include/variant

// -*- C++ -*-
//===------------------------------ variant -------------------------------===//
//
// 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);
    variant(const variant&);                // constexpr in C++20
    variant(variant&&) noexcept(see below); // constexpr in C++20

    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
    variant& operator=(const variant&);                // constexpr in C++20
    variant& operator=(variant&&) noexcept(see below); // constexpr in C++20

    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);
  };

  // 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...>&);

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

  // 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;
  constexpr bool operator<=(monostate, monostate) noexcept;
  constexpr bool operator>=(monostate, monostate) noexcept;
  constexpr bool operator==(monostate, monostate) noexcept;
  constexpr bool operator!=(monostate, monostate) noexcept;

  // 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 <__config>
#include <__availability>
#include <__tuple>
#include <array>
#include <exception>
#include <functional>
#include <initializer_list>
#include <new>
#include <tuple>
#include <type_traits>
#include <utility>
#include <limits>
#include <version>

#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_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS bad_variant_access : public exception {
public:
  virtual const char* what() const _NOEXCEPT;
};

} // namespace std

_LIBCPP_BEGIN_NAMESPACE_STD

// TODO: GCC 5 lies about its support for C++17 (it says it supports it but it
//       really doesn't). That breaks variant, which uses some C++17 features.
//       Remove this once we drop support for GCC 5.
#if _LIBCPP_STD_VER > 14 && !(defined(_LIBCPP_COMPILER_GCC) && _GNUC_VER_NEW < 6000)

_LIBCPP_NORETURN
inline _LIBCPP_INLINE_VISIBILITY
_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
void __throw_bad_variant_access() {
#ifndef _LIBCPP_NO_EXCEPTIONS
        throw bad_variant_access();
#else
        _VSTD::abort();
#endif
}

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

template <class _Tp>
struct _LIBCPP_TEMPLATE_VIS variant_size;

template <class _Tp>
_LIBCPP_INLINE_VAR 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...>;
};

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

constexpr int __choose_index_type(unsigned int __num_elem) {
  if (__num_elem < std::numeric_limits<unsigned char>::max())
    return 0;
  if (__num_elem < std::numeric_limits<unsigned short>::max())
    return 1;
  return 2;
}

template <size_t _NumAlts>
using __variant_index_t =
#ifndef _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
  unsigned int;
#else
  std::tuple_element_t<
      __choose_index_type(_NumAlts),
      std::tuple<unsigned char, unsigned short, unsigned int>
  >;
#endif

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

namespace __find_detail {

template <class _Tp, class... _Types>
inline _LIBCPP_INLINE_VISIBILITY
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 <class _Tp,
          template <class> class _IsTriviallyAvailable,
          template <class> class _IsAvailable>
constexpr _Trait __trait =
    _IsTriviallyAvailable<_Tp>::value
        ? _Trait::_TriviallyAvailable
        : _IsAvailable<_Tp>::value ? _Trait::_Available : _Trait::_Unavailable;

inline _LIBCPP_INLINE_VISIBILITY
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 <class... _Types>
struct __traits {
  static constexpr _Trait __copy_constructible_trait =
      __common_trait({__trait<_Types,
                              is_trivially_copy_constructible,
                              is_copy_constructible>...});

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

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

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

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

namespace __access {

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

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

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

struct __variant {
  template <size_t _Ip, class _Vp>
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr auto&& __get_alt(_Vp&& __v) {
    return __base::__get_alt<_Ip>(_VSTD::forward<_Vp>(__v).__impl);
  }
};

} // namespace __access

namespace __visitation {

#define _LIBCPP_VARIANT_CASES_4(_Case, _Base)                                  \
  _Case(_Base + 0)                                                             \
  _Case(_Base + 1)                                                             \
  _Case(_Base + 2)                                                             \
  _Case(_Base + 3)

