Index: include/variant
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+++ include/variant
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+// -*- C++ -*-
+//===------------------------------ variant -------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#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&);
+    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
+    variant& operator=(const variant&);
+    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>
+    void emplace(Args&&...);
+
+    template <class T, class U, class... Args>
+    void emplace(initializer_list<U>, Args&&...);
+
+    template <size_t I, class... Args>
+    void emplace(Args&&...);
+
+    template <size_t I, class U, class...  Args>
+    void 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>
+  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...>>;
+
+  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 <__tuple>
+#include <array>
+#include <exception>
+#include <functional>
+#include <initializer_list>
+#include <new>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+namespace std {  // explicitly not using versioning namespace
+
+class _LIBCPP_EXCEPTION_ABI bad_variant_access : public exception {
+public:
+  virtual const char* what() const noexcept;
+};
+
+}  // namespace std
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 14
+
+template <class... _Types>
+class _LIBCPP_TYPE_VIS variant;
+
+template <class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY variant_size;
+
+template <class _Tp>
+constexpr size_t variant_size_v = variant_size<_Tp>::value;
+
+template <class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY variant_size<const _Tp>
+    : variant_size<_Tp> {};
+
+template <class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY variant_size<volatile _Tp>
+    : variant_size<_Tp> {};
+
+template <class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY variant_size<const volatile _Tp>
+    : variant_size<_Tp> {};
+
+template <class... _Types>
+struct _LIBCPP_TYPE_VIS_ONLY variant_size<variant<_Types...>>
+    : integral_constant<size_t, sizeof...(_Types)> {};
+
+template <size_t _Ip, class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY 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_TYPE_VIS_ONLY variant_alternative<_Ip, const _Tp>
+    : add_const<variant_alternative_t<_Ip, _Tp>> {};
+
+template <size_t _Ip, class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY variant_alternative<_Ip, volatile _Tp>
+    : add_volatile<variant_alternative_t<_Ip, _Tp>> {};
+
+template <size_t _Ip, class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY variant_alternative<_Ip, const volatile _Tp>
+    : add_cv<variant_alternative_t<_Ip, _Tp>> {};
+
+template <size_t _Ip, class... _Types>
+struct _LIBCPP_TYPE_VIS_ONLY variant_alternative<_Ip, variant<_Types...>> {
+  static_assert(_Ip < sizeof...(_Types));
+  using type = __type_pack_element<_Ip, _Types...>;
+};
+
+constexpr size_t variant_npos = static_cast<size_t>(-1);
+
+struct __variant_valueless_t {};
+constexpr __variant_valueless_t __variant_valueless = __variant_valueless_t{};
+
+namespace __find_detail {
+
+  template <class _Tp, class... _Types>
+  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
+
+using __find_detail::__find_unambiguous_index_sfinae;
+
+template <class... _Types>
+class __variant_overload;
+
+template <>
+class __variant_overload<> {
+public:
+  void operator()() const;
+};
+
+template <class _Tp, class... _Types>
+class __variant_overload<_Tp, _Types...>
+    : public __variant_overload<_Types...> {
+public:
+  using __variant_overload<_Types...>::operator();
+  __identity<_Tp> operator()(_Tp) const;
+};
+
+template <class _Tp, class... _Types>
