template<typename _Derived>
    struct _Dtor_mixin<_Derived, true>
    {
      ~_Dtor_mixin() = default;
    };

  // Helps SFINAE on special member functions. Otherwise it can live in variant
  // class.
  template<typename... _Types>
    struct _Variant_base :
      _Variant_storage<_Types...>,
      _Dtor_mixin<_Variant_base<_Types...>,
		  __and_<std::is_trivially_destructible<_Types>...>::value>
    {
      using _Storage = _Variant_storage<_Types...>;

      constexpr
      _Variant_base()
      noexcept(is_nothrow_default_constructible_v<
		 variant_alternative_t<0, variant<_Types...>>>)
      : _Variant_base(in_place<0>) { }

      _Variant_base(const _Variant_base& __rhs)
      : _Storage(), _M_index(__rhs._M_index)
      {
	if (__rhs._M_valid())
	  {
	    static constexpr void (*_S_vtable[])(void*, void*) =
	      { &__erased_ctor<__storage<_Types>&,
			       const __storage<_Types>&>... };
	    _S_vtable[__rhs._M_index](_M_storage(), __rhs._M_storage());
	  }
      }

      _Variant_base(_Variant_base&& __rhs)
      noexcept(__and_<is_nothrow_move_constructible<_Types>...>::value)
      : _Storage(), _M_index(__rhs._M_index)
      {
	if (__rhs._M_valid())
	  {
	    static constexpr void (*_S_vtable[])(void*, void*) =
	      { &__erased_ctor<__storage<_Types>&, __storage<_Types>&&>... };
	    _S_vtable[__rhs._M_index](_M_storage(), __rhs._M_storage());
	  }
      }

      template<size_t _Np, typename... _Args>
	constexpr explicit
	_Variant_base(in_place_index_t<_Np> __i, _Args&&... __args)
	: _Storage(__i, forward<_Args>(__args)...), _M_index(_Np)
	{ }

      template<typename _Alloc>
	_Variant_base(const _Alloc& __a, const _Variant_base& __rhs)
	: _Storage(), _M_index(__rhs._M_index)
	{
	  if (__rhs._M_valid())
	    {
	      static constexpr void
	      (*_S_vtable[])(const _Alloc&, void*, void*) =
		{ &__erased_use_alloc_ctor<_Alloc, __storage<_Types>&,
					   const __storage<_Types>&>... };
	      _S_vtable[__rhs._M_index](__a, _M_storage(), __rhs._M_storage());
	    }
	}

      template<typename _Alloc>
	_Variant_base(const _Alloc& __a, _Variant_base&& __rhs)
	: _Storage(), _M_index(__rhs._M_index)
	{
	  if (__rhs._M_valid())
	    {
	      static constexpr void
	      (*_S_vtable[])(const _Alloc&, void*, void*) =
		{ &__erased_use_alloc_ctor<_Alloc, __storage<_Types>&,
					   __storage<_Types>&&>... };
	      _S_vtable[__rhs._M_index](__a, _M_storage(), __rhs._M_storage());
	    }
	}

      template<typename _Alloc, size_t _Np, typename... _Args>
	constexpr explicit
	_Variant_base(const _Alloc& __a, in_place_index_t<_Np>,
		      _Args&&... __args)
	: _Storage(), _M_index(_Np)
	{
	  using _Storage =
	    __storage<variant_alternative_t<_Np, variant<_Types...>>>;
	  __uses_allocator_construct(__a, static_cast<_Storage*>(_M_storage()),
				     forward<_Args>(__args)...);
	  __glibcxx_assert(_M_index == _Np);
	}

      _Variant_base&
      operator=(const _Variant_base& __rhs)
      {
	if (_M_index == __rhs._M_index)
	  {
	    if (__rhs._M_valid())
	      {
		static constexpr void (*_S_vtable[])(void*, void*) =
		  { &__erased_assign<__storage<_Types>&,
				     const __storage<_Types>&>... };
		_S_vtable[__rhs._M_index](_M_storage(), __rhs._M_storage());
	      }
	  }
	else
	  {
	    _Variant_base __tmp(__rhs);
	    this->~_Variant_base();
	    __try
	      {
		::new (this) _Variant_base(std::move(__tmp));
	      }
	    __catch (...)
	      {
		_M_index = variant_npos;
		__throw_exception_again;
	      }
	  }
	__glibcxx_assert(_M_index == __rhs._M_index);
	return *this;
      }