#define _LIBCPP_VARIANT_CASES_16(_Case, _Base)                                 \
  _LIBCPP_VARIANT_CASES_4(_Case, _Base + 4 * 0)                                \
  _LIBCPP_VARIANT_CASES_4(_Case, _Base + 4 * 1)                                \
  _LIBCPP_VARIANT_CASES_4(_Case, _Base + 4 * 2)                                \
  _LIBCPP_VARIANT_CASES_4(_Case, _Base + 4 * 3)

#define _LIBCPP_VARIANT_CASES_64(_Case, _Base)                                 \
  _LIBCPP_VARIANT_CASES_16(_Case, _Base + 16 * 0)                              \
  _LIBCPP_VARIANT_CASES_16(_Case, _Base + 16 * 1)                              \
  _LIBCPP_VARIANT_CASES_16(_Case, _Base + 16 * 2)                              \
  _LIBCPP_VARIANT_CASES_16(_Case, _Base + 16 * 3)

#define _LIBCPP_VARIANT_CASES_256(_Case, _Base)                                \
  _LIBCPP_VARIANT_CASES_64(_Case, _Base + 64 * 0)                              \
  _LIBCPP_VARIANT_CASES_64(_Case, _Base + 64 * 1)                              \
  _LIBCPP_VARIANT_CASES_64(_Case, _Base + 64 * 2)                              \
  _LIBCPP_VARIANT_CASES_64(_Case, _Base + 64 * 3)

#define _LIBCPP_VARIANT_CASES(_NumCases, _Case)                                \
  _LIBCPP_CONCAT(_LIBCPP_VARIANT_CASES_, _NumCases)(_Case, 0)

#define _LIBCPP_VARIANT_SWITCH_MAX 256

template <class _Iter, class _Fp, size_t... _Is>
inline _LIBCPP_INLINE_VISIBILITY
static constexpr void __fill_cartesian_impl(
    _Iter __iter, _Fp __f, index_sequence<_Is...>) {
  *__iter = __f(integral_constant<size_t, _Is>{}...);
}

template <class _Iter, class _Fp, size_t... _Is, size_t... _Js, class... _Ls>
inline _LIBCPP_INLINE_VISIBILITY
static constexpr void __fill_cartesian_impl(
    _Iter __iter, _Fp __f, index_sequence<_Is...>, index_sequence<_Js...>, _Ls... __ls) {
  constexpr size_t _Mp = (1 * ... * _Ls::size());
  (__fill_cartesian_impl(
      __iter + _Js * _Mp, __f, index_sequence<_Is..., _Js>{}, __ls...), ...);
}

template <size_t... _Ns, class _Iter, class _Fp>
inline _LIBCPP_INLINE_VISIBILITY
static constexpr void __fill_cartesian(_Iter __iter, _Fp __f) {
  __fill_cartesian_impl(
      __iter, __f, index_sequence<>{}, make_index_sequence<_Ns>{}...);
}

template <size_t _Np, size_t... _Ns>
struct __multi {
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr size_t __size = (_Np * ... * _Ns);

  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr size_t
  __index(const size_t (&__is)[sizeof...(_Ns) + 1]) noexcept {
    constexpr size_t __ns[] = {_Ns..., 1};
    size_t __result = 0;
    for (size_t __i = 0; __i < sizeof...(_Ns) + 1; ++__i) {
      if (__is[__i] == variant_npos) {
        return variant_npos;
      }
      __result += __is[__i];
      __result *= __ns[__i];
    }
    return __result;
  }
};

template <size_t... _Ns>
struct __indices {
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr auto __value = [] {
    using _Tp = array<size_t, sizeof...(_Ns)>;
    array<_Tp, (1 * ... * _Ns)> __result = {};
    __fill_cartesian<_Ns...>(__result.begin(),
                             [](auto... __is) -> _Tp { return {__is...}; });
    return __result;
  }();
};