+using __variant_best_match_t =
+    typename result_of_t<__variant_overload<_Types...>(_Tp&&)>::type;
+
+struct __variant_union_access {
+  template <class _Vp>
+  static constexpr auto&& __get_alt(_Vp&& __v, in_place_index_t<0>) {
+    return _VSTD::forward<_Vp>(__v).__head;
+  }
+
+  template <class _Vp, size_t _Ip>
+  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 __variant_impl_access {
+  template <size_t _Ip, class _Vp>
+  static constexpr auto&& __get_alt(_Vp&& __v) {
+    return __variant_union_access::__get_alt(_VSTD::forward<_Vp>(__v).__data,
+                                             in_place_index<_Ip>);
+  }
+};
+
+struct __variant_access {
+  template <size_t _Ip, class _Vp>
+  static constexpr auto&& __get_alt(_Vp&& __v) {
+    return __variant_impl_access::__get_alt<_Ip>(
+        _VSTD::forward<_Vp>(__v).__impl);
+  }
+};
+
+namespace __variant_impl_visitation_detail {
+
+  template <class _Tp>
+  constexpr const _Tp& __at_impl(const _Tp& __elem, const size_t*) {
+    return __elem;
+  }
+
+  template <class _Tp, size_t _Np>
+  constexpr auto&& __at_impl(const array<_Tp, _Np>& __elems,
+                             const size_t* __index) {
+    return __at_impl(__elems[*__index], __index + 1);
+  }
+
+  template <class _Tp, size_t _Np, size_t _Ip>
+  constexpr auto&& __at(const array<_Tp, _Np>& __elems,
+                        const size_t (&__indices)[_Ip]) {
+    return __at_impl(__elems, begin(__indices));
+  }
+
+  template <class _Tp, class... _Types>
+  struct __all_same : __all<is_same_v<_Tp, _Types>...> {};
+
+  template <class... _Types>
+  constexpr void __variant_visit_has_same_return_types(true_type) {}
+
+  template <class... _Types>
+  constexpr void __variant_visit_has_same_return_types(false_type) = delete;
+
+  template <class... _Args>
+  constexpr array<common_type_t<decay_t<_Args>...>, sizeof...(_Args)>
+  __make_array(_Args&&... __args) {
+    using __all_same = __all_same<decay_t<_Args>...>;
+    __variant_visit_has_same_return_types<decay_t<_Args>...>(__all_same{});
+    return {{_VSTD::forward<_Args>(__args)...}};
+  }
+
+  template <class _Fp, class... _Vs, size_t... _Is>
+  constexpr auto __make_dispatch(index_sequence<_Is...>) {
+    struct __dispatcher {
+      static constexpr decltype(auto) __dispatch(_Fp __f, _Vs... __vs) {
+        return __invoke_constexpr(
+            static_cast<_Fp>(__f),
+            __variant_impl_access::__get_alt<_Is>(static_cast<_Vs>(__vs))...);
+      }
+    };
+    return _VSTD::addressof(__dispatcher::__dispatch);
+  }
+
+  template <size_t _Ip, class _Fp, class... _Vs>
+  constexpr auto __make_fdiagonal_impl() {
+    return __make_dispatch<_Fp, _Vs...>(
+        index_sequence<(__identity<_Vs>{}, _Ip)...>{});
+  }
+
+  template <class _Fp, class... _Vs, size_t... _Is>
+  constexpr auto __make_fdiagonal_impl(index_sequence<_Is...>) {
+    return __make_array(__make_fdiagonal_impl<_Is, _Fp, _Vs...>()...);
+  }
+
+  template <class _Fp, class _Vp, class... _Vs>
+  constexpr auto __make_fdiagonal() {
+    constexpr size_t _Np = decay_t<_Vp>::__size();
+    static_assert(__all<(_Np == decay_t<_Vs>::__size())...>::value);
+    return __make_fdiagonal_impl<_Fp, _Vp, _Vs...>(make_index_sequence<_Np>{});
+  }
+
+  template <class _Fp, class... _Vs, size_t... _Is>
+  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>
+  constexpr auto __make_fmatrix_impl(index_sequence<_Is...>,
+                                     index_sequence<_Js...>,
+                                     _Ls... __ls) {
+    return __make_array(__make_fmatrix_impl<_Fp, _Vs...>(
+        index_sequence<_Is..., _Js>{}, __ls...)...);
+  }
+
+  template <class _Fp, class... _Vs>
+  constexpr auto __make_fmatrix() {
+    return __make_fmatrix_impl<_Fp, _Vs...>(