      _Variant_base&
      operator=(_Variant_base&& __rhs)
      noexcept(__and_<is_nothrow_move_constructible<_Types>...,
		      is_nothrow_move_assignable<_Types>...>::value)
      {
	if (_M_index == __rhs._M_index)
	  {
	    if (__rhs._M_valid())
	      {
		static constexpr void (*_S_vtable[])(void*, void*) =
		  { &__erased_assign<__storage<_Types>&,
				     __storage<_Types>&&>... };
		_S_vtable[__rhs._M_index](_M_storage(), __rhs._M_storage());
	      }
	  }
	else
	  {
	    this->~_Variant_base();
	    __try
	      {
		::new (this) _Variant_base(std::move(__rhs));
	      }
	    __catch (...)
	      {
		_M_index = variant_npos;
		__throw_exception_again;
	      }
	  }
	return *this;
      }

      void _M_destroy()
      {
	if (_M_valid())
	  {
	    static constexpr void (*_S_vtable[])(void*) =
	      { &__erased_dtor<__storage<_Types>&>... };
	    _S_vtable[this->_M_index](_M_storage());
	  }
      }

      constexpr void*
      _M_storage() const
      { return _Storage::_M_storage(); }

      constexpr bool
      _M_valid() const noexcept
      { return _M_index != variant_npos; }

      size_t _M_index;
    };

  // For how many times does _Tp appear in _Tuple?
  template<typename _Tp, typename _Tuple>
    struct __tuple_count;

  template<typename _Tp, typename _Tuple>
    constexpr size_t __tuple_count_v = __tuple_count<_Tp, _Tuple>::value;

  template<typename _Tp, typename... _Types>
    struct __tuple_count<_Tp, tuple<_Types...>>
    : integral_constant<size_t, 0> { };

  template<typename _Tp, typename _First, typename... _Rest>
    struct __tuple_count<_Tp, tuple<_First, _Rest...>>
    : integral_constant<
	size_t,
	__tuple_count_v<_Tp, tuple<_Rest...>> + is_same_v<_Tp, _First>> { };

  // TODO: Reuse this in <tuple> ?
  template<typename _Tp, typename... _Types>
    constexpr bool __exactly_once = __tuple_count_v<_Tp, tuple<_Types...>> == 1;

  // Takes _Types and create an overloaded _S_fun for each type.
  // If a type appears more than once in _Types, create only one overload.
  template<typename... _Types>
    struct __overload_set
    { static void _S_fun(); };

  template<typename _First, typename... _Rest>
    struct __overload_set<_First, _Rest...> : __overload_set<_Rest...>
    {
      using __overload_set<_Rest...>::_S_fun;
      static integral_constant<size_t, sizeof...(_Rest)> _S_fun(_First);
    };

  template<typename... _Rest>
    struct __overload_set<void, _Rest...> : __overload_set<_Rest...>
    {
      using __overload_set<_Rest...>::_S_fun;
    };

  // Helper for variant(_Tp&&) and variant::operator=(_Tp&&).
  // __accepted_index maps the arbitrary _Tp to an alternative type in _Variant.
  template<typename _Tp, typename _Variant, typename = void>
    struct __accepted_index
    { static constexpr size_t value = variant_npos; };

  template<typename _Tp, typename... _Types>
    struct __accepted_index<
      _Tp, variant<_Types...>,
      decltype(__overload_set<_Types...>::_S_fun(std::declval<_Tp>()),
	       std::declval<void>())>
    {
      static constexpr size_t value = sizeof...(_Types) - 1
	- decltype(__overload_set<_Types...>::
		   _S_fun(std::declval<_Tp>()))::value;
    };

  // Returns the raw storage for __v.
  template<typename _Variant>
    void* __get_storage(_Variant&& __v)
    { return __v._M_storage(); }

  // Returns the reference to the desired alternative.
  // It is as unsafe as a reinterpret_cast.
  template<typename _Tp, typename _Variant>
    decltype(auto) __access(_Variant&& __v)
    {
      return __get_alternative<__reserved_type_map<_Variant&&, __storage<_Tp>>>(
	__get_storage(forward<_Variant>(__v)));
    }

  // A helper used to create variadic number of _To types.
  template<typename _From, typename _To>
    using _To_type = _To;

  // Call the actual visitor.
  // _Args are qualified storage types.
  template<typename _Visitor, typename... _Args>
    decltype(auto) __visit_invoke(_Visitor&& __visitor,
				  _To_type<_Args, void*>... __ptrs)
    {
      return forward<_Visitor>(__visitor)(__get_alternative<_Args>(__ptrs)...);
    }

  // Used for storing multi-dimensional vtable.
  template<typename _Tp, size_t... _Dimensions>
    struct _Multi_array
    {
      constexpr const _Tp&
      _M_access() const
      { return _M_data; }

      _Tp _M_data;
    };

  template<typename _Tp, size_t __first, size_t... __rest>
    struct _Multi_array<_Tp, __first, __rest...>
    {
      template<typename... _Args>
	constexpr const _Tp&
	_M_access(size_t __first_index, _Args... __rest_indices) const
	{ return _M_arr[__first_index]._M_access(__rest_indices...); }

