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

// Copyright (C) 2016-2023 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 >= 201703L

#include <initializer_list>
#include <type_traits>
#include <bits/enable_special_members.h>
#include <bits/exception_defines.h>
#include <bits/functional_hash.h>
#include <bits/invoke.h>
#include <bits/parse_numbers.h>
#include <bits/stl_iterator_base_funcs.h>
#include <bits/stl_construct.h>
#include <bits/utility.h> // in_place_index_t
#if __cplusplus >= 202002L
# include <compare>
#endif

#if __cpp_concepts >= 202002L && __cpp_constexpr >= 201811L
// P2231R1 constexpr needs constexpr unions and constrained destructors.
# define __cpp_lib_variant 202106L
#else
# include <ext/aligned_buffer.h> // Use __aligned_membuf instead of union.
# define __cpp_lib_variant 202102L
#endif

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>
    inline constexpr size_t variant_size_v = variant_size<_Variant>::value;

  template<typename... _Types>
    inline constexpr size_t
    variant_size_v<variant<_Types...>> = sizeof...(_Types);

  template<typename... _Types>
    inline constexpr size_t
    variant_size_v<const variant<_Types...>> = sizeof...(_Types);

  template<size_t _Np, typename _Variant>
    struct variant_alternative;

  template<size_t _Np, typename... _Types>
    struct variant_alternative<_Np, variant<_Types...>>
    {
      static_assert(_Np < sizeof...(_Types));

      using type = typename _Nth_type<_Np, _Types...>::type;
    };

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

  template<size_t _Np, typename _Variant>
    struct variant_alternative<_Np, const _Variant>
    { using type = const variant_alternative_t<_Np, _Variant>; };

  template<size_t _Np, typename _Variant>
    struct variant_alternative<_Np, volatile _Variant>
    { using type = volatile variant_alternative_t<_Np, _Variant>; };

  template<size_t _Np, typename _Variant>
    struct variant_alternative<_Np, const volatile _Variant>
    { using type = const volatile variant_alternative_t<_Np, _Variant>; };

  inline constexpr size_t variant_npos = -1;

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

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

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

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

  template<typename _Result_type, typename _Visitor, typename... _Variants>
    constexpr decltype(auto)
    __do_visit(_Visitor&& __visitor, _Variants&&... __variants);

  template <typename... _Types, typename _Tp>
    _GLIBCXX20_CONSTEXPR
    decltype(auto)
    __variant_cast(_Tp&& __rhs)
    {
      if constexpr (is_lvalue_reference_v<_Tp>)
	{
	  if constexpr (is_const_v<remove_reference_t<_Tp>>)
	    return static_cast<const variant<_Types...>&>(__rhs);
	  else
	    return static_cast<variant<_Types...>&>(__rhs);
	}
      else
        return static_cast<variant<_Types...>&&>(__rhs);
    }

namespace __detail
{
namespace __variant
{
  // used for raw visitation
  struct __variant_cookie {};
  // used for raw visitation with indices passed in
  struct __variant_idx_cookie { using type = __variant_idx_cookie; };
  // Used to enable deduction (and same-type checking) for std::visit:
  template<typename _Tp> struct __deduce_visit_result { using type = _Tp; };

  // Visit variants that might be valueless.
  template<typename _Visitor, typename... _Variants>
    constexpr void
    __raw_visit(_Visitor&& __visitor, _Variants&&... __variants)
    {
      std::__do_visit<__variant_cookie>(std::forward<_Visitor>(__visitor),
				        std::forward<_Variants>(__variants)...);
    }

  // Visit variants that might be valueless, passing indices to the visitor.
  template<typename _Visitor, typename... _Variants>
    constexpr void
    __raw_idx_visit(_Visitor&& __visitor, _Variants&&... __variants)
    {
      std::__do_visit<__variant_idx_cookie>(std::forward<_Visitor>(__visitor),
	  std::forward<_Variants>(__variants)...);
    }

  // The __as function templates implement the exposition-only "as-variant"

  template<typename... _Types>
    constexpr std::variant<_Types...>&
    __as(std::variant<_Types...>& __v) noexcept
    { return __v; }

  template<typename... _Types>
    constexpr const std::variant<_Types...>&
    __as(const std::variant<_Types...>& __v) noexcept
    { return __v; }

  template<typename... _Types>
    constexpr std::variant<_Types...>&&
    __as(std::variant<_Types...>&& __v) noexcept
    { return std::move(__v); }

  template<typename... _Types>
    constexpr const std::variant<_Types...>&&
    __as(const std::variant<_Types...>&& __v) noexcept
    { return std::move(__v); }

  // For C++17:
  // _Uninitialized<T> is guaranteed to be a trivially destructible type,
  // even if T is not.
  // For C++20:
  // _Uninitialized<T> is trivially destructible iff T is, so _Variant_union
  // needs a constrained non-trivial destructor.
  template<typename _Type, bool = std::is_trivially_destructible_v<_Type>>
    struct _Uninitialized;

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

      constexpr const _Type& _M_get() const & noexcept
      { return _M_storage; }

      constexpr _Type& _M_get() & noexcept
      { return _M_storage; }

      constexpr const _Type&& _M_get() const && noexcept
      { return std::move(_M_storage); }

      constexpr _Type&& _M_get() && noexcept
      { return std::move(_M_storage); }

      _Type _M_storage;
    };

  template<typename _Type>
    struct _Uninitialized<_Type, false>
    {
#if __cpp_lib_variant >= 202106L
      template<typename... _Args>
	constexpr
	_Uninitialized(in_place_index_t<0>, _Args&&... __args)
	: _M_storage(std::forward<_Args>(__args)...)
	{ }

      constexpr ~_Uninitialized() { }

      _Uninitialized(const _Uninitialized&) = default;
      _Uninitialized(_Uninitialized&&) = default;
      _Uninitialized& operator=(const _Uninitialized&) = default;
      _Uninitialized& operator=(_Uninitialized&&) = default;

      constexpr const _Type& _M_get() const & noexcept
      { return _M_storage; }

      constexpr _Type& _M_get() & noexcept
      { return _M_storage; }

      constexpr const _Type&& _M_get() const && noexcept
      { return std::move(_M_storage); }

      constexpr _Type&& _M_get() && noexcept
      { return std::move(_M_storage); }

      struct _Empty_byte { };

      union {
	_Empty_byte _M_empty;
	_Type _M_storage;
      };
#else
      template<typename... _Args>
	constexpr
	_Uninitialized(in_place_index_t<0>, _Args&&... __args)
	{
	  ::new ((void*)std::addressof(_M_storage))
	    _Type(std::forward<_Args>(__args)...);
	}

      const _Type& _M_get() const & noexcept
      { return *_M_storage._M_ptr(); }

      _Type& _M_get() & noexcept
      { return *_M_storage._M_ptr(); }

      const _Type&& _M_get() const && noexcept
      { return std::move(*_M_storage._M_ptr()); }

      _Type&& _M_get() && noexcept
      { return std::move(*_M_storage._M_ptr()); }

      __gnu_cxx::__aligned_membuf<_Type> _M_storage;
#endif
    };

  template<size_t _Np, typename _Union>
    constexpr decltype(auto)
    __get_n(_Union&& __u) noexcept
    {
      if constexpr (_Np == 0)
	return std::forward<_Union>(__u)._M_first._M_get();
      else if constexpr (_Np == 1)
	return std::forward<_Union>(__u)._M_rest._M_first._M_get();
      else if constexpr (_Np == 2)
	return std::forward<_Union>(__u)._M_rest._M_rest._M_first._M_get();
      else
	return __variant::__get_n<_Np - 3>(
		 std::forward<_Union>(__u)._M_rest._M_rest._M_rest);
    }

  // Returns the typed storage for __v.
  template<size_t _Np, typename _Variant>
    constexpr decltype(auto)
    __get(_Variant&& __v) noexcept
    { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }

  // Gets the _Uninitialized to construct into for __u.
  template<size_t _Np, typename _Union>
    constexpr decltype(auto)
    __construct_n(_Union& __u) noexcept
    {
      if constexpr (_Np == 0)
	return &__u._M_first;
      else if constexpr (_Np == 1)
	{
	  std::_Construct(&__u._M_rest);
	  return &__u._M_rest._M_first;
	}
      else if constexpr (_Np == 2)
	{
	  std::_Construct(&__u._M_rest);
	  std::_Construct(&__u._M_rest._M_rest);
	  return &__u._M_rest._M_rest._M_first;
	}
      else
	{
	  std::_Construct(&__u._M_rest);
	  std::_Construct(&__u._M_rest._M_rest);
	  std::_Construct(&__u._M_rest._M_rest._M_rest);
	  return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
	}
    }

  template<typename... _Types>
    struct _Traits
    {
      static constexpr bool _S_default_ctor =
	  is_default_constructible_v<typename _Nth_type<0, _Types...>::type>;
      static constexpr bool _S_copy_ctor =
	  (is_copy_constructible_v<_Types> && ...);
      static constexpr bool _S_move_ctor =
	  (is_move_constructible_v<_Types> && ...);
      static constexpr bool _S_copy_assign =
	  _S_copy_ctor
	  && (is_copy_assignable_v<_Types> && ...);
      static constexpr bool _S_move_assign =
	  _S_move_ctor
	  && (is_move_assignable_v<_Types> && ...);

      static constexpr bool _S_trivial_dtor =
	  (is_trivially_destructible_v<_Types> && ...);
      static constexpr bool _S_trivial_copy_ctor =
	  (is_trivially_copy_constructible_v<_Types> && ...);
      static constexpr bool _S_trivial_move_ctor =
	  (is_trivially_move_constructible_v<_Types> && ...);
      static constexpr bool _S_trivial_copy_assign =
	  _S_trivial_dtor && _S_trivial_copy_ctor
	  && (is_trivially_copy_assignable_v<_Types> && ...);
      static constexpr bool _S_trivial_move_assign =
	  _S_trivial_dtor && _S_trivial_move_ctor
	  && (is_trivially_move_assignable_v<_Types> && ...);

      // The following nothrow traits are for non-trivial SMFs. Trivial SMFs
      // are always nothrow.
      static constexpr bool _S_nothrow_default_ctor =
	  is_nothrow_default_constructible_v<
	      typename _Nth_type<0, _Types...>::type>;
      static constexpr bool _S_nothrow_copy_ctor = false;
      static constexpr bool _S_nothrow_move_ctor =
	  (is_nothrow_move_constructible_v<_Types> && ...);
      static constexpr bool _S_nothrow_copy_assign = false;
      static constexpr bool _S_nothrow_move_assign =
	  _S_nothrow_move_ctor
	  && (is_nothrow_move_assignable_v<_Types> && ...);
    };

  // Defines members and ctors.
  template<typename... _Types>
    union _Variadic_union
    {
      _Variadic_union() = default;

      template<size_t _Np, typename... _Args>
	_Variadic_union(in_place_index_t<_Np>, _Args&&...) = delete;
    };

  template<typename _First, typename... _Rest>
    union _Variadic_union<_First, _Rest...>
    {
      constexpr _Variadic_union() : _M_rest() { }

      template<typename... _Args>
	constexpr
	_Variadic_union(in_place_index_t<0>, _Args&&... __args)
	: _M_first(in_place_index<0>, std::forward<_Args>(__args)...)
	{ }

      template<size_t _Np, typename... _Args>
	constexpr
	_Variadic_union(in_place_index_t<_Np>, _Args&&... __args)
	: _M_rest(in_place_index<_Np-1>, std::forward<_Args>(__args)...)
	{ }

#if __cpp_lib_variant >= 202106L
      _Variadic_union(const _Variadic_union&) = default;
      _Variadic_union(_Variadic_union&&) = default;
      _Variadic_union& operator=(const _Variadic_union&) = default;
      _Variadic_union& operator=(_Variadic_union&&) = default;

      ~_Variadic_union() = default;

      constexpr ~_Variadic_union()
	requires (!__has_trivial_destructor(_First))
	      || (!__has_trivial_destructor(_Variadic_union<_Rest...>))
      { }
#endif

      _Uninitialized<_First> _M_first;
      _Variadic_union<_Rest...> _M_rest;
    };

  // _Never_valueless_alt is true for variant alternatives that can
  // always be placed in a variant without it becoming valueless.

  // For suitably-small, trivially copyable types we can create temporaries
  // on the stack and then memcpy them into place.
  template<typename _Tp>
    struct _Never_valueless_alt
    : __and_<bool_constant<sizeof(_Tp) <= 256>, is_trivially_copyable<_Tp>>
    { };

  // Specialize _Never_valueless_alt for other types which have a
  // non-throwing and cheap move construction and move assignment operator,
  // so that emplacing the type will provide the strong exception-safety
  // guarantee, by creating and moving a temporary.
  // Whether _Never_valueless_alt<T> is true or not affects the ABI of a
  // variant using that alternative, so we can't change the value later!

  // True if every alternative in _Types... can be emplaced in a variant
  // without it becoming valueless. If this is true, variant<_Types...>
  // can never be valueless, which enables some minor optimizations.
  template <typename... _Types>
    constexpr bool __never_valueless()
    {
      return _Traits<_Types...>::_S_move_assign
	&& (_Never_valueless_alt<_Types>::value && ...);
    }

  // Defines index and the dtor, possibly trivial.
  template<bool __trivially_destructible, typename... _Types>
    struct _Variant_storage;

  template <typename... _Types>
    using __select_index =
      typename __select_int::_Select_int_base<sizeof...(_Types),
					      unsigned char,
					      unsigned short>::type::value_type;

  template<typename... _Types>
    struct _Variant_storage<false, _Types...>
    {
      constexpr
      _Variant_storage()
      : _M_index(static_cast<__index_type>(variant_npos))
      { }

      template<size_t _Np, typename... _Args>
	constexpr
	_Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
	: _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
	_M_index{_Np}
	{ }

      constexpr void
      _M_reset()
      {
	if (!_M_valid()) [[unlikely]]
	  return;

	std::__do_visit<void>([](auto&& __this_mem) mutable
	  {
	    std::_Destroy(std::__addressof(__this_mem));
	  }, __variant_cast<_Types...>(*this));

	_M_index = static_cast<__index_type>(variant_npos);
      }

      _GLIBCXX20_CONSTEXPR
      ~_Variant_storage()
      { _M_reset(); }

      constexpr bool
      _M_valid() const noexcept
      {
	if constexpr (__variant::__never_valueless<_Types...>())
	  return true;
	return this->_M_index != __index_type(variant_npos);
      }

      _Variadic_union<_Types...> _M_u;
      using __index_type = __select_index<_Types...>;
      __index_type _M_index;
    };

  template<typename... _Types>
    struct _Variant_storage<true, _Types...>
    {
      constexpr
      _Variant_storage()
      : _M_index(static_cast<__index_type>(variant_npos))
      { }

      template<size_t _Np, typename... _Args>
	constexpr
	_Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
	: _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
	_M_index{_Np}
	{ }

      constexpr void
      _M_reset() noexcept
      { _M_index = static_cast<__index_type>(variant_npos); }

      constexpr bool
      _M_valid() const noexcept
      {
	if constexpr (__variant::__never_valueless<_Types...>())
	  return true;
	// It would be nice if we could just return true for -fno-exceptions.
	// It's possible (but inadvisable) that a std::variant could become
	// valueless in a translation unit compiled with -fexceptions and then
	// be passed to functions compiled with -fno-exceptions. We would need
	// some #ifdef _GLIBCXX_NO_EXCEPTIONS_GLOBALLY property to elide all
	// checks for valueless_by_exception().
	return this->_M_index != static_cast<__index_type>(variant_npos);
      }