template <size_t... _Ns, class _Fp, class _Rp, class... _Args>
inline _LIBCPP_INLINE_VISIBILITY
static constexpr auto __make_vtable_impl(_Fp __f, _Rp (*)(_Args...)) {
  array<_Rp (*)(_Args...), (1 * ... * _Ns) + 1> __result = {
      [](_Args...) -> _Rp { __throw_bad_variant_access(); }
  };
  __fill_cartesian<_Ns...>(__result.begin() + 1, __f);
  return __result;
}

template <size_t... _Ns, class _Fp>
inline _LIBCPP_INLINE_VISIBILITY
static constexpr auto __make_vtable(_Fp __f) {
  using _Tp = decltype(__f(integral_constant<size_t, (_Ns, 0)>{}...));
  return __make_vtable_impl<_Ns...>(__f, _Tp{});
}

struct __base {
  template <class _Vis, class... _Vs>
  struct __dispatch {
    template <size_t... _Is>
    inline _LIBCPP_INLINE_VISIBILITY
    constexpr auto operator()(integral_constant<size_t, _Is>...) const noexcept {
      return +[](_Vis&& __vis, _Vs&&... __vs) -> decltype(auto) {
        return __invoke_constexpr(
            _VSTD::forward<_Vis>(__vis),
            __access::__base::__get_alt<_Is>(_VSTD::forward<_Vs>(__vs))...);
      };
    }
  };

  template <class _Vis, class _Vp, class _Wp>
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr decltype(auto)
  __visit_alt_at(size_t __index, _Vis&& __vis, _Vp&& __v, _Wp&& __w) {
    constexpr size_t __size = __uncvref_t<_Vp>::__size();
    static_assert(__size == __uncvref_t<_Wp>::__size());
    constexpr auto __dispatch_at = [](auto __i) {
      return __dispatch<_Vis, _Vp, _Wp>{}(__i, __i);
    };
#define _LIBCPP_VARIANT_CASE(_Ip)                                              \
  case _Ip: {                                                                  \
    if constexpr (_Ip < __size) {                                              \
      return __dispatch_at(integral_constant<size_t, _Ip>{})(                  \
          _VSTD::forward<_Vis>(__vis),                                         \
          _VSTD::forward<_Vp>(__v),                                            \
          _VSTD::forward<_Wp>(__w));                                           \
    } else {                                                                   \
      _LIBCPP_UNREACHABLE();                                                   \
    }                                                                          \
  }
    if constexpr (__size <= _LIBCPP_VARIANT_SWITCH_MAX) {
      switch (__index) {
        _LIBCPP_VARIANT_CASES(_LIBCPP_VARIANT_SWITCH_MAX, _LIBCPP_VARIANT_CASE)
        default: __throw_bad_variant_access();
      }
    } else {
      constexpr auto __vtable = __make_vtable<__size>(__dispatch_at);
      return __vtable[__index + 1](_VSTD::forward<_Vis>(__vis),
                                   _VSTD::forward<_Vp>(__v),
                                   _VSTD::forward<_Wp>(__w));
    }
#undef _LIBCPP_VARIANT_CASE
  }

  template <size_t... _Is, class _Vis, class... _Vs>
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr decltype(auto) __visit_alt(_Vis&& __vis, _Vs&&... __vs) {
    if constexpr (sizeof...(_Vs) == 0) {
      return __invoke_constexpr(_VSTD::forward<_Vis>(__vis));
    } else {
      return __visit_alt_impl(index_sequence_for<_Vs...>{},
                              _VSTD::forward<_Vis>(__vis),
                              _VSTD::forward<_Vs>(__vs)...);
    }
  }