      _Multi_array<_Tp, __rest...> _M_arr[__first];
    };

  // Creates a multi-dimensional vtable recursively.
  // _Variant_tuple is initially the input from visit(), and gets gradually
  // consumed.
  // _Arg_tuple is enumerated alternative sequence, represented by a
  // qualified storage.
  //
  // For example,
  // visit([](auto, auto){},
  //       variant<int, char>(),
  //       variant<float, double, long double>())
  // will trigger instantiations of:
  // __gen_vtable_impl<_Multi_array<void(*)(void*, void*), 2, 3>,
  //                   tuple<variant<int, char>,
  //                         variant<float, double, long double>>,
  //                   tuple<>>
  //   __gen_vtable_impl<_Multi_array<void(*)(void*, void*), 3>,
  //                     tuple<variant<float, double, long double>>,
  //                     tuple<int>>
  //     __gen_vtable_impl<_Multi_array<void(*)(void*, void*)>,
  //                       tuple<>,
  //                       tuple<int, float>>
  //     __gen_vtable_impl<_Multi_array<void(*)(void*, void*)>,
  //                       tuple<>,
  //                       tuple<int, double>>
  //     __gen_vtable_impl<_Multi_array<void(*)(void*, void*)>,
  //                       tuple<>,
  //                       tuple<int, long double>>
  //   __gen_vtable_impl<_Multi_array<void(*)(void*, void*), 3>,
  //                     tuple<variant<float, double, long double>>,
  //                     tuple<char>>
  //     __gen_vtable_impl<_Multi_array<void(*)(void*, void*)>,
  //                       tuple<>,
  //                       tuple<char, float>>
  //     __gen_vtable_impl<_Multi_array<void(*)(void*, void*)>,
  //                       tuple<>,
  //                       tuple<char, double>>
  //     __gen_vtable_impl<_Multi_array<void(*)(void*, void*)>,
  //                       tuple<>,
  //                       tuple<char, long double>>
  // The returned multi-dimensional vtable can be fast accessed by the visitor
  // using index calculation.
  template<typename _Array_type, typename _Variant_tuple, typename _Arg_tuple>
    struct __gen_vtable_impl;

  template<typename _Array_type, typename _First, typename... _Rest,
	   typename... _Args>
    struct __gen_vtable_impl<_Array_type, tuple<_First, _Rest...>,
			     tuple<_Args...>>
    {
      static constexpr _Array_type
      _S_apply()
      {
	_Array_type __vtable{};
	_S_apply_all_alts(
	  __vtable, make_index_sequence<variant_size_v<decay_t<_First>>>());
	return __vtable;
      }

      template<size_t... __indices>
	static constexpr void
	_S_apply_all_alts(_Array_type& __vtable, index_sequence<__indices...>)
	{ (_S_apply_single_alt<__indices>(__vtable._M_arr[__indices]), ...); }

      template<size_t __index>
	static constexpr void
	_S_apply_single_alt(auto& __element)
	{
	  using _Alternative = variant_alternative_t<__index, decay_t<_First>>;
	  using _Qualified_storage = __reserved_type_map<
	    _First, __storage<_Alternative>>;
	  __element = __gen_vtable_impl<
	    decay_t<decltype(__element)>, tuple<_Rest...>,
	    tuple<_Args..., _Qualified_storage>>::_S_apply();
	}
    };

  template<typename _Result_type, typename _Visitor, typename... _Args>
    struct __gen_vtable_impl<
      _Multi_array<_Result_type (*)(_Visitor, _To_type<_Args, void*>...)>,
		   tuple<>, tuple<_Args...>>
    {
      using _Array_type =
	_Multi_array<_Result_type (*)(_Visitor&&, _To_type<_Args, void*>...)>;

      static constexpr auto
      _S_apply()
      { return _Array_type{&__visit_invoke<_Visitor, _Args...>}; }
    };

  template<typename _Result_type, typename _Visitor, typename... _Variants>
    struct __gen_vtable
    {
      using _Func_ptr =
	_Result_type (*)(_Visitor&&, _To_type<_Variants, void*>...);
      using _Array_type =
	_Multi_array<_Func_ptr, variant_size_v<decay_t<_Variants>>...>;

      static constexpr _Array_type
      _S_apply()
      {
	return __gen_vtable_impl<
	  _Array_type, tuple<_Variants...>, tuple<>>::_S_apply();
      }
    };

