// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP_VARIANT
#define _LIBCPP_VARIANT

/*
   variant synopsis

namespace std {

  // 20.7.2, class template variant
  template <class... Types>
  class variant {
  public:

    // 20.7.2.1, constructors
    constexpr variant() noexcept(see below);
    constexpr variant(const variant&);
    constexpr variant(variant&&) noexcept(see below);

    template <class T> constexpr variant(T&&) noexcept(see below);

    template <class T, class... Args>
    constexpr explicit variant(in_place_type_t<T>, Args&&...);

    template <class T, class U, class... Args>
    constexpr explicit variant(
        in_place_type_t<T>, initializer_list<U>, Args&&...);

    template <size_t I, class... Args>
    constexpr explicit variant(in_place_index_t<I>, Args&&...);

    template <size_t I, class U, class... Args>
    constexpr explicit variant(
        in_place_index_t<I>, initializer_list<U>, Args&&...);

    // 20.7.2.2, destructor
    ~variant();

    // 20.7.2.3, assignment
    constexpr variant& operator=(const variant&);
    constexpr variant& operator=(variant&&) noexcept(see below);

    template <class T> variant& operator=(T&&) noexcept(see below);

    // 20.7.2.4, modifiers
    template <class T, class... Args>
    T& emplace(Args&&...);

    template <class T, class U, class... Args>
    T& emplace(initializer_list<U>, Args&&...);

    template <size_t I, class... Args>
    variant_alternative_t<I, variant>& emplace(Args&&...);

    template <size_t I, class U, class...  Args>
    variant_alternative_t<I, variant>& emplace(initializer_list<U>, Args&&...);

    // 20.7.2.5, value status
    constexpr bool valueless_by_exception() const noexcept;
    constexpr size_t index() const noexcept;

    // 20.7.2.6, swap
    void swap(variant&) noexcept(see below);

    // [variant.visit], visitation
    template<class Self, class Visitor>
      constexpr decltype(auto) visit(this Self&&, Visitor&&); // Since C++26
    template<class R, class Self, class Visitor>
      constexpr R visit(this Self&&, Visitor&&);              // Since C++26
  };

  // 20.7.3, variant helper classes
  template <class T> struct variant_size; // undefined

  template <class T>
  inline constexpr size_t variant_size_v = variant_size<T>::value;

  template <class T> struct variant_size<const T>;
  template <class T> struct variant_size<volatile T>;
  template <class T> struct variant_size<const volatile T>;

  template <class... Types>
  struct variant_size<variant<Types...>>;

  template <size_t I, class T> struct variant_alternative; // undefined

  template <size_t I, class T>
  using variant_alternative_t = typename variant_alternative<I, T>::type;

  template <size_t I, class T> struct variant_alternative<I, const T>;
  template <size_t I, class T> struct variant_alternative<I, volatile T>;
  template <size_t I, class T> struct variant_alternative<I, const volatile T>;

  template <size_t I, class... Types>
  struct variant_alternative<I, variant<Types...>>;

  inline constexpr size_t variant_npos = -1;

  // 20.7.4, value access
  template <class T, class... Types>
  constexpr bool holds_alternative(const variant<Types...>&) noexcept;

  template <size_t I, class... Types>
  constexpr variant_alternative_t<I, variant<Types...>>&
  get(variant<Types...>&);

  template <size_t I, class... Types>
  constexpr variant_alternative_t<I, variant<Types...>>&&
  get(variant<Types...>&&);

  template <size_t I, class... Types>
  constexpr variant_alternative_t<I, variant<Types...>> const&
  get(const variant<Types...>&);

  template <size_t I, class... Types>
  constexpr variant_alternative_t<I, variant<Types...>> const&&
  get(const variant<Types...>&&);

  template <class T, class...  Types>
  constexpr T& get(variant<Types...>&);

  template <class T, class... Types>
  constexpr T&& get(variant<Types...>&&);

  template <class T, class... Types>
  constexpr const T& get(const variant<Types...>&);

  template <class T, class... Types>
  constexpr const T&& get(const variant<Types...>&&);

  template <size_t I, class... Types>
  constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
  get_if(variant<Types...>*) noexcept;

  template <size_t I, class... Types>
  constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
  get_if(const variant<Types...>*) noexcept;

  template <class T, class... Types>
  constexpr add_pointer_t<T>
  get_if(variant<Types...>*) noexcept;

  template <class T, class... Types>
  constexpr add_pointer_t<const T>
  get_if(const variant<Types...>*) noexcept;

  // 20.7.5, relational operators
  template <class... Types>
  constexpr bool operator==(const variant<Types...>&, const variant<Types...>&);

  template <class... Types>
  constexpr bool operator!=(const variant<Types...>&, const variant<Types...>&);

  template <class... Types>
  constexpr bool operator<(const variant<Types...>&, const variant<Types...>&);

  template <class... Types>
  constexpr bool operator>(const variant<Types...>&, const variant<Types...>&);

  template <class... Types>
  constexpr bool operator<=(const variant<Types...>&, const variant<Types...>&);

  template <class... Types>
  constexpr bool operator>=(const variant<Types...>&, const variant<Types...>&);

  template <class... Types> requires (three_way_comparable<Types> && ...)
  constexpr common_comparison_category_t<compare_three_way_result_t<Types>...>
    operator<=>(const variant<Types...>&, const variant<Types...>&);           // since C++20

  // 20.7.6, visitation
  template <class Visitor, class... Variants>
  constexpr see below visit(Visitor&&, Variants&&...);

  template <class R, class Visitor, class... Variants>
  constexpr R visit(Visitor&&, Variants&&...); // since C++20

  // 20.7.7, class monostate
  struct monostate;

  // 20.7.8, monostate relational operators
  constexpr bool operator==(monostate, monostate) noexcept;
  constexpr bool operator!=(monostate, monostate) noexcept;             // until C++20
  constexpr bool operator<(monostate, monostate) noexcept;              // until C++20
  constexpr bool operator>(monostate, monostate) noexcept;              // until C++20
  constexpr bool operator<=(monostate, monostate) noexcept;             // until C++20
  constexpr bool operator>=(monostate, monostate) noexcept;             // until C++20
  constexpr strong_ordering operator<=>(monostate, monostate) noexcept; // since C++20

  // 20.7.9, specialized algorithms
  template <class... Types>
  void swap(variant<Types...>&, variant<Types...>&) noexcept(see below);

  // 20.7.10, class bad_variant_access
  class bad_variant_access;

  // 20.7.11, hash support
  template <class T> struct hash;
  template <class... Types> struct hash<variant<Types...>>;
  template <> struct hash<monostate>;

} // namespace std

*/

#include <__availability>
#include <__compare/common_comparison_category.h>
#include <__compare/compare_three_way_result.h>
#include <__compare/three_way_comparable.h>
#include <__config>
#include <__exception/exception.h>
#include <__functional/hash.h>
#include <__functional/invoke.h>
#include <__functional/operations.h>
#include <__functional/unary_function.h>
#include <__memory/addressof.h>
#include <__tuple/find_index.h>
#include <__tuple/sfinae_helpers.h>
#include <__type_traits/add_const.h>
#include <__type_traits/add_cv.h>
#include <__type_traits/add_pointer.h>
#include <__type_traits/add_volatile.h>
#include <__type_traits/common_type.h>
#include <__type_traits/conjunction.h>
#include <__type_traits/dependent_type.h>
#include <__type_traits/is_array.h>
#include <__type_traits/is_constructible.h>
#include <__type_traits/is_destructible.h>
#include <__type_traits/is_nothrow_assignable.h>
#include <__type_traits/is_nothrow_constructible.h>
#include <__type_traits/is_reference.h>
#include <__type_traits/is_trivially_assignable.h>
#include <__type_traits/is_trivially_constructible.h>
#include <__type_traits/is_trivially_destructible.h>
#include <__type_traits/is_void.h>
#include <__type_traits/remove_const.h>
#include <__type_traits/remove_cvref.h>
#include <__type_traits/type_identity.h>
#include <__type_traits/void_t.h>
#include <__utility/declval.h>
#include <__utility/forward.h>
#include <__utility/forward_like.h>
#include <__utility/in_place.h>
#include <__utility/integer_sequence.h>
#include <__utility/move.h>
#include <__utility/swap.h>
#include <__variant/monostate.h>
#include <__verbose_abort>
#include <initializer_list>
#include <limits>
#include <new>
#include <version>