  template <size_t... _Is, class _Vis, class... _Vs>
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr decltype(auto)
  __visit_alt_impl(index_sequence<_Is...>, _Vis&& __vis, _Vs&&... __vs) {
    using __multi = __multi<__uncvref_t<_Vs>::__size()...>;
    constexpr __dispatch<_Vis, _Vs...> __dispatch;
#define _LIBCPP_VARIANT_CASE(_Ip)                                              \
  case _Ip: {                                                                  \
    if constexpr (_Ip < __multi::__size) {                                     \
      return __dispatch(integral_constant<size_t, __itable[_Ip][_Is]>{}...)(   \
          _VSTD::forward<_Vis>(__vis), _VSTD::forward<_Vs>(__vs)...);          \
    } else {                                                                   \
      _LIBCPP_UNREACHABLE();                                                   \
    }                                                                          \
  }
    if constexpr (__multi::__size <= _LIBCPP_VARIANT_SWITCH_MAX) {
      constexpr const auto& __itable =
          __indices<__uncvref_t<_Vs>::__size()...>::__value;
      switch (__multi::__index({__vs.index()...})) {
        _LIBCPP_VARIANT_CASES(_LIBCPP_VARIANT_SWITCH_MAX, _LIBCPP_VARIANT_CASE)
        default: __throw_bad_variant_access();
      }
    } else {
      constexpr auto __vtable =
          __make_vtable<__uncvref_t<_Vs>::__size()...>(__dispatch);
      return __vtable[__multi::__index({__vs.index()...}) + 1](
          _VSTD::forward<_Vis>(__vis), _VSTD::forward<_Vs>(__vs)...);
    }
#undef _LIBCPP_VARIANT_CASE
  }
};

struct __variant {
  template <class _Vis, class _Vp, class _Wp>
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr decltype(auto)
  __visit_alt_at(size_t __index, _Vis&& __vis, _Vp&& __v, _Wp&& __w) {
    return __base::__visit_alt_at(__index,
                                  _VSTD::forward<_Vis>(__vis),
                                  _VSTD::forward<_Vp>(__v).__impl,
                                  _VSTD::forward<_Wp>(__w).__impl);
  }

  template <class _Vis, class... _Vs>
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr decltype(auto) __visit_alt(_Vis&& __vis, _Vs&&... __vs) {
    return __base::__visit_alt(_VSTD::forward<_Vis>(__vis),
                               _VSTD::forward<_Vs>(__vs).__impl...);
  }

  template <class _Vis, class _Vp, class _Wp>
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr decltype(auto)
  __visit_value_at(size_t __index, _Vis&& __vis, _Vp&& __v, _Wp&& __w) {
    return __visit_alt_at(__index,
                          __make_value_visitor(_VSTD::forward<_Vis>(__vis)),
                          _VSTD::forward<_Vp>(__v),
                          _VSTD::forward<_Wp>(__w));
  }

  template <class _Vis, class... _Vs>
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr decltype(auto) __visit_value(_Vis&& __vis, _Vs&&... __vs) {
    return __visit_alt(__make_value_visitor(_VSTD::forward<_Vis>(__vis)),
                       _VSTD::forward<_Vs>(__vs)...);
  }

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

  template <class _Vis>
  struct __value_visitor {
    template <class... _Alts>
    inline _LIBCPP_INLINE_VISIBILITY
    constexpr decltype(auto) operator()(_Alts&&... __alts) const {
      __std_visit_exhaustive_visitor_check<
          _Vis, decltype((_VSTD::forward<_Alts>(__alts).__value))...>();
      return __invoke_constexpr(_VSTD::forward<_Vis>(__vis),
                                _VSTD::forward<_Alts>(__alts).__value...);
    }
    _Vis&& __vis;
  };

  template <class _Vis>
  inline _LIBCPP_INLINE_VISIBILITY
  static constexpr auto __make_value_visitor(_Vis&& __vis) {
    return __value_visitor<_Vis>{_VSTD::forward<_Vis>(__vis)};
  }
};

#undef _LIBCPP_VARIANT_SWITCH_MAX
#undef _LIBCPP_VARIANT_CASES
#undef _LIBCPP_VARIANT_CASES_256
#undef _LIBCPP_VARIANT_CASES_64
#undef _LIBCPP_VARIANT_CASES_16
#undef _LIBCPP_VARIANT_CASES_4