} // namespace __variant
} // namespace __detail

  template<typename _Tp, typename... _Types>
    inline constexpr bool holds_alternative(const variant<_Types...>& __v)
    noexcept
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T should occur for exactly once in alternatives");
      return __v.index() == __detail::__variant::__index_of_v<_Tp, _Types...>;
    }

  template<size_t _Np, typename... _Types>
    variant_alternative_t<_Np, variant<_Types...>>&
    get(variant<_Types...>&);

  template<size_t _Np, typename... _Types>
    variant_alternative_t<_Np, variant<_Types...>>&&
    get(variant<_Types...>&&);

  template<size_t _Np, typename... _Types>
    variant_alternative_t<_Np, variant<_Types...>> const&
    get(const variant<_Types...>&);

  template<size_t _Np, typename... _Types>
    variant_alternative_t<_Np, variant<_Types...>> const&&
    get(const variant<_Types...>&&);

  template<typename _Tp, typename... _Types>
    inline _Tp& get(variant<_Types...>& __v)
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T should occur for exactly once in alternatives");
      static_assert(!is_void_v<_Tp>, "_Tp should not be void");
      return get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
    }

  template<typename _Tp, typename... _Types>
    inline _Tp&& get(variant<_Types...>&& __v)
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T should occur for exactly once in alternatives");
      static_assert(!is_void_v<_Tp>, "_Tp should not be void");
      return get<__detail::__variant::__index_of_v<_Tp, _Types...>>(
	std::move(__v));
    }

  template<typename _Tp, typename... _Types>
    inline const _Tp& get(const variant<_Types...>& __v)
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T should occur for exactly once in alternatives");
      static_assert(!is_void_v<_Tp>, "_Tp should not be void");
      return get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
    }

  template<typename _Tp, typename... _Types>
    inline const _Tp&& get(const variant<_Types...>&& __v)
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T should occur for exactly once in alternatives");
      static_assert(!is_void_v<_Tp>, "_Tp should not be void");
      return get<__detail::__variant::__index_of_v<_Tp, _Types...>>(
	std::move(__v));
    }

  template<size_t _Np, typename... _Types>
    inline add_pointer_t<variant_alternative_t<_Np, variant<_Types...>>>
    get_if(variant<_Types...>* __ptr) noexcept
    {
      using _Alternative_type = variant_alternative_t<_Np, variant<_Types...>>;
      static_assert(_Np < sizeof...(_Types),
		    "The index should be in [0, number of alternatives)");
      static_assert(!is_void_v<_Alternative_type>, "_Tp should not be void");
      if (__ptr && __ptr->index() == _Np)
	return &__detail::__variant::__access<_Alternative_type>(*__ptr);
      return nullptr;
    }

  template<size_t _Np, typename... _Types>
    inline add_pointer_t<const variant_alternative_t<_Np, variant<_Types...>>>
    get_if(const variant<_Types...>* __ptr) noexcept
    {
      using _Alternative_type = variant_alternative_t<_Np, variant<_Types...>>;
      static_assert(_Np < sizeof...(_Types),
		    "The index should be in [0, number of alternatives)");
      static_assert(!is_void_v<_Alternative_type>, "_Tp should not be void");
      if (__ptr && __ptr->index() == _Np)
	return &__detail::__variant::__access<_Alternative_type>(*__ptr);
      return nullptr;
    }

  template<typename _Tp, typename... _Types>
    inline add_pointer_t<_Tp> get_if(variant<_Types...>* __ptr) noexcept
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T should occur for exactly once in alternatives");
      static_assert(!is_void_v<_Tp>, "_Tp should not be void");
      return get_if<__detail::__variant::__index_of_v<_Tp, _Types...>>(__ptr);
    }

  template<typename _Tp, typename... _Types>
    inline add_pointer_t<const _Tp> get_if(const variant<_Types...>* __ptr)
    noexcept
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T should occur for exactly once in alternatives");
      static_assert(!is_void_v<_Tp>, "_Tp should not be void");
      return get_if<__detail::__variant::__index_of_v<_Tp, _Types...>>(__ptr);
    }

  template<typename... _Types>
    bool operator==(const variant<_Types...>& __lhs,
		    const variant<_Types...>& __rhs)
    {
      if (__lhs.index() != __rhs.index())
	return false;

      if (__lhs.valueless_by_exception())
	return true;

      using __detail::__variant::__storage;
      static constexpr bool (*_S_vtable[])(void*, void*) =
	{ &__detail::__variant::__erased_equal_to<
	  const __storage<_Types>&, const __storage<_Types>&>... };
      return _S_vtable[__lhs.index()](
	  __detail::__variant::__get_storage(__lhs),
	  __detail::__variant::__get_storage(__rhs));
    }

  template<typename... _Types>
    inline bool
    operator!=(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs)
    { return !(__lhs == __rhs); }

  template<typename... _Types>
    inline bool
    operator<(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs)
    {
      if (__lhs.index() < __rhs.index())
	return true;

      if (__lhs.index() > __rhs.index())
	return false;

      if (__lhs.valueless_by_exception())
	return false;