// standard-mandated includes

// [variant.syn]
#include <compare>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#  pragma GCC system_header
#endif

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

namespace std { // explicitly not using versioning namespace

class _LIBCPP_EXPORTED_FROM_ABI _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS bad_variant_access : public exception {
public:
  const char* what() const _NOEXCEPT override;
};

} // namespace std

_LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER >= 17

// Light N-dimensional array of function pointers. Used in place of std::array to avoid
// adding a dependency.
template <class _Tp, size_t _Size>
struct __farray {
  static_assert(_Size > 0, "N-dimensional array should never be empty in std::visit");
  _Tp __buf_[_Size] = {};

  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& operator[](size_t __n) const noexcept { return __buf_[__n]; }
};

_LIBCPP_NORETURN inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS void
__throw_bad_variant_access() {
#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
  throw bad_variant_access();
#  else
  _LIBCPP_VERBOSE_ABORT("bad_variant_access was thrown in -fno-exceptions mode");
#  endif
}

template <class... _Types>
class _LIBCPP_TEMPLATE_VIS variant;

template <class _Tp>
struct _LIBCPP_TEMPLATE_VIS variant_size;

template <class _Tp>
inline constexpr size_t variant_size_v = variant_size<_Tp>::value;

template <class _Tp>
struct _LIBCPP_TEMPLATE_VIS variant_size<const _Tp> : variant_size<_Tp> {};

template <class _Tp>
struct _LIBCPP_TEMPLATE_VIS variant_size<volatile _Tp> : variant_size<_Tp> {};

template <class _Tp>
struct _LIBCPP_TEMPLATE_VIS variant_size<const volatile _Tp> : variant_size<_Tp> {};

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

template <size_t _Ip, class _Tp>
struct _LIBCPP_TEMPLATE_VIS variant_alternative;

template <size_t _Ip, class _Tp>
using variant_alternative_t = typename variant_alternative<_Ip, _Tp>::type;

template <size_t _Ip, class _Tp>
struct _LIBCPP_TEMPLATE_VIS variant_alternative<_Ip, const _Tp> : add_const<variant_alternative_t<_Ip, _Tp>> {};

template <size_t _Ip, class _Tp>
struct _LIBCPP_TEMPLATE_VIS variant_alternative<_Ip, volatile _Tp> : add_volatile<variant_alternative_t<_Ip, _Tp>> {};

template <size_t _Ip, class _Tp>
struct _LIBCPP_TEMPLATE_VIS variant_alternative<_Ip, const volatile _Tp> : add_cv<variant_alternative_t<_Ip, _Tp>> {};

template <size_t _Ip, class... _Types>
struct _LIBCPP_TEMPLATE_VIS variant_alternative<_Ip, variant<_Types...>> {
  static_assert(_Ip < sizeof...(_Types), "Index out of bounds in std::variant_alternative<>");
  using type = __type_pack_element<_Ip, _Types...>;
};

inline constexpr size_t variant_npos = static_cast<size_t>(-1);

template <size_t _NumAlternatives>
_LIBCPP_HIDE_FROM_ABI constexpr auto __choose_index_type() {
#  ifdef _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
  if constexpr (_NumAlternatives < numeric_limits<unsigned char>::max())
    return static_cast<unsigned char>(0);
  else if constexpr (_NumAlternatives < numeric_limits<unsigned short>::max())
    return static_cast<unsigned short>(0);
  else
#  endif // _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
    return static_cast<unsigned int>(0);
}

template <size_t _NumAlts>
using __variant_index_t = decltype(std::__choose_index_type<_NumAlts>());

template <class _IndexType>
constexpr _IndexType __variant_npos = static_cast<_IndexType>(-1);

template <class... _Types>
class _LIBCPP_TEMPLATE_VIS variant;

template <class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr variant<_Types...>& __as_variant(variant<_Types...>& __vs) noexcept {
  return __vs;
}

template <class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr const variant<_Types...>& __as_variant(const variant<_Types...>& __vs) noexcept {
  return __vs;
}

template <class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr variant<_Types...>&& __as_variant(variant<_Types...>&& __vs) noexcept {
  return std::move(__vs);
}

template <class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr const variant<_Types...>&& __as_variant(const variant<_Types...>&& __vs) noexcept {
  return std::move(__vs);
}

namespace __find_detail {

template <class _Tp, class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr size_t __find_index() {
  constexpr bool __matches[] = {is_same_v<_Tp, _Types>...};
  size_t __result            = __not_found;
  for (size_t __i = 0; __i < sizeof...(_Types); ++__i) {
    if (__matches[__i]) {
      if (__result != __not_found) {
        return __ambiguous;
      }
      __result = __i;
    }
  }
  return __result;
}

template <size_t _Index>
struct __find_unambiguous_index_sfinae_impl : integral_constant<size_t, _Index> {};

template <>
struct __find_unambiguous_index_sfinae_impl<__not_found> {};

template <>
struct __find_unambiguous_index_sfinae_impl<__ambiguous> {};

template <class _Tp, class... _Types>
struct __find_unambiguous_index_sfinae : __find_unambiguous_index_sfinae_impl<__find_index<_Tp, _Types...>()> {};

} // namespace __find_detail

namespace __variant_detail {

struct __valueless_t {};

enum class _Trait { _TriviallyAvailable, _Available, _Unavailable };

template <typename _Tp, template <typename> class _IsTriviallyAvailable, template <typename> class _IsAvailable>
constexpr _Trait __trait =
    _IsTriviallyAvailable<_Tp>::value ? _Trait::_TriviallyAvailable
    : _IsAvailable<_Tp>::value
        ? _Trait::_Available
        : _Trait::_Unavailable;

_LIBCPP_HIDE_FROM_ABI constexpr _Trait __common_trait(initializer_list<_Trait> __traits) {
  _Trait __result = _Trait::_TriviallyAvailable;
  for (_Trait __t : __traits) {
    if (static_cast<int>(__t) > static_cast<int>(__result)) {
      __result = __t;
    }
  }
  return __result;
}

template <typename... _Types>
struct __traits {
  static constexpr _Trait __copy_constructible_trait =
      __variant_detail::__common_trait({__trait<_Types, is_trivially_copy_constructible, is_copy_constructible>...});

  static constexpr _Trait __move_constructible_trait =
      __variant_detail::__common_trait({__trait<_Types, is_trivially_move_constructible, is_move_constructible>...});

  static constexpr _Trait __copy_assignable_trait = __variant_detail::__common_trait(
      {__copy_constructible_trait, __trait<_Types, is_trivially_copy_assignable, is_copy_assignable>...});

  static constexpr _Trait __move_assignable_trait = __variant_detail::__common_trait(
      {__move_constructible_trait, __trait<_Types, is_trivially_move_assignable, is_move_assignable>...});

  static constexpr _Trait __destructible_trait =
      __variant_detail::__common_trait({__trait<_Types, is_trivially_destructible, is_destructible>...});
};

namespace __access {

struct __union {
  template <class _Vp>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto&& __get_alt(_Vp&& __v, in_place_index_t<0>) {
    return std::forward<_Vp>(__v).__head;
  }

  template <class _Vp, size_t _Ip>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto&& __get_alt(_Vp&& __v, in_place_index_t<_Ip>) {
    return __get_alt(std::forward<_Vp>(__v).__tail, in_place_index<_Ip - 1>);
  }
};

struct __base {
  template <size_t _Ip, class _Vp>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto&& __get_alt(_Vp&& __v) {
    return __union::__get_alt(std::forward<_Vp>(__v).__data, in_place_index<_Ip>);
  }
};

struct __variant {
  template <size_t _Ip, class _Vp>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto&& __get_alt(_Vp&& __v) {
    return __base::__get_alt<_Ip>(std::forward<_Vp>(__v).__impl_);
  }
};