} // namespace __visitation

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

  template <class... _Args>
  inline _LIBCPP_INLINE_VISIBILITY
  explicit constexpr __alt(in_place_t, _Args&&... __args)
      : __value(_VSTD::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:                                                                      \
    inline _LIBCPP_INLINE_VISIBILITY                                           \
    explicit constexpr __union(__valueless_t) noexcept : __dummy{} {}          \
                                                                               \
    template <class... _Args>                                                  \
    inline _LIBCPP_INLINE_VISIBILITY                                           \
    explicit constexpr __union(in_place_index_t<0>, _Args&&... __args)         \
        : __head(in_place, _VSTD::forward<_Args>(__args)...) {}                \
                                                                               \
    template <size_t _Ip, class... _Args>                                      \
    inline _LIBCPP_INLINE_VISIBILITY                                           \
    explicit constexpr __union(in_place_index_t<_Ip>, _Args&&... __args)       \
        : __tail(in_place_index<_Ip - 1>, _VSTD::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)>;

  inline _LIBCPP_INLINE_VISIBILITY
  explicit constexpr __base(__valueless_t tag) noexcept
      : __data(tag), __index(__variant_npos<__index_t>) {}

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

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

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

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

  inline _LIBCPP_INLINE_VISIBILITY
  constexpr auto&& __as_base() && { return _VSTD::move(*this); }

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

  inline _LIBCPP_INLINE_VISIBILITY
  constexpr auto&& __as_base() const && { return _VSTD::move(*this); }

  inline _LIBCPP_INLINE_VISIBILITY
  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_INLINE_VISIBILITY                                           \
    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 = __uncvref_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>
  inline _LIBCPP_INLINE_VISIBILITY
  static _Tp& __construct_alt(__alt<_Ip, _Tp>& __a, _Args&&... __args) {
    auto* __result = ::new ((void*)_VSTD::addressof(__a))
        __alt<_Ip, _Tp>(in_place, _VSTD::forward<_Args>(__args)...);
    return __result->__value;
  }

  template <class _Rhs>
  inline _LIBCPP_INLINE_VISIBILITY
  static void __generic_construct(__ctor& __lhs, _Rhs&& __rhs) {
    __lhs.__destroy();
    if (!__rhs.valueless_by_exception()) {
      __visitation::__base::__visit_alt_at(
          __rhs.index(),
          [](auto& __lhs_alt, auto&& __rhs_alt) {
            __construct_alt(
                __lhs_alt,
                _VSTD::forward<decltype(__rhs_alt)>(__rhs_alt).__value);
          },
          __lhs, _VSTD::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, _VSTD::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>
  inline _LIBCPP_INLINE_VISIBILITY
  auto& __emplace(_Args&&... __args) {
    this->__destroy();
    auto& __res = this->__construct_alt(__access::__base::__get_alt<_Ip>(*this),
                                        _VSTD::forward<_Args>(__args)...);
    this->__index = _Ip;
    return __res;
  }

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

  template <class _That>
  inline _LIBCPP_INLINE_VISIBILITY
  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,
                _VSTD::forward<decltype(__that_alt)>(__that_alt).__value);
          },
          *this, _VSTD::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(_VSTD::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;
  using __base_type::operator=;