      using __detail::__variant::__storage;
      static constexpr bool (*_S_vtable[])(void*, void*) =
	{ &__detail::__variant::__erased_less_than<
	    const __storage<_Types>&,
	    const __storage<_Types>&>... };
      return _S_vtable[__lhs.index()](
	  __detail::__variant::__get_storage(__lhs),
	  __detail::__variant::__get_storage(__rhs));
    }

  template<typename... _Types>
    inline bool
    operator>(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs)
    { return __rhs < __lhs; }

  template<typename... _Types>
    inline bool
    operator<=(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs)
    { return !(__lhs > __rhs); }

  template<typename... _Types>
    inline bool
    operator>=(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs)
    { return !(__lhs < __rhs); }

  template<typename _Visitor, typename... _Variants>
    decltype(auto) visit(_Visitor&&, _Variants&&...);

  struct monostate { };

  constexpr bool operator<(monostate, monostate) noexcept
  { return false; }

  constexpr bool operator>(monostate, monostate) noexcept
  { return false; }

  constexpr bool operator<=(monostate, monostate) noexcept
  { return true; }

  constexpr bool operator>=(monostate, monostate) noexcept
  { return true; }

  constexpr bool operator==(monostate, monostate) noexcept
  { return true; }

  constexpr bool operator!=(monostate, monostate) noexcept
  { return false; }

  template<typename... _Types>
    inline auto swap(variant<_Types...>& __lhs, variant<_Types...>& __rhs)
    noexcept(noexcept(__lhs.swap(__rhs))) -> decltype(__lhs.swap(__rhs))
    { __lhs.swap(__rhs); }

  class bad_variant_access : public exception
  {
  public:
    bad_variant_access() noexcept : _M_reason("Unknown reason") { }
    const char* what() const noexcept override
    { return _M_reason; }

  private:
    bad_variant_access(const char* __reason) : _M_reason(__reason) { }

    const char* _M_reason;

    friend void __throw_bad_variant_access(const char* __what);
  };

  inline void
  __throw_bad_variant_access(const char* __what)
  { _GLIBCXX_THROW_OR_ABORT(bad_variant_access(__what)); }

  template<typename... _Types>
    class variant
    : private __detail::__variant::_Variant_base<_Types...>,
      private _Enable_default_constructor<
	is_default_constructible_v<
	  variant_alternative_t<0, variant<_Types...>>>, variant<_Types...>>,
      private _Enable_copy_move<
	__and_<is_copy_constructible<_Types>...>::value,
	__and_<is_copy_constructible<_Types>...,
	       is_move_constructible<_Types>...,
	       is_copy_assignable<_Types>...>::value,
	__and_<is_move_constructible<_Types>...>::value,
	__and_<is_move_constructible<_Types>...,
	       is_move_assignable<_Types>...>::value,
	variant<_Types...>>
    {
    private:
      using _Base = __detail::__variant::_Variant_base<_Types...>;
      using _Default_ctor_enabler =
	_Enable_default_constructor<
	  is_default_constructible_v<
	    variant_alternative_t<0, variant<_Types...>>>, variant<_Types...>>;

      template<typename _Tp>
	static constexpr bool
	__exactly_once = __detail::__variant::__exactly_once<_Tp, _Types...>;

      template<typename _Tp>
	static constexpr size_t __accepted_index =
	  __detail::__variant::__accepted_index<_Tp&&, variant>::value;

      template<size_t _Np, bool = _Np < sizeof...(_Types)>
	struct __to_type_impl;

      template<size_t _Np>
	struct __to_type_impl<_Np, true>
	{ using type = variant_alternative_t<_Np, variant>; };

      template<size_t _Np>
	using __to_type = typename __to_type_impl<_Np>::type;

      template<typename _Tp>
	using __accepted_type = __to_type<__accepted_index<_Tp>>;

      template<typename _Tp>
	using __storage = __detail::__variant::__storage<_Tp>;

      template<typename _Tp>
	static constexpr size_t __index_of =
	  __detail::__variant::__index_of_v<_Tp, _Types...>;

    public:
      constexpr variant()
      noexcept(is_nothrow_default_constructible_v<__to_type<0>>) = default;
      variant(const variant&) = default;
      variant(variant&&)
      noexcept(__and_<
	is_nothrow_move_constructible<_Types>...>::value) = default;

      template<typename _Tp,
	       typename = enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
			  && is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>
			  && !is_same_v<decay_t<_Tp>, variant>>>
	constexpr
	variant(_Tp&& __t)
	noexcept(is_nothrow_constructible_v<__accepted_type<_Tp&&>, _Tp&&>)
	: variant(in_place<__accepted_index<_Tp&&>>, forward<_Tp>(__t))
	{ __glibcxx_assert(holds_alternative<__accepted_type<_Tp&&>>(*this)); }