} // namespace __access

namespace __visitation {

struct __base {
  template <class _Visitor, class... _Vs>
  _LIBCPP_HIDE_FROM_ABI static constexpr decltype(auto)
  __visit_alt_at(size_t __index, _Visitor&& __visitor, _Vs&&... __vs) {
    constexpr auto __fdiagonal = __make_fdiagonal<_Visitor&&, decltype(std::forward<_Vs>(__vs).__as_base())...>();
    return __fdiagonal[__index](std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs).__as_base()...);
  }

  template <class _Visitor, class... _Vs>
  _LIBCPP_HIDE_FROM_ABI static constexpr decltype(auto) __visit_alt(_Visitor&& __visitor, _Vs&&... __vs) {
    constexpr auto __fmatrix = __make_fmatrix<_Visitor&&, decltype(std::forward<_Vs>(__vs).__as_base())...>();
    return __at(__fmatrix, __vs.index()...)(std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs).__as_base()...);
  }

private:
  template <class _Tp>
  _LIBCPP_HIDE_FROM_ABI static constexpr const _Tp& __at(const _Tp& __elem) {
    return __elem;
  }

  template <class _Tp, size_t _Np, typename... _Indices>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto&&
  __at(const __farray<_Tp, _Np>& __elems, size_t __index, _Indices... __indices) {
    return __at(__elems[__index], __indices...);
  }

  template <class _Fp, class... _Fs>
  static _LIBCPP_HIDE_FROM_ABI constexpr void __std_visit_visitor_return_type_check() {
    static_assert(
        __all<is_same_v<_Fp, _Fs>...>::value, "`std::visit` requires the visitor to have a single return type.");
  }

  template <class... _Fs>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto __make_farray(_Fs&&... __fs) {
    __std_visit_visitor_return_type_check<__remove_cvref_t<_Fs>...>();
    using __result = __farray<common_type_t<__remove_cvref_t<_Fs>...>, sizeof...(_Fs)>;
    return __result{{std::forward<_Fs>(__fs)...}};
  }

  template <size_t... _Is>
  struct __dispatcher {
    template <class _Fp, class... _Vs>
    _LIBCPP_HIDE_FROM_ABI static constexpr decltype(auto) __dispatch(_Fp __f, _Vs... __vs) {
      return std::__invoke(static_cast<_Fp>(__f), __access::__base::__get_alt<_Is>(static_cast<_Vs>(__vs))...);
    }
  };

  template <class _Fp, class... _Vs, size_t... _Is>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto __make_dispatch(index_sequence<_Is...>) {
    return __dispatcher<_Is...>::template __dispatch<_Fp, _Vs...>;
  }

  template <size_t _Ip, class _Fp, class... _Vs>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto __make_fdiagonal_impl() {
    return __make_dispatch<_Fp, _Vs...>(index_sequence<((void)__type_identity<_Vs>{}, _Ip)...>{});
  }

  template <class _Fp, class... _Vs, size_t... _Is>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto __make_fdiagonal_impl(index_sequence<_Is...>) {
    return __base::__make_farray(__make_fdiagonal_impl<_Is, _Fp, _Vs...>()...);
  }

  template <class _Fp, class _Vp, class... _Vs>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto __make_fdiagonal() {
    constexpr size_t __np = __remove_cvref_t<_Vp>::__size();
    static_assert(__all<(__np == __remove_cvref_t<_Vs>::__size())...>::value);
    return __make_fdiagonal_impl<_Fp, _Vp, _Vs...>(make_index_sequence<__np>{});
  }

  template <class _Fp, class... _Vs, size_t... _Is>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto __make_fmatrix_impl(index_sequence<_Is...> __is) {
    return __make_dispatch<_Fp, _Vs...>(__is);
  }

  template <class _Fp, class... _Vs, size_t... _Is, size_t... _Js, class... _Ls>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto
  __make_fmatrix_impl(index_sequence<_Is...>, index_sequence<_Js...>, _Ls... __ls) {
    return __base::__make_farray(__make_fmatrix_impl<_Fp, _Vs...>(index_sequence<_Is..., _Js>{}, __ls...)...);
  }

  template <class _Fp, class... _Vs>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto __make_fmatrix() {
    return __make_fmatrix_impl<_Fp, _Vs...>(
        index_sequence<>{}, make_index_sequence<__remove_cvref_t<_Vs>::__size()>{}...);
  }
};

struct __variant {
  template <class _Visitor, class... _Vs>
  _LIBCPP_HIDE_FROM_ABI static constexpr decltype(auto)
  __visit_alt_at(size_t __index, _Visitor&& __visitor, _Vs&&... __vs) {
    return __base::__visit_alt_at(__index, std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs).__impl_...);
  }

  template <class _Visitor, class... _Vs>
  _LIBCPP_HIDE_FROM_ABI static constexpr decltype(auto) __visit_alt(_Visitor&& __visitor, _Vs&&... __vs) {
    return __base::__visit_alt(
        std::forward<_Visitor>(__visitor), std::__as_variant(std::forward<_Vs>(__vs)).__impl_...);
  }

  template <class _Visitor, class... _Vs>
  _LIBCPP_HIDE_FROM_ABI static constexpr decltype(auto)
  __visit_value_at(size_t __index, _Visitor&& __visitor, _Vs&&... __vs) {
    return __visit_alt_at(__index, __make_value_visitor(std::forward<_Visitor>(__visitor)), std::forward<_Vs>(__vs)...);
  }

  template <class _Visitor, class... _Vs>
  _LIBCPP_HIDE_FROM_ABI static constexpr decltype(auto) __visit_value(_Visitor&& __visitor, _Vs&&... __vs) {
    return __visit_alt(__make_value_visitor(std::forward<_Visitor>(__visitor)), std::forward<_Vs>(__vs)...);
  }

#  if _LIBCPP_STD_VER >= 20
  template <class _Rp, class _Visitor, class... _Vs>
  _LIBCPP_HIDE_FROM_ABI static constexpr _Rp __visit_value(_Visitor&& __visitor, _Vs&&... __vs) {
    return __visit_alt(__make_value_visitor<_Rp>(std::forward<_Visitor>(__visitor)), std::forward<_Vs>(__vs)...);
  }
#  endif

private:
  template <class _Visitor, class... _Values>
  static _LIBCPP_HIDE_FROM_ABI constexpr void __std_visit_exhaustive_visitor_check() {
    static_assert(is_invocable_v<_Visitor, _Values...>, "`std::visit` requires the visitor to be exhaustive.");
  }

  template <class _Visitor>
  struct __value_visitor {
    template <class... _Alts>
    _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator()(_Alts&&... __alts) const {
      __std_visit_exhaustive_visitor_check< _Visitor, decltype((std::forward<_Alts>(__alts).__value))...>();
      return std::__invoke(std::forward<_Visitor>(__visitor), std::forward<_Alts>(__alts).__value...);
    }
    _Visitor&& __visitor;
  };

#  if _LIBCPP_STD_VER >= 20
  template <class _Rp, class _Visitor>
  struct __value_visitor_return_type {
    template <class... _Alts>
    _LIBCPP_HIDE_FROM_ABI constexpr _Rp operator()(_Alts&&... __alts) const {
      __std_visit_exhaustive_visitor_check< _Visitor, decltype((std::forward<_Alts>(__alts).__value))...>();
      if constexpr (is_void_v<_Rp>) {
        std::__invoke(std::forward<_Visitor>(__visitor), std::forward<_Alts>(__alts).__value...);
      } else {
        return std::__invoke(std::forward<_Visitor>(__visitor), std::forward<_Alts>(__alts).__value...);
      }
    }

    _Visitor&& __visitor;
  };
#  endif

  template <class _Visitor>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto __make_value_visitor(_Visitor&& __visitor) {
    return __value_visitor<_Visitor>{std::forward<_Visitor>(__visitor)};
  }

#  if _LIBCPP_STD_VER >= 20
  template <class _Rp, class _Visitor>
  _LIBCPP_HIDE_FROM_ABI static constexpr auto __make_value_visitor(_Visitor&& __visitor) {
    return __value_visitor_return_type<_Rp, _Visitor>{std::forward<_Visitor>(__visitor)};
  }
#  endif
};

} // namespace __visitation

template <size_t _Index, class _Tp>
struct _LIBCPP_TEMPLATE_VIS __alt {
  using __value_type = _Tp;

  template <class... _Args>
  _LIBCPP_HIDE_FROM_ABI explicit constexpr __alt(in_place_t, _Args&&... __args)
      : __value(std::forward<_Args>(__args)...) {}

  __value_type __value;
};

template <_Trait _DestructibleTrait, size_t _Index, class... _Types>
union _LIBCPP_TEMPLATE_VIS __union;

template <_Trait _DestructibleTrait, size_t _Index>
union _LIBCPP_TEMPLATE_VIS __union<_DestructibleTrait, _Index> {};