      _Variadic_union<_Types...> _M_u;
      using __index_type = __select_index<_Types...>;
      __index_type _M_index;
    };

  // Implementation of v.emplace<N>(args...).
  template<size_t _Np, bool _Triv, typename... _Types, typename... _Args>
    _GLIBCXX20_CONSTEXPR
    inline void
    __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
    {
      __v._M_reset();
      auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
      std::_Construct(__addr, in_place_index<0>,
		      std::forward<_Args>(__args)...);
      // Construction didn't throw, so can set the new index now:
      __v._M_index = _Np;
    }

  template<typename... _Types>
    using _Variant_storage_alias =
	_Variant_storage<_Traits<_Types...>::_S_trivial_dtor, _Types...>;

  // The following are (Copy|Move) (ctor|assign) layers for forwarding
  // triviality and handling non-trivial SMF behaviors.

  template<bool, typename... _Types>
    struct _Copy_ctor_base : _Variant_storage_alias<_Types...>
    {
      using _Base = _Variant_storage_alias<_Types...>;
      using _Base::_Base;

      _GLIBCXX20_CONSTEXPR
      _Copy_ctor_base(const _Copy_ctor_base& __rhs)
	  noexcept(_Traits<_Types...>::_S_nothrow_copy_ctor)
      {
	__variant::__raw_idx_visit(
	  [this](auto&& __rhs_mem, auto __rhs_index) mutable
	  {
	    constexpr size_t __j = __rhs_index;
	    if constexpr (__j != variant_npos)
	      std::_Construct(std::__addressof(this->_M_u),
			      in_place_index<__j>, __rhs_mem);
	  }, __variant_cast<_Types...>(__rhs));
	this->_M_index = __rhs._M_index;
      }

      _Copy_ctor_base(_Copy_ctor_base&&) = default;
      _Copy_ctor_base& operator=(const _Copy_ctor_base&) = default;
      _Copy_ctor_base& operator=(_Copy_ctor_base&&) = default;
    };

  template<typename... _Types>
    struct _Copy_ctor_base<true, _Types...> : _Variant_storage_alias<_Types...>
    {
      using _Base = _Variant_storage_alias<_Types...>;
      using _Base::_Base;
    };

  template<typename... _Types>
    using _Copy_ctor_alias =
	_Copy_ctor_base<_Traits<_Types...>::_S_trivial_copy_ctor, _Types...>;

  template<bool, typename... _Types>
    struct _Move_ctor_base : _Copy_ctor_alias<_Types...>
    {
      using _Base = _Copy_ctor_alias<_Types...>;
      using _Base::_Base;

      _GLIBCXX20_CONSTEXPR
      _Move_ctor_base(_Move_ctor_base&& __rhs)
	  noexcept(_Traits<_Types...>::_S_nothrow_move_ctor)
      {
	__variant::__raw_idx_visit(
	  [this](auto&& __rhs_mem, auto __rhs_index) mutable
	  {
	    constexpr size_t __j = __rhs_index;
	    if constexpr (__j != variant_npos)
	      std::_Construct(std::__addressof(this->_M_u),
			      in_place_index<__j>,
			      std::forward<decltype(__rhs_mem)>(__rhs_mem));
	  }, __variant_cast<_Types...>(std::move(__rhs)));
	this->_M_index = __rhs._M_index;
      }

      _Move_ctor_base(const _Move_ctor_base&) = default;
      _Move_ctor_base& operator=(const _Move_ctor_base&) = default;
      _Move_ctor_base& operator=(_Move_ctor_base&&) = default;
    };

  template<typename... _Types>
    struct _Move_ctor_base<true, _Types...> : _Copy_ctor_alias<_Types...>
    {
      using _Base = _Copy_ctor_alias<_Types...>;
      using _Base::_Base;
    };

  template<typename... _Types>
    using _Move_ctor_alias =
	_Move_ctor_base<_Traits<_Types...>::_S_trivial_move_ctor, _Types...>;

  template<bool, typename... _Types>
    struct _Copy_assign_base : _Move_ctor_alias<_Types...>
    {
      using _Base = _Move_ctor_alias<_Types...>;
      using _Base::_Base;

      _GLIBCXX20_CONSTEXPR
      _Copy_assign_base&
      operator=(const _Copy_assign_base& __rhs)
	  noexcept(_Traits<_Types...>::_S_nothrow_copy_assign)
      {
	__variant::__raw_idx_visit(
	  [this](auto&& __rhs_mem, auto __rhs_index) mutable
	  {
	    constexpr size_t __j = __rhs_index;
	    if constexpr (__j == variant_npos)
	      this->_M_reset(); // Make *this valueless.
	    else if (this->_M_index == __j)
	      __variant::__get<__j>(*this) = __rhs_mem;
	    else
	      {
		using _Tj = typename _Nth_type<__j, _Types...>::type;
		if constexpr (is_nothrow_copy_constructible_v<_Tj>
			      || !is_nothrow_move_constructible_v<_Tj>)
		  __variant::__emplace<__j>(*this, __rhs_mem);
		else
		  {
		    using _Variant = variant<_Types...>;
		    _Variant& __self = __variant_cast<_Types...>(*this);
		    __self = _Variant(in_place_index<__j>, __rhs_mem);
		  }
	      }
	  }, __variant_cast<_Types...>(__rhs));
	return *this;
      }

      _Copy_assign_base(const _Copy_assign_base&) = default;
      _Copy_assign_base(_Copy_assign_base&&) = default;
      _Copy_assign_base& operator=(_Copy_assign_base&&) = default;
    };

  template<typename... _Types>
    struct _Copy_assign_base<true, _Types...> : _Move_ctor_alias<_Types...>
    {
      using _Base = _Move_ctor_alias<_Types...>;
      using _Base::_Base;
    };

  template<typename... _Types>
    using _Copy_assign_alias =
      _Copy_assign_base<_Traits<_Types...>::_S_trivial_copy_assign, _Types...>;

  template<bool, typename... _Types>
    struct _Move_assign_base : _Copy_assign_alias<_Types...>
    {
      using _Base = _Copy_assign_alias<_Types...>;
      using _Base::_Base;

      _GLIBCXX20_CONSTEXPR
      _Move_assign_base&
      operator=(_Move_assign_base&& __rhs)
	  noexcept(_Traits<_Types...>::_S_nothrow_move_assign)
      {
	__variant::__raw_idx_visit(
	  [this](auto&& __rhs_mem, auto __rhs_index) mutable
	  {
	    constexpr size_t __j = __rhs_index;
	    if constexpr (__j != variant_npos)
	      {
		if (this->_M_index == __j)
		  __variant::__get<__j>(*this) = std::move(__rhs_mem);
		else
		  {
		    using _Tj = typename _Nth_type<__j, _Types...>::type;
		    if constexpr (is_nothrow_move_constructible_v<_Tj>)
		      __variant::__emplace<__j>(*this, std::move(__rhs_mem));
		    else
		      {
			using _Variant = variant<_Types...>;
			_Variant& __self = __variant_cast<_Types...>(*this);
			__self.template emplace<__j>(std::move(__rhs_mem));
		      }
		  }
	      }
	    else
	      this->_M_reset();
	  }, __variant_cast<_Types...>(__rhs));
	return *this;
      }

      _Move_assign_base(const _Move_assign_base&) = default;
      _Move_assign_base(_Move_assign_base&&) = default;
      _Move_assign_base& operator=(const _Move_assign_base&) = default;
    };

  template<typename... _Types>
    struct _Move_assign_base<true, _Types...> : _Copy_assign_alias<_Types...>
    {
      using _Base = _Copy_assign_alias<_Types...>;
      using _Base::_Base;
    };

  template<typename... _Types>
    using _Move_assign_alias =
      _Move_assign_base<_Traits<_Types...>::_S_trivial_move_assign, _Types...>;

  template<typename... _Types>
    struct _Variant_base : _Move_assign_alias<_Types...>
    {
      using _Base = _Move_assign_alias<_Types...>;

      constexpr
      _Variant_base() noexcept(_Traits<_Types...>::_S_nothrow_default_ctor)
      : _Variant_base(in_place_index<0>) { }