      template<typename _Tp, typename... _Args,
	       typename = enable_if_t<__exactly_once<_Tp>
			  && is_constructible_v<_Tp, _Args&&...>>>
	constexpr explicit
	variant(in_place_type_t<_Tp>, _Args&&... __args)
	: variant(in_place<__index_of<_Tp>>, forward<_Args>(__args)...)
	{ __glibcxx_assert(holds_alternative<_Tp>(*this)); }

      template<typename _Tp, typename _Up, typename... _Args,
	       typename = enable_if_t<__exactly_once<_Tp>
			  && is_constructible_v<
			    _Tp, initializer_list<_Up>&, _Args&&...>>>
	constexpr explicit
	variant(in_place_type_t<_Tp>, initializer_list<_Up> __il,
		_Args&&... __args)
	: variant(in_place<__index_of<_Tp>>, __il,
		  forward<_Args>(__args)...)
	{ __glibcxx_assert(holds_alternative<_Tp>(*this)); }

      template<size_t _Np, typename... _Args,
	       typename = enable_if_t<
		 is_constructible_v<__to_type<_Np>, _Args&&...>>>
	constexpr explicit
	variant(in_place_index_t<_Np>, _Args&&... __args)
	: _Base(in_place<_Np>, forward<_Args>(__args)...),
	_Default_ctor_enabler(_Enable_default_constructor_tag{})
	{ __glibcxx_assert(index() == _Np); }

      template<size_t _Np, typename _Up, typename... _Args,
	       typename = enable_if_t<is_constructible_v<__to_type<_Np>,
				      initializer_list<_Up>&, _Args&&...>>>
	constexpr explicit
	variant(in_place_index_t<_Np>, initializer_list<_Up> __il,
		_Args&&... __args)
	: _Base(in_place<_Np>, __il, forward<_Args>(__args)...),
	_Default_ctor_enabler(_Enable_default_constructor_tag{})
	{ __glibcxx_assert(index() == _Np); }

      template<typename _Alloc,
	       typename = enable_if_t<
		 __is_uses_allocator_constructible_v<__to_type<0>, _Alloc>>>
	variant(allocator_arg_t, const _Alloc& __a)
	: variant(allocator_arg, __a, in_place<0>)
	{ }

      template<typename _Alloc,
	       typename = enable_if_t<__and_<__is_uses_allocator_constructible<
		 _Types, _Alloc,
		 add_lvalue_reference_t<add_const_t<_Types>>>...>::value>>
	variant(allocator_arg_t, const _Alloc& __a, const variant& __rhs)
	: _Base(__a, __rhs),
	_Default_ctor_enabler(_Enable_default_constructor_tag{})
	{ }

      template<typename _Alloc,
	       typename = enable_if_t<__and_<
		 __is_uses_allocator_constructible<
		   _Types, _Alloc, add_rvalue_reference_t<_Types>>...>::value>>
	variant(allocator_arg_t, const _Alloc& __a, variant&& __rhs)
	: _Base(__a, std::move(__rhs)),
	_Default_ctor_enabler(_Enable_default_constructor_tag{})
	{ }

      template<typename _Alloc, typename _Tp,
	       typename = enable_if_t<
		 __exactly_once<__accepted_type<_Tp&&>>
		 && __is_uses_allocator_constructible_v<
		   __accepted_type<_Tp&&>, _Alloc, _Tp&&>
		 && !is_same_v<decay_t<_Tp>, variant>, variant&>>
	variant(allocator_arg_t, const _Alloc& __a, _Tp&& __t)
	: variant(allocator_arg, __a, in_place<__accepted_index<_Tp&&>>,
		  forward<_Tp>(__t))
	{ __glibcxx_assert(holds_alternative<__accepted_type<_Tp&&>>(*this)); }

      template<typename _Alloc, typename _Tp, typename... _Args,
	       typename = enable_if_t<
		 __exactly_once<_Tp>
		 && __is_uses_allocator_constructible_v<
		   _Tp, _Alloc, _Args&&...>>>
	variant(allocator_arg_t, const _Alloc& __a, in_place_type_t<_Tp>,
		_Args&&... __args)
	: variant(allocator_arg, __a, in_place<__index_of<_Tp>>,
		  forward<_Args>(__args)...)
	{ __glibcxx_assert(holds_alternative<_Tp>(*this)); }

      template<typename _Alloc, typename _Tp, typename _Up, typename... _Args,
	       typename = enable_if_t<
		 __exactly_once<_Tp>
		 && __is_uses_allocator_constructible_v<
		   _Tp, _Alloc, initializer_list<_Up>&, _Args&&...>>>
	variant(allocator_arg_t, const _Alloc& __a, in_place_type_t<_Tp>,
		initializer_list<_Up> __il, _Args&&... __args)
	: variant(allocator_arg, __a, in_place<__index_of<_Tp>>, __il,
		  forward<_Args>(__args)...)
	{ __glibcxx_assert(holds_alternative<_Tp>(*this)); }