#  define _LIBCPP_VARIANT_UNION(destructible_trait, destructor)                                                        \
    template <size_t _Index, class _Tp, class... _Types>                                                               \
    union _LIBCPP_TEMPLATE_VIS __union<destructible_trait, _Index, _Tp, _Types...> {                                   \
    public:                                                                                                            \
      _LIBCPP_HIDE_FROM_ABI explicit constexpr __union(__valueless_t) noexcept : __dummy{} {}                          \
                                                                                                                       \
      template <class... _Args>                                                                                        \
      _LIBCPP_HIDE_FROM_ABI explicit constexpr __union(in_place_index_t<0>, _Args&&... __args)                         \
          : __head(in_place, std::forward<_Args>(__args)...) {}                                                        \
                                                                                                                       \
      template <size_t _Ip, class... _Args>                                                                            \
      _LIBCPP_HIDE_FROM_ABI explicit constexpr __union(in_place_index_t<_Ip>, _Args&&... __args)                       \
          : __tail(in_place_index<_Ip - 1>, std::forward<_Args>(__args)...) {}                                         \
                                                                                                                       \
      __union(const __union&) = default;                                                                               \
      __union(__union&&)      = default;                                                                               \
                                                                                                                       \
      destructor                                                                                                       \
                                                                                                                       \
          __union&                                                                                                     \
          operator=(const __union&) = default;                                                                         \
      __union& operator=(__union&&) = default;                                                                         \
                                                                                                                       \
    private:                                                                                                           \
      char __dummy;                                                                                                    \
      __alt<_Index, _Tp> __head;                                                                                       \
      __union<destructible_trait, _Index + 1, _Types...> __tail;                                                       \
                                                                                                                       \
      friend struct __access::__union;                                                                                 \
    }

_LIBCPP_VARIANT_UNION(_Trait::_TriviallyAvailable, ~__union() = default;);
_LIBCPP_VARIANT_UNION(_Trait::_Available, ~__union(){});
_LIBCPP_VARIANT_UNION(_Trait::_Unavailable, ~__union() = delete;);

#  undef _LIBCPP_VARIANT_UNION

template <_Trait _DestructibleTrait, class... _Types>
class _LIBCPP_TEMPLATE_VIS __base {
public:
  using __index_t = __variant_index_t<sizeof...(_Types)>;

  _LIBCPP_HIDE_FROM_ABI explicit constexpr __base(__valueless_t __tag) noexcept
      : __data(__tag), __index(__variant_npos<__index_t>) {}

  template <size_t _Ip, class... _Args>
  _LIBCPP_HIDE_FROM_ABI explicit constexpr __base(in_place_index_t<_Ip>, _Args&&... __args)
      : __data(in_place_index<_Ip>, std::forward<_Args>(__args)...), __index(_Ip) {}

  _LIBCPP_HIDE_FROM_ABI constexpr bool valueless_by_exception() const noexcept { return index() == variant_npos; }

  _LIBCPP_HIDE_FROM_ABI constexpr size_t index() const noexcept {
    return __index == __variant_npos<__index_t> ? variant_npos : __index;
  }

protected:
  _LIBCPP_HIDE_FROM_ABI constexpr auto&& __as_base() & { return *this; }

  _LIBCPP_HIDE_FROM_ABI constexpr auto&& __as_base() && { return std::move(*this); }

  _LIBCPP_HIDE_FROM_ABI constexpr auto&& __as_base() const& { return *this; }

  _LIBCPP_HIDE_FROM_ABI constexpr auto&& __as_base() const&& { return std::move(*this); }

  _LIBCPP_HIDE_FROM_ABI static constexpr size_t __size() { return sizeof...(_Types); }

  __union<_DestructibleTrait, 0, _Types...> __data;
  __index_t __index;

  friend struct __access::__base;
  friend struct __visitation::__base;
};

template <class _Traits, _Trait = _Traits::__destructible_trait>
class _LIBCPP_TEMPLATE_VIS __dtor;

#  define _LIBCPP_VARIANT_DESTRUCTOR(destructible_trait, destructor, destroy)                                          \
    template <class... _Types>                                                                                         \
    class _LIBCPP_TEMPLATE_VIS __dtor<__traits<_Types...>, destructible_trait>                                         \
        : public __base<destructible_trait, _Types...> {                                                               \
      using __base_type = __base<destructible_trait, _Types...>;                                                       \
      using __index_t   = typename __base_type::__index_t;                                                             \
                                                                                                                       \
    public:                                                                                                            \
      using __base_type::__base_type;                                                                                  \
      using __base_type::operator=;                                                                                    \
                                                                                                                       \
      __dtor(const __dtor&)                       = default;                                                           \
      __dtor(__dtor&&)                            = default;                                                           \
      destructor __dtor& operator=(const __dtor&) = default;                                                           \
      __dtor& operator=(__dtor&&)                 = default;                                                           \
                                                                                                                       \
    protected:                                                                                                         \
      inline _LIBCPP_HIDE_FROM_ABI destroy                                                                             \
    }

_LIBCPP_VARIANT_DESTRUCTOR(
    _Trait::_TriviallyAvailable, ~__dtor() = default;
    , void __destroy() noexcept { this->__index = __variant_npos<__index_t>; });

_LIBCPP_VARIANT_DESTRUCTOR(
    _Trait::_Available,
    ~__dtor() { __destroy(); },
    void __destroy() noexcept {
      if (!this->valueless_by_exception()) {
        __visitation::__base::__visit_alt(
            [](auto& __alt) noexcept {
              using __alt_type = __remove_cvref_t<decltype(__alt)>;
              __alt.~__alt_type();
            },
            *this);
      }
      this->__index = __variant_npos<__index_t>;
    });

_LIBCPP_VARIANT_DESTRUCTOR(_Trait::_Unavailable, ~__dtor() = delete;, void __destroy() noexcept = delete;);

#  undef _LIBCPP_VARIANT_DESTRUCTOR

template <class _Traits>
class _LIBCPP_TEMPLATE_VIS __ctor : public __dtor<_Traits> {
  using __base_type = __dtor<_Traits>;

public:
  using __base_type::__base_type;
  using __base_type::operator=;

protected:
  template <size_t _Ip, class _Tp, class... _Args>
  _LIBCPP_HIDE_FROM_ABI static _Tp& __construct_alt(__alt<_Ip, _Tp>& __a, _Args&&... __args) {
    ::new ((void*)std::addressof(__a)) __alt<_Ip, _Tp>(in_place, std::forward<_Args>(__args)...);
    return __a.__value;
  }

  template <class _Rhs>
  _LIBCPP_HIDE_FROM_ABI static void __generic_construct(__ctor& __lhs, _Rhs&& __rhs) {
    __lhs.__destroy();
    if (!__rhs.valueless_by_exception()) {
      auto __rhs_index = __rhs.index();
      __visitation::__base::__visit_alt_at(
          __rhs_index,
          [](auto& __lhs_alt, auto&& __rhs_alt) {
            __construct_alt(__lhs_alt, std::forward<decltype(__rhs_alt)>(__rhs_alt).__value);
          },
          __lhs,
          std::forward<_Rhs>(__rhs));
      __lhs.__index = __rhs_index;
    }
  }
};

template <class _Traits, _Trait = _Traits::__move_constructible_trait>
class _LIBCPP_TEMPLATE_VIS __move_constructor;

#  define _LIBCPP_VARIANT_MOVE_CONSTRUCTOR(move_constructible_trait, move_constructor)                                 \
    template <class... _Types>                                                                                         \
    class _LIBCPP_TEMPLATE_VIS __move_constructor<__traits<_Types...>, move_constructible_trait>                       \
        : public __ctor<__traits<_Types...>> {                                                                         \
      using __base_type = __ctor<__traits<_Types...>>;                                                                 \
                                                                                                                       \
    public:                                                                                                            \
      using __base_type::__base_type;                                                                                  \
      using __base_type::operator=;                                                                                    \
                                                                                                                       \
      __move_constructor(const __move_constructor&)            = default;                                              \
      move_constructor ~__move_constructor()                   = default;                                              \
      __move_constructor& operator=(const __move_constructor&) = default;                                              \
      __move_constructor& operator=(__move_constructor&&)      = default;                                              \
    }

_LIBCPP_VARIANT_MOVE_CONSTRUCTOR(_Trait::_TriviallyAvailable,
                                 __move_constructor(__move_constructor&& __that) = default;);

_LIBCPP_VARIANT_MOVE_CONSTRUCTOR(
    _Trait::_Available,
    __move_constructor(__move_constructor&& __that) noexcept(__all<is_nothrow_move_constructible_v<_Types>...>::value)
    : __move_constructor(__valueless_t{}) { this->__generic_construct(*this, std::move(__that)); });