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

  inline _LIBCPP_INLINE_VISIBILITY
  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 _VSTD::swap;
            swap(__this_alt.__value, __that_alt.__value);
          },
          *this,
          __that);
    } else {
      __impl* __lhs = this;
      __impl* __rhs = _VSTD::addressof(__that);
      if (__lhs->__move_nothrow() && !__rhs->__move_nothrow()) {
        _VSTD::swap(__lhs, __rhs);
      }
      __impl __tmp(_VSTD::move(*__rhs));
#ifndef _LIBCPP_NO_EXCEPTIONS
      if constexpr (__all<is_nothrow_move_constructible_v<_Types>...>::value) {
        this->__generic_construct(*__rhs, _VSTD::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, _VSTD::move(*__lhs));
        } catch (...) {
          if (__tmp.__move_nothrow()) {
            this->__generic_construct(*__rhs, _VSTD::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, _VSTD::move(*__lhs));
#endif
      this->__generic_construct(*__lhs, _VSTD::move(__tmp));
    }
  }

private:
  inline _LIBCPP_INLINE_VISIBILITY
  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 = __identity<_Dest>;
};

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

template <class _Dest, class _Source>
using __check_for_narrowing _LIBCPP_NODEBUG_TYPE =
  typename _If<
#ifdef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
    false &&
#endif
    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 _Tp, size_t>
struct __overload_bool  {
  template <class _Up, class _Ap = __uncvref_t<_Up>>
  auto operator()(bool, _Up&&) const
      -> enable_if_t<is_same_v<_Ap, bool>, __identity<_Tp>>;
};

template <size_t _Idx>
struct __overload<bool, _Idx> : __overload_bool<bool, _Idx> {};
template <size_t _Idx>
struct __overload<bool const, _Idx> : __overload_bool<bool const, _Idx> {};
template <size_t _Idx>
struct __overload<bool volatile, _Idx> : __overload_bool<bool volatile, _Idx> {};
template <size_t _Idx>
struct __overload<bool const volatile, _Idx> : __overload_bool<bool const volatile, _Idx> {};

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_TYPE = __all_overloads<__overload<_Types, _Idx>...>;
};

template <class ..._Types>
using _MakeOverloads _LIBCPP_NODEBUG_TYPE = 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;

} // __variant_detail

template <class... _Types>
class _LIBCPP_TEMPLATE_VIS 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>
  inline _LIBCPP_INLINE_VISIBILITY
  constexpr variant() noexcept(is_nothrow_default_constructible_v<__first_type>)
      : __impl(in_place_index<0>) {}

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

  template <
      class _Arg,
      enable_if_t<!is_same_v<__uncvref_t<_Arg>, variant>, int> = 0,
      enable_if_t<!__is_inplace_type<__uncvref_t<_Arg>>::value, int> = 0,
      enable_if_t<!__is_inplace_index<__uncvref_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>
  inline _LIBCPP_INLINE_VISIBILITY
  constexpr variant(_Arg&& __arg) noexcept(
      is_nothrow_constructible_v<_Tp, _Arg>)
      : __impl(in_place_index<_Ip>, _VSTD::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>
  inline _LIBCPP_INLINE_VISIBILITY
  explicit constexpr variant(
      in_place_index_t<_Ip>,
      _Args&&... __args) noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
      : __impl(in_place_index<_Ip>, _VSTD::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>
  inline _LIBCPP_INLINE_VISIBILITY
  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, _VSTD::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>
  inline _LIBCPP_INLINE_VISIBILITY
  explicit constexpr variant(in_place_type_t<_Tp>, _Args&&... __args) noexcept(
      is_nothrow_constructible_v<_Tp, _Args...>)
      : __impl(in_place_index<_Ip>, _VSTD::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>
  inline _LIBCPP_INLINE_VISIBILITY
  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, _VSTD::forward<_Args>(__args)...) {}

  ~variant() = default;