      template<typename _Alloc, size_t _Np, typename... _Args,
	       typename = enable_if_t<
		 __is_uses_allocator_constructible_v<
		   __to_type<_Np>, _Alloc, _Args&&...>>>
	variant(allocator_arg_t, const _Alloc& __a, in_place_index_t<_Np>,
		_Args&&... __args)
	: _Base(__a, in_place<_Np>, forward<_Args>(__args)...),
	_Default_ctor_enabler(_Enable_default_constructor_tag{})
	{ __glibcxx_assert(index() == _Np); }

      template<typename _Alloc, size_t _Np, typename _Up, typename... _Args,
	       typename = enable_if_t<
		 __is_uses_allocator_constructible_v<
		   __to_type<_Np>, _Alloc, initializer_list<_Up>&, _Args&&...>>>
	variant(allocator_arg_t, const _Alloc& __a, in_place_index_t<_Np>,
		initializer_list<_Up> __il, _Args&&... __args)
	: _Base(__a, in_place<_Np>, __il, forward<_Args>(__args)...),
	_Default_ctor_enabler(_Enable_default_constructor_tag{})
	{ __glibcxx_assert(index() == _Np); }

      ~variant() = default;

      variant& operator=(const variant&) = default;
      variant& operator=(variant&&)
      noexcept(__and_<is_nothrow_move_constructible<_Types>...,
		      is_nothrow_move_assignable<_Types>...>::value) = default;

      template<typename _Tp>
	enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
		    && is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>
		    && is_assignable_v<__accepted_type<_Tp&&>&, _Tp&&>
		    && !is_same_v<decay_t<_Tp>, variant>, variant&>
	operator=(_Tp&& __rhs)
	noexcept(is_nothrow_assignable_v<__accepted_type<_Tp&&>&, _Tp&&>
		 && is_nothrow_constructible_v<__accepted_type<_Tp&&>, _Tp&&>)
	{
	  constexpr auto __index = __accepted_index<_Tp&&>;
	  if (index() == __index)
	    *static_cast<__storage<__to_type<__index>>*>(this->_M_storage())
	      = forward<_Tp>(__rhs);
	  else
	    this->emplace<__index>(forward<_Tp>(__rhs));
	  __glibcxx_assert(holds_alternative<__accepted_type<_Tp&&>>(*this));
	  return *this;
	}

      template<typename _Tp, typename... _Args>
	void emplace(_Args&&... __args)
	{
	  static_assert(__exactly_once<_Tp>,
			"T should occur for exactly once in alternatives");
	  this->emplace<__index_of<_Tp>>(forward<_Args>(__args)...);
	  __glibcxx_assert(holds_alternative<_Tp>(*this));
	}

      template<typename _Tp, typename _Up, typename... _Args>
	void emplace(initializer_list<_Up> __il, _Args&&... __args)
	{
	  static_assert(__exactly_once<_Tp>,
			"T should occur for exactly once in alternatives");
	  this->emplace<__index_of<_Tp>>(__il, forward<_Args>(__args)...);
	  __glibcxx_assert(holds_alternative<_Tp>(*this));
	}

      template<size_t _Np, typename... _Args>
	void emplace(_Args&&... __args)
	{
	  static_assert(_Np < sizeof...(_Types),
			"The index should be in [0, number of alternatives)");
	  this->~variant();
	  __try
	    {
	      ::new (this) variant(in_place<_Np>,
				   forward<_Args>(__args)...);
	    }
	  __catch (...)
	    {
	      this->_M_index = variant_npos;
	      __throw_exception_again;
	    }
	  __glibcxx_assert(index() == _Np);
	}

      template<size_t _Np, typename _Up, typename... _Args>
	void emplace(initializer_list<_Up> __il, _Args&&... __args)
	{
	  static_assert(_Np < sizeof...(_Types),
			"The index should be in [0, number of alternatives)");
	  this->~variant();
	  __try
	    {
	      ::new (this) variant(in_place<_Np>, __il,
				   forward<_Args>(__args)...);
	    }
	  __catch (...)
	    {
	      this->_M_index = variant_npos;
	      __throw_exception_again;
	    }
	  __glibcxx_assert(index() == _Np);
	}

      constexpr bool valueless_by_exception() const noexcept
      { return !this->_M_valid(); }