_LIBCPP_VARIANT_MOVE_CONSTRUCTOR(_Trait::_Unavailable, __move_constructor(__move_constructor&&) = delete;);

#  undef _LIBCPP_VARIANT_MOVE_CONSTRUCTOR

template <class _Traits, _Trait = _Traits::__copy_constructible_trait>
class _LIBCPP_TEMPLATE_VIS __copy_constructor;

#  define _LIBCPP_VARIANT_COPY_CONSTRUCTOR(copy_constructible_trait, copy_constructor)                                 \
    template <class... _Types>                                                                                         \
    class _LIBCPP_TEMPLATE_VIS __copy_constructor<__traits<_Types...>, copy_constructible_trait>                       \
        : public __move_constructor<__traits<_Types...>> {                                                             \
      using __base_type = __move_constructor<__traits<_Types...>>;                                                     \
                                                                                                                       \
    public:                                                                                                            \
      using __base_type::__base_type;                                                                                  \
      using __base_type::operator=;                                                                                    \
                                                                                                                       \
      copy_constructor __copy_constructor(__copy_constructor&&) = default;                                             \
      ~__copy_constructor()                                     = default;                                             \
      __copy_constructor& operator=(const __copy_constructor&)  = default;                                             \
      __copy_constructor& operator=(__copy_constructor&&)       = default;                                             \
    }

_LIBCPP_VARIANT_COPY_CONSTRUCTOR(_Trait::_TriviallyAvailable,
                                 __copy_constructor(const __copy_constructor& __that) = default;);

_LIBCPP_VARIANT_COPY_CONSTRUCTOR(
    _Trait::_Available, __copy_constructor(const __copy_constructor& __that)
    : __copy_constructor(__valueless_t{}) { this->__generic_construct(*this, __that); });

_LIBCPP_VARIANT_COPY_CONSTRUCTOR(_Trait::_Unavailable, __copy_constructor(const __copy_constructor&) = delete;);

#  undef _LIBCPP_VARIANT_COPY_CONSTRUCTOR

template <class _Traits>
class _LIBCPP_TEMPLATE_VIS __assignment : public __copy_constructor<_Traits> {
  using __base_type = __copy_constructor<_Traits>;

public:
  using __base_type::__base_type;
  using __base_type::operator=;

  template <size_t _Ip, class... _Args>
  _LIBCPP_HIDE_FROM_ABI auto& __emplace(_Args&&... __args) {
    this->__destroy();
    auto& __res   = this->__construct_alt(__access::__base::__get_alt<_Ip>(*this), std::forward<_Args>(__args)...);
    this->__index = _Ip;
    return __res;
  }

protected:
  template <size_t _Ip, class _Tp, class _Arg>
  _LIBCPP_HIDE_FROM_ABI void __assign_alt(__alt<_Ip, _Tp>& __a, _Arg&& __arg) {
    if (this->index() == _Ip) {
      __a.__value = std::forward<_Arg>(__arg);
    } else {
      struct {
        _LIBCPP_HIDE_FROM_ABI void operator()(true_type) const { __this->__emplace<_Ip>(std::forward<_Arg>(__arg)); }
        _LIBCPP_HIDE_FROM_ABI void operator()(false_type) const {
          __this->__emplace<_Ip>(_Tp(std::forward<_Arg>(__arg)));
        }
        __assignment* __this;
        _Arg&& __arg;
      } __impl{this, std::forward<_Arg>(__arg)};
      __impl(bool_constant < is_nothrow_constructible_v<_Tp, _Arg> || !is_nothrow_move_constructible_v < _Tp >> {});
    }
  }

  template <class _That>
  _LIBCPP_HIDE_FROM_ABI void __generic_assign(_That&& __that) {
    if (this->valueless_by_exception() && __that.valueless_by_exception()) {
      // do nothing.
    } else if (__that.valueless_by_exception()) {
      this->__destroy();
    } else {
      __visitation::__base::__visit_alt_at(
          __that.index(),
          [this](auto& __this_alt, auto&& __that_alt) {
            this->__assign_alt(__this_alt, std::forward<decltype(__that_alt)>(__that_alt).__value);
          },
          *this,
          std::forward<_That>(__that));
    }
  }
};

template <class _Traits, _Trait = _Traits::__move_assignable_trait>
class _LIBCPP_TEMPLATE_VIS __move_assignment;

#  define _LIBCPP_VARIANT_MOVE_ASSIGNMENT(move_assignable_trait, move_assignment)                                      \
    template <class... _Types>                                                                                         \
    class _LIBCPP_TEMPLATE_VIS __move_assignment<__traits<_Types...>, move_assignable_trait>                           \
        : public __assignment<__traits<_Types...>> {                                                                   \
      using __base_type = __assignment<__traits<_Types...>>;                                                           \
                                                                                                                       \
    public:                                                                                                            \
      using __base_type::__base_type;                                                                                  \
      using __base_type::operator=;                                                                                    \
                                                                                                                       \
      __move_assignment(const __move_assignment&)            = default;                                                \
      __move_assignment(__move_assignment&&)                 = default;                                                \
      ~__move_assignment()                                   = default;                                                \
      __move_assignment& operator=(const __move_assignment&) = default;                                                \
      move_assignment                                                                                                  \
    }

_LIBCPP_VARIANT_MOVE_ASSIGNMENT(_Trait::_TriviallyAvailable,
                                __move_assignment& operator=(__move_assignment&& __that) = default;);

_LIBCPP_VARIANT_MOVE_ASSIGNMENT(
    _Trait::_Available,
    __move_assignment&
    operator=(__move_assignment&& __that) noexcept(
        __all<(is_nothrow_move_constructible_v<_Types> && is_nothrow_move_assignable_v<_Types>)...>::value) {
      this->__generic_assign(std::move(__that));
      return *this;
    });

_LIBCPP_VARIANT_MOVE_ASSIGNMENT(_Trait::_Unavailable, __move_assignment& operator=(__move_assignment&&) = delete;);

#  undef _LIBCPP_VARIANT_MOVE_ASSIGNMENT

template <class _Traits, _Trait = _Traits::__copy_assignable_trait>
class _LIBCPP_TEMPLATE_VIS __copy_assignment;

#  define _LIBCPP_VARIANT_COPY_ASSIGNMENT(copy_assignable_trait, copy_assignment)                                      \
    template <class... _Types>                                                                                         \
    class _LIBCPP_TEMPLATE_VIS __copy_assignment<__traits<_Types...>, copy_assignable_trait>                           \
        : public __move_assignment<__traits<_Types...>> {                                                              \
      using __base_type = __move_assignment<__traits<_Types...>>;                                                      \
                                                                                                                       \
    public:                                                                                                            \
      using __base_type::__base_type;                                                                                  \
      using __base_type::operator=;                                                                                    \
                                                                                                                       \
      __copy_assignment(const __copy_assignment&)                       = default;                                     \
      __copy_assignment(__copy_assignment&&)                            = default;                                     \
      ~__copy_assignment()                                              = default;                                     \
      copy_assignment __copy_assignment& operator=(__copy_assignment&&) = default;                                     \
    }

_LIBCPP_VARIANT_COPY_ASSIGNMENT(_Trait::_TriviallyAvailable,
                                __copy_assignment& operator=(const __copy_assignment& __that) = default;);

_LIBCPP_VARIANT_COPY_ASSIGNMENT(
    _Trait::_Available, __copy_assignment& operator=(const __copy_assignment& __that) {
      this->__generic_assign(__that);
      return *this;
    });

_LIBCPP_VARIANT_COPY_ASSIGNMENT(_Trait::_Unavailable, __copy_assignment& operator=(const __copy_assignment&) = delete;);

