Variant.h

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#ifdef IPPL_USE_ALTERNATIVE_VARIANT
// <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/>.

 
#ifndef _GLIBCXX_VARIANT
#define _GLIBCXX_VARIANT 1
 
#pragma GCC system_header
 
#if __cplusplus >= 201703L
 
#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_construct.h>
#include <bits/stl_iterator_base_funcs.h>
#include <bits/utility.h>  // in_place_index_t
#include <initializer_list>
#include <type_traits>
#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);
            }
 
            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(){};
 
                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() = 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");
        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>;
 
        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));
    }

    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::__gen_vtable_impl;
                using __detail::__variant::_Multi_array;
                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_cookie;
                        using __detail::__variant::__variant_idx_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
            }
        }
    }
 
    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

    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...>;
 
    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 _GLIBCXX_VISIBILITY(default)
 
#endif  // C++17
 
#endif  // _GLIBCXX_VARIANT
#else
#include <variant>
#endif