variant

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// <variant> -*- C++ -*-
 
// Copyright (C) 2016-2022 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_types.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 = add_const_t<variant_alternative_t<_Np, _Variant>>; };
 
  template<size_t _Np, typename _Variant>
    struct variant_alternative<_Np, volatile _Variant>
    { using type = add_volatile_t<variant_alternative_t<_Np, _Variant>>; };
 
  template<size_t _Np, typename _Variant>
    struct variant_alternative<_Np, const volatile _Variant>
    { using type = add_cv_t<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); }
 
  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 = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
      std::_Construct(__addr, 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();
    };
 
  // "... 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>
    struct __accepted_index
    : integral_constant<size_t, variant_npos>
    { };
 
  template<typename _Tp, typename _Variant>
    struct __accepted_index<_Tp, _Variant, void_t<_FUN_type<_Tp, _Variant>>>
    : _FUN_type<_Tp, _Variant>
    { };
 
  template <typename _Maybe_variant_cookie, typename _Variant>
    struct _Extra_visit_slot_needed
    {
      template <typename> struct _Variant_never_valueless;
 
      template <typename... _Types>
        struct _Variant_never_valueless<variant<_Types...>>
        : bool_constant<__variant::__never_valueless<_Types...>()> {};
 
      static constexpr bool value =
        (is_same_v<_Maybe_variant_cookie, __variant_cookie>
         || is_same_v<_Maybe_variant_cookie, __variant_idx_cookie>)
        && !_Variant_never_valueless<__remove_cvref_t<_Variant>>::value;
    };
 
  // 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; };
 
      template <typename... _Args>
        struct __untag_result<const void(*)(_Args...)>
        : false_type
        { using element_type = void(*)(_Args...); };
 
      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>::value ? 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>::value)
            (_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");
      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<__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");
      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<__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");
      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<__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");
      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<__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>::value;
 
      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
                operator=(variant(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)
        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