#  undef _LIBCPP_VARIANT_COPY_ASSIGNMENT

template <class... _Types>
class _LIBCPP_TEMPLATE_VIS __impl : public __copy_assignment<__traits<_Types...>> {
  using __base_type = __copy_assignment<__traits<_Types...>>;

public:
  using __base_type::__base_type; // get in_place_index_t constructor & friends
  _LIBCPP_HIDE_FROM_ABI __impl(__impl const&)            = default;
  _LIBCPP_HIDE_FROM_ABI __impl(__impl&&)                 = default;
  _LIBCPP_HIDE_FROM_ABI __impl& operator=(__impl const&) = default;
  _LIBCPP_HIDE_FROM_ABI __impl& operator=(__impl&&)      = default;

  template <size_t _Ip, class _Arg>
  _LIBCPP_HIDE_FROM_ABI void __assign(_Arg&& __arg) {
    this->__assign_alt(__access::__base::__get_alt<_Ip>(*this), std::forward<_Arg>(__arg));
  }

  inline _LIBCPP_HIDE_FROM_ABI void __swap(__impl& __that) {
    if (this->valueless_by_exception() && __that.valueless_by_exception()) {
      // do nothing.
    } else if (this->index() == __that.index()) {
      __visitation::__base::__visit_alt_at(
          this->index(),
          [](auto& __this_alt, auto& __that_alt) {
            using std::swap;
            swap(__this_alt.__value, __that_alt.__value);
          },
          *this,
          __that);
    } else {
      __impl* __lhs = this;
      __impl* __rhs = std::addressof(__that);
      if (__lhs->__move_nothrow() && !__rhs->__move_nothrow()) {
        std::swap(__lhs, __rhs);
      }
      __impl __tmp(std::move(*__rhs));
#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
      if constexpr (__all<is_nothrow_move_constructible_v<_Types>...>::value) {
        this->__generic_construct(*__rhs, std::move(*__lhs));
      } else {
        // EXTENSION: When the move construction of `__lhs` into `__rhs` throws
        // and `__tmp` is nothrow move constructible then we move `__tmp` back
        // into `__rhs` and provide the strong exception safety guarantee.
        try {
          this->__generic_construct(*__rhs, std::move(*__lhs));
        } catch (...) {
          if (__tmp.__move_nothrow()) {
            this->__generic_construct(*__rhs, std::move(__tmp));
          }
          throw;
        }
      }
#  else
      // this isn't consolidated with the `if constexpr` branch above due to
      // `throw` being ill-formed with exceptions disabled even when discarded.
      this->__generic_construct(*__rhs, std::move(*__lhs));
#  endif
      this->__generic_construct(*__lhs, std::move(__tmp));
    }
  }

private:
  inline _LIBCPP_HIDE_FROM_ABI bool __move_nothrow() const {
    constexpr bool __results[] = {is_nothrow_move_constructible_v<_Types>...};
    return this->valueless_by_exception() || __results[this->index()];
  }
};

struct __no_narrowing_check {
  template <class _Dest, class _Source>
  using _Apply = __type_identity<_Dest>;
};

struct __narrowing_check {
  template <class _Dest>
  static auto __test_impl(_Dest (&&)[1]) -> __type_identity<_Dest>;
  template <class _Dest, class _Source>
  using _Apply _LIBCPP_NODEBUG = decltype(__test_impl<_Dest>({std::declval<_Source>()}));
};

template <class _Dest, class _Source>
using __check_for_narrowing _LIBCPP_NODEBUG =
    typename _If< is_arithmetic<_Dest>::value, __narrowing_check, __no_narrowing_check >::template _Apply<_Dest,
                                                                                                          _Source>;

template <class _Tp, size_t _Idx>
struct __overload {
  template <class _Up>
  auto operator()(_Tp, _Up&&) const -> __check_for_narrowing<_Tp, _Up>;
};

template <class... _Bases>
struct __all_overloads : _Bases... {
  void operator()() const;
  using _Bases::operator()...;
};

template <class _IdxSeq>
struct __make_overloads_imp;

template <size_t... _Idx>
struct __make_overloads_imp<__tuple_indices<_Idx...> > {
  template <class... _Types>
  using _Apply _LIBCPP_NODEBUG = __all_overloads<__overload<_Types, _Idx>...>;
};

template <class... _Types>
using _MakeOverloads _LIBCPP_NODEBUG =
    typename __make_overloads_imp< __make_indices_imp<sizeof...(_Types), 0> >::template _Apply<_Types...>;

template <class _Tp, class... _Types>
using __best_match_t = typename invoke_result_t<_MakeOverloads<_Types...>, _Tp, _Tp>::type;

} // namespace __variant_detail

template <class _Visitor, class... _Vs, typename = void_t<decltype(std::__as_variant(std::declval<_Vs>()))...>>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr decltype(auto)
visit(_Visitor&& __visitor, _Vs&&... __vs);

#  if _LIBCPP_STD_VER >= 20
template <class _Rp,
          class _Visitor,
          class... _Vs,
          typename = void_t<decltype(std::__as_variant(std::declval<_Vs>()))...>>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr _Rp
visit(_Visitor&& __visitor, _Vs&&... __vs);
#  endif

template <class... _Types>
class _LIBCPP_TEMPLATE_VIS _LIBCPP_DECLSPEC_EMPTY_BASES variant
    : private __sfinae_ctor_base< __all<is_copy_constructible_v<_Types>...>::value,
                                  __all<is_move_constructible_v<_Types>...>::value>,
      private __sfinae_assign_base<
          __all<(is_copy_constructible_v<_Types> && is_copy_assignable_v<_Types>)...>::value,
          __all<(is_move_constructible_v<_Types> && is_move_assignable_v<_Types>)...>::value> {
  static_assert(0 < sizeof...(_Types), "variant must consist of at least one alternative.");

  static_assert(__all<!is_array_v<_Types>...>::value, "variant can not have an array type as an alternative.");

  static_assert(__all<!is_reference_v<_Types>...>::value, "variant can not have a reference type as an alternative.");

  static_assert(__all<!is_void_v<_Types>...>::value, "variant can not have a void type as an alternative.");

  using __first_type = variant_alternative_t<0, variant>;

public:
  template <bool _Dummy                                                                               = true,
            enable_if_t<__dependent_type<is_default_constructible<__first_type>, _Dummy>::value, int> = 0>
  _LIBCPP_HIDE_FROM_ABI constexpr variant() noexcept(is_nothrow_default_constructible_v<__first_type>)
      : __impl_(in_place_index<0>) {}

  _LIBCPP_HIDE_FROM_ABI constexpr variant(const variant&) = default;
  _LIBCPP_HIDE_FROM_ABI constexpr variant(variant&&)      = default;

  template < class _Arg,
             enable_if_t<!is_same_v<__remove_cvref_t<_Arg>, variant>, int>        = 0,
             enable_if_t<!__is_inplace_type<__remove_cvref_t<_Arg>>::value, int>  = 0,
             enable_if_t<!__is_inplace_index<__remove_cvref_t<_Arg>>::value, int> = 0,
             class _Tp  = __variant_detail::__best_match_t<_Arg, _Types...>,
             size_t _Ip = __find_detail::__find_unambiguous_index_sfinae<_Tp, _Types...>::value,
             enable_if_t<is_constructible_v<_Tp, _Arg>, int> = 0>
  _LIBCPP_HIDE_FROM_ABI constexpr variant(_Arg&& __arg) noexcept(is_nothrow_constructible_v<_Tp, _Arg>)
      : __impl_(in_place_index<_Ip>, std::forward<_Arg>(__arg)) {}

  template <size_t _Ip,
            class... _Args,
            class                                               = enable_if_t<(_Ip < sizeof...(_Types)), int>,
            class _Tp                                           = variant_alternative_t<_Ip, variant<_Types...>>,
            enable_if_t<is_constructible_v<_Tp, _Args...>, int> = 0>
  _LIBCPP_HIDE_FROM_ABI explicit constexpr variant(in_place_index_t<_Ip>, _Args&&... __args) noexcept(
      is_nothrow_constructible_v<_Tp, _Args...>)
      : __impl_(in_place_index<_Ip>, std::forward<_Args>(__args)...) {}

  template < size_t _Ip,
             class _Up,
             class... _Args,
             enable_if_t<(_Ip < sizeof...(_Types)), int> = 0,
             class _Tp                                   = variant_alternative_t<_Ip, variant<_Types...>>,
             enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>, int> = 0>
  _LIBCPP_HIDE_FROM_ABI explicit constexpr variant(
      in_place_index_t<_Ip>,
      initializer_list<_Up> __il,
      _Args&&... __args) noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _Args...>)
      : __impl_(in_place_index<_Ip>, __il, std::forward<_Args>(__args)...) {}

  template < class _Tp,
             class... _Args,
             size_t _Ip = __find_detail::__find_unambiguous_index_sfinae<_Tp, _Types...>::value,
             enable_if_t<is_constructible_v<_Tp, _Args...>, int> = 0>
  _LIBCPP_HIDE_FROM_ABI explicit constexpr variant(in_place_type_t<_Tp>, _Args&&... __args) noexcept(
      is_nothrow_constructible_v<_Tp, _Args...>)
      : __impl_(in_place_index<_Ip>, std::forward<_Args>(__args)...) {}

  template < class _Tp,
             class _Up,
             class... _Args,
             size_t _Ip = __find_detail::__find_unambiguous_index_sfinae<_Tp, _Types...>::value,
             enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>, int> = 0>
  _LIBCPP_HIDE_FROM_ABI explicit constexpr variant(
      in_place_type_t<_Tp>,
      initializer_list<_Up> __il,
      _Args&&... __args) noexcept(is_nothrow_constructible_v<_Tp, initializer_list< _Up>&, _Args...>)
      : __impl_(in_place_index<_Ip>, __il, std::forward<_Args>(__args)...) {}