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

      _Variant_base(const _Variant_base&) = default;
      _Variant_base(_Variant_base&&) = default;
      _Variant_base& operator=(const _Variant_base&) = default;
      _Variant_base& operator=(_Variant_base&&) = default;
    };

  template<typename _Tp, typename... _Types>
    inline constexpr bool __exactly_once
      = std::__find_uniq_type_in_pack<_Tp, _Types...>() < sizeof...(_Types);

  // Helper used to check for valid conversions that don't involve narrowing.
  template<typename _Ti> struct _Arr { _Ti _M_x[1]; };

  // "Build an imaginary function FUN(Ti) for each alternative type Ti"
  template<size_t _Ind, typename _Tp, typename _Ti, typename = void>
    struct _Build_FUN
    {
      // This function means 'using _Build_FUN<I, T, Ti>::_S_fun;' is valid,
      // but only static functions will be considered in the call below.
      void _S_fun() = delete;
    };

  // "... for which Ti x[] = {std::forward<T>(t)}; is well-formed."
  template<size_t _Ind, typename _Tp, typename _Ti>
    struct _Build_FUN<_Ind, _Tp, _Ti,
		      void_t<decltype(_Arr<_Ti>{{std::declval<_Tp>()}})>>
    {
      // This is the FUN function for type _Ti, with index _Ind
      static integral_constant<size_t, _Ind> _S_fun(_Ti);
    };

  template<typename _Tp, typename _Variant,
	   typename = make_index_sequence<variant_size_v<_Variant>>>
    struct _Build_FUNs;

  template<typename _Tp, typename... _Ti, size_t... _Ind>
    struct _Build_FUNs<_Tp, variant<_Ti...>, index_sequence<_Ind...>>
    : _Build_FUN<_Ind, _Tp, _Ti>...
    {
      using _Build_FUN<_Ind, _Tp, _Ti>::_S_fun...;
    };

  // The index j of the overload FUN(Tj) selected by overload resolution
  // for FUN(std::forward<_Tp>(t))
  template<typename _Tp, typename _Variant>
    using _FUN_type
      = decltype(_Build_FUNs<_Tp, _Variant>::_S_fun(std::declval<_Tp>()));

  // The index selected for FUN(std::forward<T>(t)), or variant_npos if none.
  template<typename _Tp, typename _Variant, typename = void>
    inline constexpr size_t
    __accepted_index = variant_npos;

  template<typename _Tp, typename _Variant>
    inline constexpr size_t
    __accepted_index<_Tp, _Variant, void_t<_FUN_type<_Tp, _Variant>>>
      = _FUN_type<_Tp, _Variant>::value;

  template<typename _Maybe_variant_cookie, typename _Variant,
	   typename = __remove_cvref_t<_Variant>>
    inline constexpr bool
    __extra_visit_slot_needed = false;

  template<typename _Var, typename... _Types>
    inline constexpr bool
    __extra_visit_slot_needed<__variant_cookie, _Var, variant<_Types...>>
      = !__variant::__never_valueless<_Types...>();

  template<typename _Var, typename... _Types>
    inline constexpr bool
    __extra_visit_slot_needed<__variant_idx_cookie, _Var, variant<_Types...>>
      = !__variant::__never_valueless<_Types...>();

  // Used for storing a multi-dimensional vtable.
  template<typename _Tp, size_t... _Dimensions>
    struct _Multi_array;

  // Partial specialization with rank zero, stores a single _Tp element.
  template<typename _Tp>
    struct _Multi_array<_Tp>
    {
      template<typename>
	struct __untag_result
	: false_type
	{ using element_type = _Tp; };

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wignored-qualifiers"
      template <typename... _Args>
	struct __untag_result<const void(*)(_Args...)>
	: false_type
	{ using element_type = void(*)(_Args...); };
#pragma GCC diagnostic pop

      template <typename... _Args>
	struct __untag_result<__variant_cookie(*)(_Args...)>
	: false_type
	{ using element_type = void(*)(_Args...); };

      template <typename... _Args>
	struct __untag_result<__variant_idx_cookie(*)(_Args...)>
	: false_type
	{ using element_type = void(*)(_Args...); };

      template <typename _Res, typename... _Args>
	struct __untag_result<__deduce_visit_result<_Res>(*)(_Args...)>
	: true_type
	{ using element_type = _Res(*)(_Args...); };

      using __result_is_deduced = __untag_result<_Tp>;

      constexpr const typename __untag_result<_Tp>::element_type&
      _M_access() const
      { return _M_data; }

      typename __untag_result<_Tp>::element_type _M_data;
    };

  // Partial specialization with rank >= 1.
  template<typename _Ret,
	   typename _Visitor,
	   typename... _Variants,
	   size_t __first, size_t... __rest>
    struct _Multi_array<_Ret(*)(_Visitor, _Variants...), __first, __rest...>
    {
      static constexpr size_t __index =
	sizeof...(_Variants) - sizeof...(__rest) - 1;

      using _Variant = typename _Nth_type<__index, _Variants...>::type;

      static constexpr int __do_cookie =
	__extra_visit_slot_needed<_Ret, _Variant> ? 1 : 0;

      using _Tp = _Ret(*)(_Visitor, _Variants...);

      template<typename... _Args>
	constexpr decltype(auto)
	_M_access(size_t __first_index, _Args... __rest_indices) const
        {
	  return _M_arr[__first_index + __do_cookie]
	    ._M_access(__rest_indices...);
	}

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

  // Creates a multi-dimensional vtable recursively.
  //
  // For example,
  // visit([](auto, auto){},
  //       variant<int, char>(),  // typedef'ed as V1
  //       variant<float, double, long double>())  // typedef'ed as V2
  // will trigger instantiations of:
  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 2, 3>,
  //                   tuple<V1&&, V2&&>, std::index_sequence<>>
  //   __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
  //                     tuple<V1&&, V2&&>, std::index_sequence<0>>
  //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
  //                       tuple<V1&&, V2&&>, std::index_sequence<0, 0>>
  //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
  //                       tuple<V1&&, V2&&>, std::index_sequence<0, 1>>
  //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
  //                       tuple<V1&&, V2&&>, std::index_sequence<0, 2>>
  //   __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
  //                     tuple<V1&&, V2&&>, std::index_sequence<1>>
  //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
  //                       tuple<V1&&, V2&&>, std::index_sequence<1, 0>>
  //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
  //                       tuple<V1&&, V2&&>, std::index_sequence<1, 1>>
  //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
  //                       tuple<V1&&, V2&&>, std::index_sequence<1, 2>>
  // The returned multi-dimensional vtable can be fast accessed by the visitor
  // using index calculation.
  template<typename _Array_type, typename _Index_seq>
    struct __gen_vtable_impl;