+        index_sequence<>{}, make_index_sequence<decay_t<_Vs>::__size()>{}...);
+  }
+
+}  // namespace __variant_impl_visitation_detail
+
+struct __variant_impl_visitation {
+  template <class _Visitor, class... _Vs>
+  static constexpr decltype(auto) __visit_alt_at(size_t __index,
+                                                 _Visitor&& __visitor,
+                                                 _Vs&&... __vs) {
+    using namespace __variant_impl_visitation_detail;
+    constexpr auto __fdiagonal = __make_fdiagonal<
+        _Visitor&&,
+        decltype(_VSTD::forward<_Vs>(__vs).__as_variant_base())...>();
+    return __fdiagonal[__index](
+        _VSTD::forward<_Visitor>(__visitor),
+        _VSTD::forward<_Vs>(__vs).__as_variant_base()...);
+  }
+
+  template <class _Visitor, class... _Vs>
+  static constexpr decltype(auto) __visit_alt(_Visitor&& __visitor,
+                                              _Vs&&... __vs) {
+    using namespace __variant_impl_visitation_detail;
+    constexpr auto __fmatrix = __make_fmatrix<
+        _Visitor&&,
+        decltype(_VSTD::forward<_Vs>(__vs).__as_variant_base())...>();
+    const size_t __indices[] = {__vs.index()...};
+    return __at(__fmatrix, __indices)(
+        _VSTD::forward<_Visitor>(__visitor),
+        _VSTD::forward<_Vs>(__vs).__as_variant_base()...);
+  }
+};
+
+struct __variant_visitation {
+  template <class _Visitor, class... _Vs>
+  static constexpr decltype(auto) __visit_alt_at(size_t __index,
+                                                 _Visitor&& __visitor,
+                                                 _Vs&&... __vs) {
+    return __variant_impl_visitation::__visit_alt_at(
+        __index,
+        _VSTD::forward<_Visitor>(__visitor),
+        _VSTD::forward<_Vs>(__vs).__impl...);
+  }
+
+  template <class _Visitor, class... _Vs>
+  static constexpr decltype(auto) __visit_alt(_Visitor&& __visitor,
+                                              _Vs&&... __vs) {
+    return __variant_impl_visitation::__visit_alt(
+        _VSTD::forward<_Visitor>(__visitor),
+        _VSTD::forward<_Vs>(__vs).__impl...);
+  }
+
+  template <class _Visitor>
+  struct __value_visitor {
+    template <class... _Alts>
+    constexpr decltype(auto) operator()(_Alts&&... __alts) const {
+      return __invoke_constexpr(static_cast<_Visitor>(__visitor),
+                                _VSTD::forward<_Alts>(__alts).__value...);
+    }
+
+    _Visitor __visitor;
+  };
+
+  template <class _Visitor>
+  static constexpr auto __make_value_visitor(_Visitor&& __visitor) {
+    return __value_visitor<_Visitor&&>{_VSTD::forward<_Visitor>(__visitor)};
+  }
+
+  template <class _Visitor, class... _Vs>
+  static constexpr decltype(auto) __visit_value_at(size_t __index,
+                                                   _Visitor&& __visitor,
+                                                   _Vs&&... __vs) {
+    return __visit_alt_at(
+        __index,
+        __make_value_visitor(_VSTD::forward<_Visitor>(__visitor)),
+        _VSTD::forward<_Vs>(__vs)...);
+  }
+
+  template <class _Visitor, class... _Vs>
+  static constexpr decltype(auto) __visit_value(_Visitor&& __visitor,
+                                                _Vs&&... __vs) {
+    return __visit_alt(
+        __make_value_visitor(_VSTD::forward<_Visitor>(__visitor)),
+        _VSTD::forward<_Vs>(__vs)...);
+  }
+};
+
+template <size_t _Index, class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY __variant_alt {
+  using __value_type = _Tp;
+
+  __variant_alt() = default;
+  __variant_alt(const __variant_alt&) = delete;
+  __variant_alt(__variant_alt&&) = delete;
+  __variant_alt& operator=(const __variant_alt&) = delete;
+  __variant_alt& operator=(__variant_alt&&) = delete;