  _LIBCPP_HIDE_FROM_ABI ~variant() = default;

  _LIBCPP_HIDE_FROM_ABI constexpr variant& operator=(const variant&) = default;
  _LIBCPP_HIDE_FROM_ABI constexpr variant& operator=(variant&&)      = default;

  template < class _Arg,
             enable_if_t<!is_same_v<__remove_cvref_t<_Arg>, variant>, int> = 0,
             class _Tp  = __variant_detail::__best_match_t<_Arg, _Types...>,
             size_t _Ip = __find_detail::__find_unambiguous_index_sfinae<_Tp, _Types...>::value,
             enable_if_t<is_assignable_v<_Tp&, _Arg> && is_constructible_v<_Tp, _Arg>, int> = 0>
  _LIBCPP_HIDE_FROM_ABI variant&
  operator=(_Arg&& __arg) noexcept(is_nothrow_assignable_v<_Tp&, _Arg> && is_nothrow_constructible_v<_Tp, _Arg>) {
    __impl_.template __assign<_Ip>(std::forward<_Arg>(__arg));
    return *this;
  }

  template < size_t _Ip,
             class... _Args,
             enable_if_t<(_Ip < sizeof...(_Types)), int>         = 0,
             class _Tp                                           = variant_alternative_t<_Ip, variant<_Types...>>,
             enable_if_t<is_constructible_v<_Tp, _Args...>, int> = 0>
  _LIBCPP_HIDE_FROM_ABI _Tp& emplace(_Args&&... __args) {
    return __impl_.template __emplace<_Ip>(std::forward<_Args>(__args)...);
  }

  template < size_t _Ip,
             class _Up,
             class... _Args,
             enable_if_t<(_Ip < sizeof...(_Types)), int> = 0,
             class _Tp                                   = variant_alternative_t<_Ip, variant<_Types...>>,
             enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>, int> = 0>
  _LIBCPP_HIDE_FROM_ABI _Tp& emplace(initializer_list<_Up> __il, _Args&&... __args) {
    return __impl_.template __emplace<_Ip>(__il, std::forward<_Args>(__args)...);
  }

  template < class _Tp,
             class... _Args,
             size_t _Ip = __find_detail::__find_unambiguous_index_sfinae<_Tp, _Types...>::value,
             enable_if_t<is_constructible_v<_Tp, _Args...>, int> = 0>
  _LIBCPP_HIDE_FROM_ABI _Tp& emplace(_Args&&... __args) {
    return __impl_.template __emplace<_Ip>(std::forward<_Args>(__args)...);
  }

  template < class _Tp,
             class _Up,
             class... _Args,
             size_t _Ip = __find_detail::__find_unambiguous_index_sfinae<_Tp, _Types...>::value,
             enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>, int> = 0>
  _LIBCPP_HIDE_FROM_ABI _Tp& emplace(initializer_list<_Up> __il, _Args&&... __args) {
    return __impl_.template __emplace<_Ip>(__il, std::forward<_Args>(__args)...);
  }

  _LIBCPP_HIDE_FROM_ABI constexpr bool valueless_by_exception() const noexcept {
    return __impl_.valueless_by_exception();
  }

  _LIBCPP_HIDE_FROM_ABI constexpr size_t index() const noexcept { return __impl_.index(); }

  template < bool _Dummy       = true,
             enable_if_t< __all<(__dependent_type<is_move_constructible<_Types>, _Dummy>::value &&
                                 __dependent_type<is_swappable<_Types>, _Dummy>::value)...>::value,
                          int> = 0>
  _LIBCPP_HIDE_FROM_ABI void swap(variant& __that) noexcept(
      __all<(is_nothrow_move_constructible_v<_Types> && is_nothrow_swappable_v<_Types>)...>::value) {
    __impl_.__swap(__that.__impl_);
  }

#  if _LIBCPP_STD_VER >= 26 && defined(_LIBCPP_HAS_EXPLICIT_THIS_PARAMETER)
  // Helper class to implement [variant.visit]/10
  //   Constraints: The call to visit does not use an explicit template-argument-list
  //   that begins with a type template-argument.
  struct __variant_visit_barrier_tag {
    _LIBCPP_HIDE_FROM_ABI explicit __variant_visit_barrier_tag() = default;
  };

  template <__variant_visit_barrier_tag = __variant_visit_barrier_tag{}, class _Self, class _Visitor>
  _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) visit(this _Self&& __self, _Visitor&& __visitor) {
    using _VariantT = _OverrideRef<_Self&&, _CopyConst<remove_reference_t<_Self>, variant>>;
    return std::visit(std::forward<_Visitor>(__visitor), (_VariantT)__self);
  }

  template <class _Rp, class _Self, class _Visitor>
  _LIBCPP_HIDE_FROM_ABI constexpr _Rp visit(this _Self&& __self, _Visitor&& __visitor) {
    using _VariantT = _OverrideRef<_Self&&, _CopyConst<remove_reference_t<_Self>, variant>>;
    return std::visit<_Rp>(std::forward<_Visitor>(__visitor), (_VariantT)__self);
  }
#  endif

private:
  __variant_detail::__impl<_Types...> __impl_;

  friend struct __variant_detail::__access::__variant;
  friend struct __variant_detail::__visitation::__variant;
};

template <size_t _Ip, class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr bool __holds_alternative(const variant<_Types...>& __v) noexcept {
  return __v.index() == _Ip;
}

template <class _Tp, class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr bool holds_alternative(const variant<_Types...>& __v) noexcept {
  return std::__holds_alternative<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}

template <size_t _Ip, class _Vp>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr auto&& __generic_get(_Vp&& __v) {
  using __variant_detail::__access::__variant;
  if (!std::__holds_alternative<_Ip>(__v)) {
    __throw_bad_variant_access();
  }
  return __variant::__get_alt<_Ip>(std::forward<_Vp>(__v)).__value;
}

template <size_t _Ip, class... _Types>
_LIBCPP_HIDE_FROM_ABI
    _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr variant_alternative_t<_Ip, variant<_Types...>>&
    get(variant<_Types...>& __v) {
  static_assert(_Ip < sizeof...(_Types));
  static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
  return std::__generic_get<_Ip>(__v);
}

template <size_t _Ip, class... _Types>
_LIBCPP_HIDE_FROM_ABI
    _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr variant_alternative_t<_Ip, variant<_Types...>>&&
    get(variant<_Types...>&& __v) {
  static_assert(_Ip < sizeof...(_Types));
  static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
  return std::__generic_get<_Ip>(std::move(__v));
}

template <size_t _Ip, class... _Types>
_LIBCPP_HIDE_FROM_ABI
    _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr const variant_alternative_t<_Ip, variant<_Types...>>&
    get(const variant<_Types...>& __v) {
  static_assert(_Ip < sizeof...(_Types));
  static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
  return std::__generic_get<_Ip>(__v);
}

template <size_t _Ip, class... _Types>
_LIBCPP_HIDE_FROM_ABI
    _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr const variant_alternative_t<_Ip, variant<_Types...>>&&
    get(const variant<_Types...>&& __v) {
  static_assert(_Ip < sizeof...(_Types));
  static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
  return std::__generic_get<_Ip>(std::move(__v));
}

template <class _Tp, class... _Types>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr _Tp& get(variant<_Types...>& __v) {
  static_assert(!is_void_v<_Tp>);
  return std::get<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}

template <class _Tp, class... _Types>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr _Tp&& get(variant<_Types...>&& __v) {
  static_assert(!is_void_v<_Tp>);
  return std::get<__find_exactly_one_t<_Tp, _Types...>::value>(std::move(__v));
}

template <class _Tp, class... _Types>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr const _Tp&
get(const variant<_Types...>& __v) {
  static_assert(!is_void_v<_Tp>);
  return std::get<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}