  // Defines the _S_apply() member that returns a _Multi_array populated
  // with function pointers that perform the visitation expressions e(m)
  // for each valid pack of indexes into the variant types _Variants.
  //
  // This partial specialization builds up the index sequences by recursively
  // calling _S_apply() on the next specialization of __gen_vtable_impl.
  // The base case of the recursion defines the actual function pointers.
  template<typename _Result_type, typename _Visitor, size_t... __dimensions,
	   typename... _Variants, size_t... __indices>
    struct __gen_vtable_impl<
	_Multi_array<_Result_type (*)(_Visitor, _Variants...), __dimensions...>,
	std::index_sequence<__indices...>>
    {
      using _Next =
	  remove_reference_t<typename _Nth_type<sizeof...(__indices),
			     _Variants...>::type>;
      using _Array_type =
	  _Multi_array<_Result_type (*)(_Visitor, _Variants...),
		       __dimensions...>;

      static constexpr _Array_type
      _S_apply()
      {
	_Array_type __vtable{};
	_S_apply_all_alts(
	  __vtable, make_index_sequence<variant_size_v<_Next>>());
	return __vtable;
      }

      template<size_t... __var_indices>
	static constexpr void
	_S_apply_all_alts(_Array_type& __vtable,
			  std::index_sequence<__var_indices...>)
	{
	  if constexpr (__extra_visit_slot_needed<_Result_type, _Next>)
	    (_S_apply_single_alt<true, __var_indices>(
	      __vtable._M_arr[__var_indices + 1],
	      &(__vtable._M_arr[0])), ...);
	  else
	    (_S_apply_single_alt<false, __var_indices>(
	      __vtable._M_arr[__var_indices]), ...);
	}

      template<bool __do_cookie, size_t __index, typename _Tp>
	static constexpr void
	_S_apply_single_alt(_Tp& __element, _Tp* __cookie_element = nullptr)
	{
	  if constexpr (__do_cookie)
	    {
	      __element = __gen_vtable_impl<
		_Tp,
		std::index_sequence<__indices..., __index>>::_S_apply();
	      *__cookie_element = __gen_vtable_impl<
		_Tp,
		std::index_sequence<__indices..., variant_npos>>::_S_apply();
	    }
	  else
	    {
	      auto __tmp_element = __gen_vtable_impl<
		remove_reference_t<decltype(__element)>,
		std::index_sequence<__indices..., __index>>::_S_apply();
	      static_assert(is_same_v<_Tp, decltype(__tmp_element)>,
			    "std::visit requires the visitor to have the same "
			    "return type for all alternatives of a variant");
	      __element = __tmp_element;
	    }
	}
    };

  // This partial specialization is the base case for the recursion.
  // It populates a _Multi_array element with the address of a function
  // that invokes the visitor with the alternatives specified by __indices.
  template<typename _Result_type, typename _Visitor, typename... _Variants,
	   size_t... __indices>
    struct __gen_vtable_impl<
      _Multi_array<_Result_type (*)(_Visitor, _Variants...)>,
		   std::index_sequence<__indices...>>
    {
      using _Array_type =
	  _Multi_array<_Result_type (*)(_Visitor, _Variants...)>;

      template<size_t __index, typename _Variant>
	static constexpr decltype(auto)
	__element_by_index_or_cookie(_Variant&& __var) noexcept
        {
	  if constexpr (__index != variant_npos)
	    return __variant::__get<__index>(std::forward<_Variant>(__var));
	  else
	    return __variant_cookie{};
	}

      static constexpr decltype(auto)
      __visit_invoke(_Visitor&& __visitor, _Variants... __vars)
      {
	if constexpr (is_same_v<_Result_type, __variant_idx_cookie>)
	  // For raw visitation using indices, pass the indices to the visitor
	  // and discard the return value:
	  std::__invoke(std::forward<_Visitor>(__visitor),
	      __element_by_index_or_cookie<__indices>(
		std::forward<_Variants>(__vars))...,
	      integral_constant<size_t, __indices>()...);
	else if constexpr (is_same_v<_Result_type, __variant_cookie>)
	  // For raw visitation without indices, and discard the return value:
	  std::__invoke(std::forward<_Visitor>(__visitor),
	      __element_by_index_or_cookie<__indices>(
		std::forward<_Variants>(__vars))...);
	else if constexpr (_Array_type::__result_is_deduced::value)
	  // For the usual std::visit case deduce the return value:
	  return std::__invoke(std::forward<_Visitor>(__visitor),
	      __element_by_index_or_cookie<__indices>(
		std::forward<_Variants>(__vars))...);
	else // for std::visit<R> use INVOKE<R>
	  return std::__invoke_r<_Result_type>(
	      std::forward<_Visitor>(__visitor),
	      __variant::__get<__indices>(std::forward<_Variants>(__vars))...);
      }

      static constexpr auto
      _S_apply()
      {
	if constexpr (_Array_type::__result_is_deduced::value)
	  {
	    constexpr bool __visit_ret_type_mismatch =
	      !is_same_v<typename _Result_type::type,
			 decltype(__visit_invoke(std::declval<_Visitor>(),
				    std::declval<_Variants>()...))>;
	    if constexpr (__visit_ret_type_mismatch)
	      {
		struct __cannot_match {};
		return __cannot_match{};
	      }
	    else
	      return _Array_type{&__visit_invoke};
	  }
	else
	  return _Array_type{&__visit_invoke};
      }
    };

  template<typename _Result_type, typename _Visitor, typename... _Variants>
    struct __gen_vtable
    {
      using _Array_type =
	  _Multi_array<_Result_type (*)(_Visitor, _Variants...),
		       variant_size_v<remove_reference_t<_Variants>>...>;

      static constexpr _Array_type _S_vtable
	= __gen_vtable_impl<_Array_type, std::index_sequence<>>::_S_apply();
    };

  template<size_t _Np, typename _Tp>
    struct _Base_dedup : public _Tp { };

  template<typename _Variant, typename __indices>
    struct _Variant_hash_base;

  template<typename... _Types, size_t... __indices>
    struct _Variant_hash_base<variant<_Types...>,
			      std::index_sequence<__indices...>>
    : _Base_dedup<__indices, __poison_hash<remove_const_t<_Types>>>... { };

  // Equivalent to decltype(get<_Np>(as-variant(declval<_Variant>())))
  template<size_t _Np, typename _Variant,
      typename _AsV = decltype(__variant::__as(std::declval<_Variant>())),
      typename _Tp = variant_alternative_t<_Np, remove_reference_t<_AsV>>>
    using __get_t
      = __conditional_t<is_lvalue_reference_v<_Variant>, _Tp&, _Tp&&>;

  // Return type of std::visit.
  template<typename _Visitor, typename... _Variants>
    using __visit_result_t
      = invoke_result_t<_Visitor, __get_t<0, _Variants>...>;

  template<typename _Tp, typename... _Types>
    constexpr inline bool __same_types = (is_same_v<_Tp, _Types> && ...);

  template <typename _Visitor, typename _Variant, size_t... _Idxs>
    constexpr bool __check_visitor_results(std::index_sequence<_Idxs...>)
    {
      return __same_types<
	invoke_result_t<_Visitor, __get_t<_Idxs, _Variant>>...
	>;
    }

} // namespace __variant
} // namespace __detail

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

  template<typename _Tp, typename... _Types>
    constexpr _Tp&
    get(variant<_Types...>& __v)
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T must occur exactly once in alternatives");
      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
      return std::get<__n>(__v);
    }

  template<typename _Tp, typename... _Types>
    constexpr _Tp&&
    get(variant<_Types...>&& __v)
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T must occur exactly once in alternatives");
      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
      return std::get<__n>(std::move(__v));
    }

  template<typename _Tp, typename... _Types>
    constexpr const _Tp&
    get(const variant<_Types...>& __v)
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T must occur exactly once in alternatives");
      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
      return std::get<__n>(__v);
    }

  template<typename _Tp, typename... _Types>
    constexpr const _Tp&&
    get(const variant<_Types...>&& __v)
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T must occur exactly once in alternatives");
      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
      return std::get<__n>(std::move(__v));
    }

  template<size_t _Np, typename... _Types>
    constexpr 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 must be in [0, number of alternatives)");
      static_assert(!is_void_v<_Alternative_type>, "_Tp must not be void");
      if (__ptr && __ptr->index() == _Np)
	return std::addressof(__detail::__variant::__get<_Np>(*__ptr));
      return nullptr;
    }

  template<size_t _Np, typename... _Types>
    constexpr
    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 must be in [0, number of alternatives)");
      static_assert(!is_void_v<_Alternative_type>, "_Tp must not be void");
      if (__ptr && __ptr->index() == _Np)
	return std::addressof(__detail::__variant::__get<_Np>(*__ptr));
      return nullptr;
    }

  template<typename _Tp, typename... _Types>
    constexpr add_pointer_t<_Tp>
    get_if(variant<_Types...>* __ptr) noexcept
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T must occur exactly once in alternatives");
      static_assert(!is_void_v<_Tp>, "_Tp must not be void");
      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
      return std::get_if<__n>(__ptr);
    }

  template<typename _Tp, typename... _Types>
    constexpr add_pointer_t<const _Tp>
    get_if(const variant<_Types...>* __ptr) noexcept
    {
      static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
		    "T must occur exactly once in alternatives");
      static_assert(!is_void_v<_Tp>, "_Tp must not be void");
      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
      return std::get_if<__n>(__ptr);
    }

  struct monostate { };