      constexpr size_t index() const noexcept
      { return this->_M_index; }

      void
      swap(variant& __rhs)
      noexcept(__and_<__is_nothrow_swappable<_Types>...>::value
	       && is_nothrow_move_assignable_v<variant>)
      {
	if (this->index() == __rhs.index())
	  {
	    if (this->_M_valid())
	      {
		static constexpr void (*_S_vtable[])(void*, void*) =
		  { &__detail::__variant::__erased_swap<
		      __storage<_Types>&, __storage<_Types>&>... };
		_S_vtable[__rhs._M_index](this->_M_storage(),
					  __rhs._M_storage());
	      }
	  }
	else if (!this->_M_valid())
	  {
	    *this = std::move(__rhs);
	  }
	else if (!__rhs._M_valid())
	  {
	    __rhs = std::move(*this);
	  }
	else
	  {
	    auto __tmp = std::move(__rhs);
	    __rhs = std::move(*this);
	    *this = std::move(__tmp);
	  }
      }

      template<typename _Vp>
	friend void* __detail::__variant::__get_storage(_Vp&& __v);
    };

  // To honor algebraic data type, variant<> should be a bottom type, which
  // is 0 (as opposed to a void type, which is 1). Use incomplete type to model
  // bottom type.
  template<> class variant<>;

  template<size_t _Np, typename... _Types>
    variant_alternative_t<_Np, variant<_Types...>>&
    get(variant<_Types...>& __v)
    {
      static_assert(_Np < sizeof...(_Types),
		    "The index should be in [0, number of alternatives)");
      if (__v.index() != _Np)
	__throw_bad_variant_access("Unexpected index");
      return __detail::__variant::__access<
	variant_alternative_t<_Np, variant<_Types...>>>(__v);
    }

  template<size_t _Np, typename... _Types>
    variant_alternative_t<_Np, variant<_Types...>>&&
    get(variant<_Types...>&& __v)
    {
      static_assert(_Np < sizeof...(_Types),
		    "The index should be in [0, number of alternatives)");
      if (__v.index() != _Np)
	__throw_bad_variant_access("Unexpected index");
      return __detail::__variant::__access<
	variant_alternative_t<_Np, variant<_Types...>>>(std::move(__v));
    }

  template<size_t _Np, typename... _Types>
    const variant_alternative_t<_Np, variant<_Types...>>&
    get(const variant<_Types...>& __v)
    {
      static_assert(_Np < sizeof...(_Types),
		    "The index should be in [0, number of alternatives)");
      if (__v.index() != _Np)
	__throw_bad_variant_access("Unexpected index");
      return __detail::__variant::__access<
	variant_alternative_t<_Np, variant<_Types...>>>(__v);
    }

  template<size_t _Np, typename... _Types>
    const variant_alternative_t<_Np, variant<_Types...>>&&
    get(const variant<_Types...>&& __v)
    {
      static_assert(_Np < sizeof...(_Types),
		    "The index should be in [0, number of alternatives)");
      if (__v.index() != _Np)
	__throw_bad_variant_access("Unexpected index");
      return __detail::__variant::__access<
	variant_alternative_t<_Np, variant<_Types...>>>(std::move(__v));
    }

  template<typename _Visitor, typename... _Variants>
    decltype(auto)
    visit(_Visitor&& __visitor, _Variants&&... __variants)
    {
      using _Result_type =
	decltype(forward<_Visitor>(__visitor)(get<0>(__variants)...));
      static constexpr auto _S_vtable =
	__detail::__variant::__gen_vtable<
	  _Result_type, _Visitor&&, _Variants&&...>::_S_apply();
      auto __func_ptr = _S_vtable._M_access(__variants.index()...);
      return (*__func_ptr)(forward<_Visitor>(__visitor),
			   __detail::__variant::__get_storage(__variants)...);
    }

  template<typename... _Types, typename _Alloc>
    struct uses_allocator<variant<_Types...>, _Alloc>
    : true_type { };

  template<typename... _Types>
    struct hash<variant<_Types...>>
    {
      using result_type = size_t;
      using argument_type = variant<_Types...>;

      size_t
      operator()(const variant<_Types...>& __t) const
      noexcept((... && noexcept(hash<decay_t<_Types>>{}(std::declval<_Types>()))))
      {
	if (!__t.valueless_by_exception())
	  {
	    namespace __edv = __detail::__variant;
	    static constexpr size_t (*_S_vtable[])(void*) =
	      { &__edv::__erased_hash<const __edv::__storage<_Types>&>... };
	    return hash<size_t>{}(__t.index())
	      + _S_vtable[__t.index()](__edv::__get_storage(__t));
	  }
	return hash<size_t>{}(__t.index());
      }
    };

  template<>
    struct hash<monostate>
    {
      using result_type = size_t;
      using argument_type = monostate;

      size_t
      operator()(const monostate& __t) const noexcept
      {
	constexpr size_t __magic_monostate_hash = -7777;
	return __magic_monostate_hash;
      }
    };

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++17

#endif // _GLIBCXX_VARIANT