template <class _Tp, class... _Types>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr const _Tp&&
get(const variant<_Types...>&& __v) {
  static_assert(!is_void_v<_Tp>);
  return std::get<__find_exactly_one_t<_Tp, _Types...>::value>(std::move(__v));
}

template <size_t _Ip, class _Vp>
_LIBCPP_HIDE_FROM_ABI constexpr auto* __generic_get_if(_Vp* __v) noexcept {
  using __variant_detail::__access::__variant;
  return __v && std::__holds_alternative<_Ip>(*__v) ? std::addressof(__variant::__get_alt<_Ip>(*__v).__value) : nullptr;
}

template <size_t _Ip, class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<variant_alternative_t<_Ip, variant<_Types...>>>
get_if(variant<_Types...>* __v) noexcept {
  static_assert(_Ip < sizeof...(_Types));
  static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
  return std::__generic_get_if<_Ip>(__v);
}

template <size_t _Ip, class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<const variant_alternative_t<_Ip, variant<_Types...>>>
get_if(const variant<_Types...>* __v) noexcept {
  static_assert(_Ip < sizeof...(_Types));
  static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
  return std::__generic_get_if<_Ip>(__v);
}

template <class _Tp, class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<_Tp> get_if(variant<_Types...>* __v) noexcept {
  static_assert(!is_void_v<_Tp>);
  return std::get_if<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}

template <class _Tp, class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<const _Tp> get_if(const variant<_Types...>* __v) noexcept {
  static_assert(!is_void_v<_Tp>);
  return std::get_if<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}

template <class _Operator>
struct __convert_to_bool {
  template <class _T1, class _T2>
  _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(_T1&& __t1, _T2&& __t2) const {
    static_assert(is_convertible<decltype(_Operator{}(std::forward<_T1>(__t1), std::forward<_T2>(__t2))), bool>::value,
                  "the relational operator does not return a type which is implicitly convertible to bool");
    return _Operator{}(std::forward<_T1>(__t1), std::forward<_T2>(__t2));
  }
};

template <class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr bool operator==(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs) {
  using __variant_detail::__visitation::__variant;
  if (__lhs.index() != __rhs.index())
    return false;
  if (__lhs.valueless_by_exception())
    return true;
  return __variant::__visit_value_at(__lhs.index(), __convert_to_bool<equal_to<>>{}, __lhs, __rhs);
}

#  if _LIBCPP_STD_VER >= 20

template <class... _Types>
  requires(three_way_comparable<_Types> && ...)
_LIBCPP_HIDE_FROM_ABI constexpr common_comparison_category_t<compare_three_way_result_t<_Types>...>
operator<=>(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs) {
  using __variant_detail::__visitation::__variant;
  using __result_t = common_comparison_category_t<compare_three_way_result_t<_Types>...>;
  if (__lhs.valueless_by_exception() && __rhs.valueless_by_exception())
    return strong_ordering::equal;
  if (__lhs.valueless_by_exception())
    return strong_ordering::less;
  if (__rhs.valueless_by_exception())
    return strong_ordering::greater;
  if (auto __c = __lhs.index() <=> __rhs.index(); __c != 0)
    return __c;
  auto __three_way = []<class _Type>(const _Type& __v, const _Type& __w) -> __result_t { return __v <=> __w; };
  return __variant::__visit_value_at(__lhs.index(), __three_way, __lhs, __rhs);
}

#  endif // _LIBCPP_STD_VER >= 20

template <class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr bool operator!=(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs) {
  using __variant_detail::__visitation::__variant;
  if (__lhs.index() != __rhs.index())
    return true;
  if (__lhs.valueless_by_exception())
    return false;
  return __variant::__visit_value_at(__lhs.index(), __convert_to_bool<not_equal_to<>>{}, __lhs, __rhs);
}

template <class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr bool operator<(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs) {
  using __variant_detail::__visitation::__variant;
  if (__rhs.valueless_by_exception())
    return false;
  if (__lhs.valueless_by_exception())
    return true;
  if (__lhs.index() < __rhs.index())
    return true;
  if (__lhs.index() > __rhs.index())
    return false;
  return __variant::__visit_value_at(__lhs.index(), __convert_to_bool<less<>>{}, __lhs, __rhs);
}

template <class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr bool operator>(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs) {
  using __variant_detail::__visitation::__variant;
  if (__lhs.valueless_by_exception())
    return false;
  if (__rhs.valueless_by_exception())
    return true;
  if (__lhs.index() > __rhs.index())
    return true;
  if (__lhs.index() < __rhs.index())
    return false;
  return __variant::__visit_value_at(__lhs.index(), __convert_to_bool<greater<>>{}, __lhs, __rhs);
}

template <class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr bool operator<=(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs) {
  using __variant_detail::__visitation::__variant;
  if (__lhs.valueless_by_exception())
    return true;
  if (__rhs.valueless_by_exception())
    return false;
  if (__lhs.index() < __rhs.index())
    return true;
  if (__lhs.index() > __rhs.index())
    return false;
  return __variant::__visit_value_at(__lhs.index(), __convert_to_bool<less_equal<>>{}, __lhs, __rhs);
}

template <class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr bool operator>=(const variant<_Types...>& __lhs, const variant<_Types...>& __rhs) {
  using __variant_detail::__visitation::__variant;
  if (__rhs.valueless_by_exception())
    return true;
  if (__lhs.valueless_by_exception())
    return false;
  if (__lhs.index() > __rhs.index())
    return true;
  if (__lhs.index() < __rhs.index())
    return false;
  return __variant::__visit_value_at(__lhs.index(), __convert_to_bool<greater_equal<>>{}, __lhs, __rhs);
}

template <class... _Vs>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr void __throw_if_valueless(_Vs&&... __vs) {
  const bool __valueless = (... || std::__as_variant(__vs).valueless_by_exception());
  if (__valueless) {
    __throw_bad_variant_access();
  }
}

template < class _Visitor, class... _Vs, typename>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr decltype(auto)
visit(_Visitor&& __visitor, _Vs&&... __vs) {
  using __variant_detail::__visitation::__variant;
  std::__throw_if_valueless(std::forward<_Vs>(__vs)...);
  return __variant::__visit_value(std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs)...);
}

#  if _LIBCPP_STD_VER >= 20
template < class _Rp, class _Visitor, class... _Vs, typename>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS constexpr _Rp
visit(_Visitor&& __visitor, _Vs&&... __vs) {
  using __variant_detail::__visitation::__variant;
  std::__throw_if_valueless(std::forward<_Vs>(__vs)...);
  return __variant::__visit_value<_Rp>(std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs)...);
}
#  endif

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

template <class... _Types>
struct _LIBCPP_TEMPLATE_VIS hash< __enable_hash_helper<variant<_Types...>, remove_const_t<_Types>...>> {
  using argument_type = variant<_Types...>;
  using result_type   = size_t;

  _LIBCPP_HIDE_FROM_ABI result_type operator()(const argument_type& __v) const {
    using __variant_detail::__visitation::__variant;
    size_t __res =
        __v.valueless_by_exception()
            ? 299792458 // Random value chosen by the universe upon creation
            : __variant::__visit_alt(
                  [](const auto& __alt) {
                    using __alt_type   = __remove_cvref_t<decltype(__alt)>;
                    using __value_type = remove_const_t< typename __alt_type::__value_type>;
                    return hash<__value_type>{}(__alt.__value);
                  },
                  __v);
    return std::__hash_combine(__res, hash<size_t>{}(__v.index()));
  }
};

// __unchecked_get is the same as std::get, except, it is UB to use it with the wrong
// type whereas std::get will throw or returning nullptr. This makes it faster than
// std::get.
template <size_t _Ip, class _Vp>
_LIBCPP_HIDE_FROM_ABI constexpr auto&& __unchecked_get(_Vp&& __v) noexcept {
  using __variant_detail::__access::__variant;
  return __variant::__get_alt<_Ip>(std::forward<_Vp>(__v)).__value;
}

template <class _Tp, class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr auto&& __unchecked_get(const variant<_Types...>& __v) noexcept {
  return std::__unchecked_get<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}

template <class _Tp, class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr auto&& __unchecked_get(variant<_Types...>& __v) noexcept {
  return std::__unchecked_get<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}

#endif // _LIBCPP_STD_VER >= 17

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
#  include <exception>
#  include <tuple>
#  include <type_traits>
#  include <typeinfo>
#  include <utility>
#endif

#endif // _LIBCPP_VARIANT