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

  template <
      class _Arg,
      enable_if_t<!is_same_v<__uncvref_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>
  inline _LIBCPP_INLINE_VISIBILITY
  variant& operator=(_Arg&& __arg) noexcept(
      is_nothrow_assignable_v<_Tp&, _Arg> &&
      is_nothrow_constructible_v<_Tp, _Arg>) {
    __impl.template __assign<_Ip>(_VSTD::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>
  inline _LIBCPP_INLINE_VISIBILITY
  _Tp& emplace(_Args&&... __args) {
    return __impl.template __emplace<_Ip>(_VSTD::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>
  inline _LIBCPP_INLINE_VISIBILITY
  _Tp& emplace(initializer_list<_Up> __il, _Args&&... __args) {
    return __impl.template __emplace<_Ip>(__il, _VSTD::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>
  inline _LIBCPP_INLINE_VISIBILITY
  _Tp& emplace(_Args&&... __args) {
    return __impl.template __emplace<_Ip>(_VSTD::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>
  inline _LIBCPP_INLINE_VISIBILITY
  _Tp& emplace(initializer_list<_Up> __il, _Args&&... __args) {
    return __impl.template __emplace<_Ip>(__il, _VSTD::forward<_Args>(__args)...);
  }

  inline _LIBCPP_INLINE_VISIBILITY
  constexpr bool valueless_by_exception() const noexcept {
    return __impl.valueless_by_exception();
  }

  inline _LIBCPP_INLINE_VISIBILITY
  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>
  inline _LIBCPP_INLINE_VISIBILITY
  void swap(variant& __that) noexcept(
      __all<(is_nothrow_move_constructible_v<_Types> &&
             is_nothrow_swappable_v<_Types>)...>::value) {
    __impl.__swap(__that.__impl);
  }

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

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

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

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

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

template <size_t _Ip, class... _Types>
inline _LIBCPP_INLINE_VISIBILITY
_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 __generic_get<_Ip>(__v);
}

template <size_t _Ip, class... _Types>
inline _LIBCPP_INLINE_VISIBILITY
_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 __generic_get<_Ip>(_VSTD::move(__v));
}

template <size_t _Ip, class... _Types>
inline _LIBCPP_INLINE_VISIBILITY
_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 __generic_get<_Ip>(__v);
}

template <size_t _Ip, class... _Types>
inline _LIBCPP_INLINE_VISIBILITY
_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 __generic_get<_Ip>(_VSTD::move(__v));
}

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

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

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

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

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

template <size_t _Ip, class... _Types>
inline _LIBCPP_INLINE_VISIBILITY
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 __generic_get_if<_Ip>(__v);
}

template <size_t _Ip, class... _Types>
inline _LIBCPP_INLINE_VISIBILITY
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 __generic_get_if<_Ip>(__v);
}

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

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

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

template <class... _Types>
inline _LIBCPP_INLINE_VISIBILITY
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);
}

template <class... _Types>
inline _LIBCPP_INLINE_VISIBILITY
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>
inline _LIBCPP_INLINE_VISIBILITY
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>
inline _LIBCPP_INLINE_VISIBILITY
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>
inline _LIBCPP_INLINE_VISIBILITY
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>
inline _LIBCPP_INLINE_VISIBILITY
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 _Vis, class... _Vs>
inline _LIBCPP_INLINE_VISIBILITY
_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
constexpr decltype(auto) visit(_Vis&& __vis, _Vs&&... __vs) {
  using __variant_detail::__visitation::__variant;
  return __variant::__visit_value(_VSTD::forward<_Vis>(__vis),
                                  _VSTD::forward<_Vs>(__vs)...);
}

struct _LIBCPP_TEMPLATE_VIS monostate {};

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

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

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

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

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

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

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

  inline _LIBCPP_INLINE_VISIBILITY
  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 = __uncvref_t<decltype(__alt)>;
                       using __value_type = remove_const_t<
                         typename __alt_type::__value_type>;
                       return hash<__value_type>{}(__alt.__value);
                     },
                     __v);
    return __hash_combine(__res, hash<size_t>{}(__v.index()));
  }
};

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

  inline _LIBCPP_INLINE_VISIBILITY
  result_type operator()(const argument_type&) const _NOEXCEPT {
    return 66740831; // return a fundamentally attractive random value.
  }
};

#endif  // _LIBCPP_STD_VER > 14

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#endif  // _LIBCPP_VARIANT