#define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP, __NAME) \
  template<typename... _Types> \
    constexpr bool operator __OP(const variant<_Types...>& __lhs, \
				 const variant<_Types...>& __rhs) \
    { \
      bool __ret = true; \
      __detail::__variant::__raw_idx_visit( \
        [&__ret, &__lhs] (auto&& __rhs_mem, auto __rhs_index) mutable \
        { \
	  if constexpr (__rhs_index != variant_npos) \
	    { \
	      if (__lhs.index() == __rhs_index) \
	        { \
		  auto& __this_mem = std::get<__rhs_index>(__lhs);	\
                  __ret = __this_mem __OP __rhs_mem; \
                } \
	      else \
		__ret = (__lhs.index() + 1) __OP (__rhs_index + 1); \
            } \
          else \
            __ret = (__lhs.index() + 1) __OP (__rhs_index + 1); \
	}, __rhs); \
      return __ret; \
    }

  _VARIANT_RELATION_FUNCTION_TEMPLATE(<, less)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(<=, less_equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(==, equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(!=, not_equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(>=, greater_equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(>, greater)

#undef _VARIANT_RELATION_FUNCTION_TEMPLATE

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

#ifdef __cpp_lib_three_way_comparison
  template<typename... _Types>
    requires (three_way_comparable<_Types> && ...)
    constexpr
    common_comparison_category_t<compare_three_way_result_t<_Types>...>
    operator<=>(const variant<_Types...>& __v, const variant<_Types...>& __w)
    {
      common_comparison_category_t<compare_three_way_result_t<_Types>...> __ret
	= strong_ordering::equal;

      __detail::__variant::__raw_idx_visit(
	[&__ret, &__v] (auto&& __w_mem, auto __w_index) mutable
	{
	  if constexpr (__w_index != variant_npos)
	    {
	      if (__v.index() == __w_index)
		{
		  auto& __this_mem = std::get<__w_index>(__v);
		  __ret = __this_mem <=> __w_mem;
		  return;
		}
	    }
	  __ret = (__v.index() + 1) <=> (__w_index + 1);
	}, __w);
      return __ret;
    }

  constexpr strong_ordering
  operator<=>(monostate, monostate) noexcept { return strong_ordering::equal; }
#else
  constexpr bool operator!=(monostate, monostate) noexcept { return false; }
  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; }
#endif

  template<typename _Visitor, typename... _Variants>
    constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...>
    visit(_Visitor&&, _Variants&&...);

  template<typename... _Types>
    _GLIBCXX20_CONSTEXPR
    inline enable_if_t<(is_move_constructible_v<_Types> && ...)
			&& (is_swappable_v<_Types> && ...)>
    swap(variant<_Types...>& __lhs, variant<_Types...>& __rhs)
    noexcept(noexcept(__lhs.swap(__rhs)))
    { __lhs.swap(__rhs); }

  template<typename... _Types>
    enable_if_t<!((is_move_constructible_v<_Types> && ...)
		   && (is_swappable_v<_Types> && ...))>
    swap(variant<_Types...>&, variant<_Types...>&) = delete;

  class bad_variant_access : public exception
  {
  public:
    bad_variant_access() noexcept { }

    const char* what() const noexcept override
    { return _M_reason; }

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

    // Must point to a string with static storage duration:
    const char* _M_reason = "bad variant access";

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

  // Must only be called with a string literal
  inline void
  __throw_bad_variant_access(const char* __what)
  { _GLIBCXX_THROW_OR_ABORT(bad_variant_access(__what)); }

  inline void
  __throw_bad_variant_access(bool __valueless)
  {
    if (__valueless) [[__unlikely__]]
      __throw_bad_variant_access("std::get: variant is valueless");
    else
      __throw_bad_variant_access("std::get: wrong index for variant");
  }

  template<typename... _Types>
    class variant
    : private __detail::__variant::_Variant_base<_Types...>,
      private _Enable_default_constructor<
	__detail::__variant::_Traits<_Types...>::_S_default_ctor,
	  variant<_Types...>>,
      private _Enable_copy_move<
	__detail::__variant::_Traits<_Types...>::_S_copy_ctor,
	__detail::__variant::_Traits<_Types...>::_S_copy_assign,
	__detail::__variant::_Traits<_Types...>::_S_move_ctor,
	__detail::__variant::_Traits<_Types...>::_S_move_assign,
	variant<_Types...>>
    {
    private:
      template <typename... _UTypes, typename _Tp>
	friend _GLIBCXX20_CONSTEXPR decltype(auto)
	__variant_cast(_Tp&&);

      static_assert(sizeof...(_Types) > 0,
		    "variant must have at least one alternative");
      static_assert(!(std::is_reference_v<_Types> || ...),
		    "variant must have no reference alternative");
      static_assert(!(std::is_void_v<_Types> || ...),
		    "variant must have no void alternative");

      using _Base = __detail::__variant::_Variant_base<_Types...>;
      using _Default_ctor_enabler =
	_Enable_default_constructor<
	  __detail::__variant::_Traits<_Types...>::_S_default_ctor,
	    variant<_Types...>>;

      template<typename _Tp>
	static constexpr bool __not_self
	  = !is_same_v<__remove_cvref_t<_Tp>, variant>;

      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>;

      template<size_t _Np, typename = enable_if_t<(_Np < sizeof...(_Types))>>
	using __to_type = typename _Nth_type<_Np, _Types...>::type;

      template<typename _Tp, typename = enable_if_t<__not_self<_Tp>>>
	using __accepted_type = __to_type<__accepted_index<_Tp>>;

      template<typename _Tp>
	static constexpr size_t __index_of
	  = std::__find_uniq_type_in_pack<_Tp, _Types...>();

      using _Traits = __detail::__variant::_Traits<_Types...>;

      template<typename _Tp>
	struct __is_in_place_tag : false_type { };
      template<typename _Tp>
	struct __is_in_place_tag<in_place_type_t<_Tp>> : true_type { };
      template<size_t _Np>
	struct __is_in_place_tag<in_place_index_t<_Np>> : true_type { };

      template<typename _Tp>
	static constexpr bool __not_in_place_tag
	  = !__is_in_place_tag<__remove_cvref_t<_Tp>>::value;

    public:
      variant() = default;
      variant(const variant& __rhs) = default;
      variant(variant&&) = default;
      variant& operator=(const variant&) = default;
      variant& operator=(variant&&) = default;
      _GLIBCXX20_CONSTEXPR ~variant() = default;

      template<typename _Tp,
	       typename = enable_if_t<sizeof...(_Types) != 0>,
	       typename = enable_if_t<__not_in_place_tag<_Tp>>,
	       typename _Tj = __accepted_type<_Tp&&>,
	       typename = enable_if_t<__exactly_once<_Tj>
				      && is_constructible_v<_Tj, _Tp>>>
	constexpr
	variant(_Tp&& __t)
	noexcept(is_nothrow_constructible_v<_Tj, _Tp>)
	: variant(in_place_index<__accepted_index<_Tp>>,
		  std::forward<_Tp>(__t))
	{ }