+
+  template <class... _Args>
+  explicit constexpr __variant_alt(_Args&&... __args)
+      : __value(_VSTD::forward<_Args>(__args)...) {}
+
+  __value_type __value;
+};
+
+template <bool _TriviallyDestructible, size_t _Index, class... _Types>
+union __variant_union;
+
+template <bool _TriviallyDestructible, size_t _Index>
+union __variant_union<_TriviallyDestructible, _Index> {};
+
+#define _LIBCPP_VARIANT_UNION(__trivially_destructible, dtor)                  \
+  template <size_t _Index, class _Tp, class... _Types>                         \
+  union __variant_union<__trivially_destructible, _Index, _Tp, _Types...> {    \
+  public:                                                                      \
+    explicit constexpr __variant_union(__variant_valueless_t) noexcept         \
+        : __dummy{} {}                                                         \
+                                                                               \
+    template <class... _Args>                                                  \
+    explicit constexpr __variant_union(in_place_index_t<0>,                    \
+                                       _Args&&... __args)                      \
+        : __head(_VSTD::forward<_Args>(__args)...) {}                          \
+                                                                               \
+    template <size_t _Ip, class... _Args>                                      \
+    explicit constexpr __variant_union(in_place_index_t<_Ip>,                  \
+                                       _Args&&... __args)                      \
+        : __tail(in_place_index<_Ip - 1>, _VSTD::forward<_Args>(__args)...) {} \
+                                                                               \
+    dtor                                                                       \
+                                                                               \
+  private:                                                                     \
+    char __dummy;                                                              \
+    __variant_alt<_Index, _Tp> __head;                                         \
+    __variant_union<__trivially_destructible, _Index + 1, _Types...> __tail;   \
+                                                                               \
+    friend struct __variant_union_access;                                      \
+  }
+
+_LIBCPP_VARIANT_UNION(true , ~__variant_union() = default;);
+_LIBCPP_VARIANT_UNION(false, ~__variant_union() {});
+
+#undef _LIBCPP_VARIANT_UNION
+
+template <bool _TriviallyDestructible, class... _Types>
+class __variant_base {
+public:
+  static constexpr size_t __size() { return sizeof...(_Types); }
+
+  explicit constexpr __variant_base(__variant_valueless_t) noexcept
+      : __index(variant_npos), __data(__variant_valueless) {}
+
+  template <size_t _Ip, class... _Args>
+  explicit constexpr __variant_base(in_place_index_t<_Ip>, _Args&&... __args)
+      : __index(_Ip), __data(in_place_index<_Ip>, _VSTD::forward<_Args>(__args)...) {}
+
+  constexpr bool valueless_by_exception() const noexcept {
+    return index() == variant_npos;
+  }
+
+  constexpr size_t index() const noexcept { return __index; }
+
+protected:
+  constexpr auto&& __as_variant_base() & { return *this; }
+  constexpr auto&& __as_variant_base() && { return _VSTD::move(*this); }
+  constexpr auto&& __as_variant_base() const & { return *this; }
+  constexpr auto&& __as_variant_base() const && { return _VSTD::move(*this); }
+
+  size_t __index;
+  __variant_union<__trivially_destructible, 0, _Types...> __data;
+
+  friend struct __variant_impl_access;
+  friend struct __variant_impl_visitation;
+};
+
+template <bool _TriviallyDestructible, class... _Types>
+class __variant_dtor;
+
+template <class... _Types>
+class __variant_dtor</* __trivially_destructible */ true, _Types...>
+    : public __variant_base<true, _Types...> {
+  using __base_type = __variant_base<true, _Types...>;
+
+public:
+  using __base_type::__base_type;
+  ~__variant_dtor() = default;
+
+protected:
+  void __destroy() noexcept { this->__index = variant_npos; }
+};
+
+template <class... _Types>
+class __variant_dtor</* __trivially_destructible */ false, _Types...>
+    : public __variant_base<false, _Types...> {
+  using __base_type = __variant_base<false, _Types...>;
+
+public:
+  using __base_type::__base_type;
+  ~__variant_dtor() { __destroy(); }
+
+protected:
+  void __destroy() noexcept {
+    if (!this->valueless_by_exception()) {
+      __variant_impl_visitation::__visit_alt(
+          [](auto& __alt) noexcept {
+            using __alt_type = decay_t<decltype(__alt)>;
+            __alt.~__alt_type();
+          },
+          *this);
+    }
+    this->__index = variant_npos;
+  }
+};
+
+template <class... _Types>
+struct __variant_impl
+    : public __variant_dtor<
+          __all<is_trivially_destructible_v<_Types>...>::value, _Types...> {
+  using __base_type =
+      __variant_dtor<__all<is_trivially_destructible_v<_Types>...>::value,
+                     _Types...>;
+
+public:
+  using __base_type::__base_type;
+
+  __variant_impl(
+      const conditional_t<__all<is_copy_constructible_v<_Types>...>::value,
+                          __variant_impl,
+                          __nat>& __that)
+      : __variant_impl(__variant_valueless) {
+    static_assert(!is_same_v<decay_t<decltype(__that)>, __nat>);
+    __generic_construct(*this, __that);
+  }
+
+  __variant_impl(
+      conditional_t<__all<is_move_constructible_v<_Types>...>::value,
+                    __variant_impl,
+                    __nat>&&
+          __that)
+    noexcept(__all<is_nothrow_move_constructible_v<_Types>...>::value)
+      : __variant_impl(__variant_valueless) {
+    static_assert(!is_same_v<decay_t<decltype(__that)>, __nat>);
+    __generic_construct(*this, _VSTD::move(__that));
+  }
+
+  __variant_impl& operator=(
+      const conditional_t<__all<(is_copy_constructible_v<_Types> &&
+                                 is_move_constructible_v<_Types> &&
+                                 is_copy_assignable_v<_Types>)...>::value,
+                          __variant_impl,
+                          __nat>& __that) {
+    static_assert(!is_same_v<decay_t<decltype(__that)>, __nat>);
+    return __generic_assign(__that);
+  }
+
+  __variant_impl& operator=(
+      conditional_t<__all<(is_move_constructible_v<_Types> &&
+                           is_move_assignable_v<_Types>)...>::value,
+                    __variant_impl,
+                    __nat>&&
+          __that) noexcept(__all<(is_nothrow_move_constructible_v<_Types> &&
+                                  is_nothrow_move_assignable_v<_Types>)...>::value) {
+    static_assert(!is_same_v<decay_t<decltype(__that)>, __nat>);
+    return __generic_assign(_VSTD::move(__that));
+  }
+
+  template <size_t _Ip, class _Arg>
+  void __assign(_Arg&& __arg) {
+    __assign_alt(__variant_impl_access::__get_alt<_Ip>(*this),
+                 _VSTD::forward<_Arg>(__arg),
+                 false_type{});
+  }
+
+  template <size_t _Ip, class... _Args>
+  void __emplace(_Args&&... __args) {
+    this->__destroy();
+    __construct_alt(__variant_impl_access::__get_alt<_Ip>(*this),
+                    _VSTD::forward<_Args>(__args)...);
+    this->__index = _Ip;
+  }
+
+  void __swap(__variant_impl& __that)  {
+    if (this->valueless_by_exception() && __that.valueless_by_exception()) {
+      // do nothing.