      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<__index_of<_Tp>>,
		  std::forward<_Args>(__args)...)
	{ }

      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<__index_of<_Tp>>, __il,
		  std::forward<_Args>(__args)...)
	{ }

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

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

      template<typename _Tp>
	_GLIBCXX20_CONSTEXPR
	enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
		    && is_constructible_v<__accepted_type<_Tp&&>, _Tp>
		    && is_assignable_v<__accepted_type<_Tp&&>&, _Tp>,
		    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)
	    std::get<__index>(*this) = std::forward<_Tp>(__rhs);
	  else
	    {
	      using _Tj = __accepted_type<_Tp&&>;
	      if constexpr (is_nothrow_constructible_v<_Tj, _Tp>
			    || !is_nothrow_move_constructible_v<_Tj>)
		this->emplace<__index>(std::forward<_Tp>(__rhs));
	      else
		// _GLIBCXX_RESOLVE_LIB_DEFECTS
		// 3585. converting assignment with immovable alternative
		this->emplace<__index>(_Tj(std::forward<_Tp>(__rhs)));
	    }
	  return *this;
	}

      template<typename _Tp, typename... _Args>
	_GLIBCXX20_CONSTEXPR
	enable_if_t<is_constructible_v<_Tp, _Args...> && __exactly_once<_Tp>,
		    _Tp&>
	emplace(_Args&&... __args)
	{
	  constexpr size_t __index = __index_of<_Tp>;
	  return this->emplace<__index>(std::forward<_Args>(__args)...);
	}

      template<typename _Tp, typename _Up, typename... _Args>
	_GLIBCXX20_CONSTEXPR
	enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>
		    && __exactly_once<_Tp>,
		    _Tp&>
	emplace(initializer_list<_Up> __il, _Args&&... __args)
	{
	  constexpr size_t __index = __index_of<_Tp>;
	  return this->emplace<__index>(__il, std::forward<_Args>(__args)...);
	}

      template<size_t _Np, typename... _Args>
	_GLIBCXX20_CONSTEXPR
	enable_if_t<is_constructible_v<__to_type<_Np>, _Args...>,
		    __to_type<_Np>&>
	emplace(_Args&&... __args)
	{
	  namespace __variant = std::__detail::__variant;
	  using type = typename _Nth_type<_Np, _Types...>::type;
	  // Provide the strong exception-safety guarantee when possible,
	  // to avoid becoming valueless.
	  if constexpr (is_nothrow_constructible_v<type, _Args...>)
	    {
	      __variant::__emplace<_Np>(*this, std::forward<_Args>(__args)...);
	    }
	  else if constexpr (is_scalar_v<type>)
	    {
	      // This might invoke a potentially-throwing conversion operator:
	      const type __tmp(std::forward<_Args>(__args)...);
	      // But this won't throw:
	      __variant::__emplace<_Np>(*this, __tmp);
	    }
	  else if constexpr (__variant::_Never_valueless_alt<type>()
	      && _Traits::_S_move_assign)
	    {
	      // This construction might throw:
	      variant __tmp(in_place_index<_Np>,
			    std::forward<_Args>(__args)...);
	      // But _Never_valueless_alt<type> means this won't:
	      *this = std::move(__tmp);
	    }
	  else
	    {
	      // This case only provides the basic exception-safety guarantee,
	      // i.e. the variant can become valueless.
	      __variant::__emplace<_Np>(*this, std::forward<_Args>(__args)...);
	    }
	  return std::get<_Np>(*this);
	}

      template<size_t _Np, typename _Up, typename... _Args>
	_GLIBCXX20_CONSTEXPR
	enable_if_t<is_constructible_v<__to_type<_Np>,
				       initializer_list<_Up>&, _Args...>,
		    __to_type<_Np>&>
	emplace(initializer_list<_Up> __il, _Args&&... __args)
	{
	  namespace __variant = std::__detail::__variant;
	  using type = typename _Nth_type<_Np, _Types...>::type;
	  // Provide the strong exception-safety guarantee when possible,
	  // to avoid becoming valueless.
	  if constexpr (is_nothrow_constructible_v<type,
						   initializer_list<_Up>&,
						   _Args...>)
	    {
	      __variant::__emplace<_Np>(*this, __il,
					std::forward<_Args>(__args)...);
	    }
	  else if constexpr (__variant::_Never_valueless_alt<type>()
	      && _Traits::_S_move_assign)
	    {
	      // This construction might throw:
	      variant __tmp(in_place_index<_Np>, __il,
			    std::forward<_Args>(__args)...);
	      // But _Never_valueless_alt<type> means this won't:
	      *this = std::move(__tmp);
	    }
	  else
	    {
	      // This case only provides the basic exception-safety guarantee,
	      // i.e. the variant can become valueless.
	      __variant::__emplace<_Np>(*this, __il,
					std::forward<_Args>(__args)...);
	    }
	  return std::get<_Np>(*this);
	}

      template<size_t _Np, typename... _Args>
	enable_if_t<!(_Np < sizeof...(_Types))> emplace(_Args&&...) = delete;

      template<typename _Tp, typename... _Args>
	enable_if_t<!__exactly_once<_Tp>> emplace(_Args&&...) = delete;

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

      constexpr size_t index() const noexcept
      {
	using __index_type = typename _Base::__index_type;
	if constexpr (__detail::__variant::__never_valueless<_Types...>())
	  return this->_M_index;
	else if constexpr (sizeof...(_Types) <= __index_type(-1) / 2)
	  return make_signed_t<__index_type>(this->_M_index);
	else
	  return size_t(__index_type(this->_M_index + 1)) - 1;
      }

      _GLIBCXX20_CONSTEXPR
      void
      swap(variant& __rhs)
      noexcept((__is_nothrow_swappable<_Types>::value && ...)
	       && is_nothrow_move_constructible_v<variant>)
      {
	static_assert((is_move_constructible_v<_Types> && ...));

	// Handle this here to simplify the visitation.
	if (__rhs.valueless_by_exception()) [[__unlikely__]]
	  {
	    if (!this->valueless_by_exception()) [[__likely__]]
	      __rhs.swap(*this);
	    return;
	  }

	namespace __variant = __detail::__variant;

	__variant::__raw_idx_visit(
	  [this, &__rhs](auto&& __rhs_mem, auto __rhs_index) mutable
	  {
	    constexpr size_t __j = __rhs_index;
	    if constexpr (__j != variant_npos)
	      {
		if (this->index() == __j)
		  {
		    using std::swap;
		    swap(std::get<__j>(*this), __rhs_mem);
		  }
		else
		  {
		    auto __tmp(std::move(__rhs_mem));

		    if constexpr (_Traits::_S_trivial_move_assign)
		      __rhs = std::move(*this);
		    else
		      __variant::__raw_idx_visit(
			[&__rhs](auto&& __this_mem, auto __this_index) mutable
			{
			  constexpr size_t __k = __this_index;
			  if constexpr (__k != variant_npos)
			    __variant::__emplace<__k>(__rhs,
						      std::move(__this_mem));
			}, *this);

		    __variant::__emplace<__j>(*this, std::move(__tmp));
		  }
	      }
	  }, __rhs);
      }

#if defined(__clang__) && __clang_major__ <= 7
    public:
      using _Base::_M_u; // See https://bugs.llvm.org/show_bug.cgi?id=31852
#endif

    private:
      template<size_t _Np, typename _Vp>
	friend constexpr decltype(auto)
	__detail::__variant::__get(_Vp&& __v) noexcept;

#define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP) \
      template<typename... _Tp> \
	friend constexpr bool \
	operator __OP(const variant<_Tp...>& __lhs, \
		      const variant<_Tp...>& __rhs);

      _VARIANT_RELATION_FUNCTION_TEMPLATE(<)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(<=)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(==)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(!=)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(>=)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(>)

#undef _VARIANT_RELATION_FUNCTION_TEMPLATE
    };

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

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

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

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

  /// @cond undocumented
  template<typename _Result_type, typename _Visitor, typename... _Variants>
    constexpr decltype(auto)
    __do_visit(_Visitor&& __visitor, _Variants&&... __variants)
    {
      // Get the silly case of visiting no variants out of the way first.
      if constexpr (sizeof...(_Variants) == 0)
	{
	  if constexpr (is_void_v<_Result_type>)
	    return (void) std::forward<_Visitor>(__visitor)();
	  else
	    return std::forward<_Visitor>(__visitor)();
	}
      else
	{
	  constexpr size_t __max = 11; // "These go to eleven."

	  // The type of the first variant in the pack.
	  using _V0 = typename _Nth_type<0, _Variants...>::type;
	  // The number of alternatives in that first variant.
	  constexpr auto __n = variant_size_v<remove_reference_t<_V0>>;

	  if constexpr (sizeof...(_Variants) > 1 || __n > __max)
	    {
	      // Use a jump table for the general case.
	      constexpr auto& __vtable = __detail::__variant::__gen_vtable<
		_Result_type, _Visitor&&, _Variants&&...>::_S_vtable;

	      auto __func_ptr = __vtable._M_access(__variants.index()...);
	      return (*__func_ptr)(std::forward<_Visitor>(__visitor),
				   std::forward<_Variants>(__variants)...);
	    }
	  else // We have a single variant with a small number of alternatives.
	    {
	      // A name for the first variant in the pack.
	      _V0& __v0
		= [](_V0& __v, ...) -> _V0& { return __v; }(__variants...);

	      using __detail::__variant::_Multi_array;
	      using __detail::__variant::__gen_vtable_impl;
	      using _Ma = _Multi_array<_Result_type (*)(_Visitor&&, _V0&&)>;

#ifdef _GLIBCXX_DEBUG
# define _GLIBCXX_VISIT_UNREACHABLE __builtin_trap
#else
# define _GLIBCXX_VISIT_UNREACHABLE __builtin_unreachable
#endif

#define _GLIBCXX_VISIT_CASE(N)						\
  case N:								\
  {									\
    if constexpr (N < __n)						\
      {									\
	return __gen_vtable_impl<_Ma, index_sequence<N>>::		\
	  __visit_invoke(std::forward<_Visitor>(__visitor),		\
			 std::forward<_V0>(__v0));		\
      }									\
    else _GLIBCXX_VISIT_UNREACHABLE();					\
  }

	      switch (__v0.index())
		{
		  _GLIBCXX_VISIT_CASE(0)
		  _GLIBCXX_VISIT_CASE(1)
		  _GLIBCXX_VISIT_CASE(2)
		  _GLIBCXX_VISIT_CASE(3)
		  _GLIBCXX_VISIT_CASE(4)
		  _GLIBCXX_VISIT_CASE(5)
		  _GLIBCXX_VISIT_CASE(6)
		  _GLIBCXX_VISIT_CASE(7)
		  _GLIBCXX_VISIT_CASE(8)
		  _GLIBCXX_VISIT_CASE(9)
		  _GLIBCXX_VISIT_CASE(10)
		case variant_npos:
		  using __detail::__variant::__variant_idx_cookie;
		  using __detail::__variant::__variant_cookie;
		  if constexpr (is_same_v<_Result_type, __variant_idx_cookie>
				|| is_same_v<_Result_type, __variant_cookie>)
		    {
		      using _Npos = index_sequence<variant_npos>;
		      return __gen_vtable_impl<_Ma, _Npos>::
			__visit_invoke(std::forward<_Visitor>(__visitor),
				       std::forward<_V0>(__v0));
		    }
		  else
		    _GLIBCXX_VISIT_UNREACHABLE();
		default:
		  _GLIBCXX_VISIT_UNREACHABLE();
		}
#undef _GLIBCXX_VISIT_CASE
#undef _GLIBCXX_VISIT_UNREACHABLE
	    }
	}
    }
  /// @endcond

  template<typename _Visitor, typename... _Variants>
    constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...>
    visit(_Visitor&& __visitor, _Variants&&... __variants)
    {
      namespace __variant = std::__detail::__variant;

      if ((__variant::__as(__variants).valueless_by_exception() || ...))
	__throw_bad_variant_access("std::visit: variant is valueless");

      using _Result_type
	= __detail::__variant::__visit_result_t<_Visitor, _Variants...>;

      using _Tag = __detail::__variant::__deduce_visit_result<_Result_type>;

      if constexpr (sizeof...(_Variants) == 1)
	{
	  using _Vp = decltype(__variant::__as(std::declval<_Variants>()...));

	  constexpr bool __visit_rettypes_match = __detail::__variant::
	    __check_visitor_results<_Visitor, _Vp>(
	      make_index_sequence<variant_size_v<remove_reference_t<_Vp>>>());
	  if constexpr (!__visit_rettypes_match)
	    {
	      static_assert(__visit_rettypes_match,
			  "std::visit requires the visitor to have the same "
			  "return type for all alternatives of a variant");
	      return;
	    }
	  else
	    return std::__do_visit<_Tag>(
	      std::forward<_Visitor>(__visitor),
	      static_cast<_Vp>(__variants)...);
	}
      else
	return std::__do_visit<_Tag>(
	  std::forward<_Visitor>(__visitor),
	  __variant::__as(std::forward<_Variants>(__variants))...);
    }

#if __cplusplus > 201703L
  template<typename _Res, typename _Visitor, typename... _Variants>
    constexpr _Res
    visit(_Visitor&& __visitor, _Variants&&... __variants)
    {
      namespace __variant = std::__detail::__variant;

      if ((__variant::__as(__variants).valueless_by_exception() || ...))
	__throw_bad_variant_access("std::visit<R>: variant is valueless");

      return std::__do_visit<_Res>(std::forward<_Visitor>(__visitor),
	  __variant::__as(std::forward<_Variants>(__variants))...);
    }
#endif

  /// @cond undocumented
  template<bool, typename... _Types>
    struct __variant_hash_call_base_impl
    {
      size_t
      operator()(const variant<_Types...>& __t) const
      noexcept((is_nothrow_invocable_v<hash<decay_t<_Types>>, _Types> && ...))
      {
	size_t __ret;
	__detail::__variant::__raw_visit(
	  [&__t, &__ret](auto&& __t_mem) mutable
	  {
	    using _Type = __remove_cvref_t<decltype(__t_mem)>;
	    if constexpr (!is_same_v<_Type,
			             __detail::__variant::__variant_cookie>)
	      __ret = std::hash<size_t>{}(__t.index())
		      + std::hash<_Type>{}(__t_mem);
	    else
	      __ret = std::hash<size_t>{}(__t.index());
	  }, __t);
	return __ret;
      }
    };

  template<typename... _Types>
    struct __variant_hash_call_base_impl<false, _Types...> {};

  template<typename... _Types>
    using __variant_hash_call_base =
    __variant_hash_call_base_impl<(__poison_hash<remove_const_t<_Types>>::
				   __enable_hash_call &&...), _Types...>;
  /// @endcond

  template<typename... _Types>
    struct hash<variant<_Types...>>
    : private __detail::__variant::_Variant_hash_base<
	variant<_Types...>, std::index_sequence_for<_Types...>>,
      public __variant_hash_call_base<_Types...>
    {
      using result_type [[__deprecated__]] = size_t;
      using argument_type [[__deprecated__]] = variant<_Types...>;
    };

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

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

  template<typename... _Types>
    struct __is_fast_hash<hash<variant<_Types...>>>
    : bool_constant<(__is_fast_hash<_Types>::value && ...)>
    { };

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++17

#endif // _GLIBCXX_VARIANT