+    } else if (this->index() == __that.index()) {
+      __variant_impl_visitation::__visit_alt_at(
+          this->index(),
+          [](auto& __this_alt, auto& __that_alt) {
+            using _VSTD::swap;
+            swap(__this_alt.__value, __that_alt.__value);
+          },
+          *this, __that);
+    } else {
+      __variant_impl* __lhs = this;
+      __variant_impl* __rhs = _VSTD::addressof(__that);
+      if (__lhs->__move_nothrow() && !__rhs->__move_nothrow()) {
+        _VSTD::swap(__lhs, __rhs);
+      }
+      __variant_impl __tmp(_VSTD::move(*__rhs));
+#ifndef _LIBCPP_NO_EXCEPTIONS
+      // 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 guarentee.
+      try {
+        __generic_construct(*__rhs, _VSTD::move(*__lhs));
+      } catch (...) {
+        if (__tmp.__move_nothrow()) {
+          __generic_construct(*__rhs, _VSTD::move(__tmp));
+        }
+        throw;
+      }
+#else
+      __generic_construct(*__rhs, _VSTD::move(*__lhs));
+#endif
+      __generic_construct(*__lhs, _VSTD::move(__tmp));
+    }
+  }
+
+private:
+  bool __move_nothrow() const {
+    constexpr bool __results[] = {is_nothrow_move_constructible_v<_Types>...};
+    return this->valueless_by_exception() || __results[this->index()];
+  }
+
+  template <bool _CopyAssign, size_t _Ip, class _Tp, class _Arg>
+  void __assign_alt(__variant_alt<_Ip, _Tp>& __alt,
+                    _Arg&& __arg,
+                    bool_constant<_CopyAssign> __tag) {
+    if (this->index() == _Ip) {
+      __alt.__value = _VSTD::forward<_Arg>(__arg);
+    } else {
+      struct {
+        void operator()(true_type) const {
+          __this->__emplace<_Ip>(_Tp(static_cast<_Arg&&>(__arg)));
+        }
+        void operator()(false_type) const {
+          __this->__emplace<_Ip>(static_cast<_Arg&&>(__arg));
+        }
+        __variant_impl* __this;
+        _Arg&& __arg;
+      } __impl{this, _VSTD::forward<_Arg>(__arg)};
+      __impl(__tag);
+    }
+  }
+
+  template <class _That>
+  __variant_impl& __generic_assign(_That&& __that) {
+    static_assert(is_same_v<decay_t<_That>, __variant_impl>);
+    if (this->valueless_by_exception() && __that.valueless_by_exception()) {
+      // do nothing.
+    } else if (__that.valueless_by_exception()) {
+      this->__destroy();
+    } else {
+      __variant_impl_visitation::__visit_alt_at(
+          __that.index(),
+          [this](auto& __this_alt, auto&& __that_alt) {
+            __assign_alt(
+                __this_alt,
+                _VSTD::forward<decltype(__that_alt)>(__that_alt).__value,
+                is_lvalue_reference<_That&&>{});
+          },
+          *this, _VSTD::forward<_That>(__that));
+    }
+    return *this;
+  }
+
+  template <size_t _Ip, class _Tp, class... _Args>
+  static void __construct_alt(__variant_alt<_Ip, _Tp>& __alt,
+                              _Args&&... __args) {
+    ::new (_VSTD::addressof(__alt))
+        __variant_alt<_Ip, _Tp>(_VSTD::forward<_Args>(__args)...);
+  }
+
+  template <class _Rhs>
+  static void __generic_construct(__variant_impl& __lhs, _Rhs&& __rhs) {
+    static_assert(is_same_v<decay_t<_Rhs>, __variant_impl>);
+    __lhs.__destroy();
+    if (!__rhs.valueless_by_exception()) {
+      __variant_impl_visitation::__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... _Types>
+class variant {
+  static_assert(sizeof...(_Types) > 0,
+                "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.");
+
+public:
+  template <size_t _Ip = 0,
+            enable_if_t<(_Ip < sizeof...(_Types)), int> = 0,
+            class _Tp = variant_alternative_t<_Ip, variant<_Types...>>,
+            enable_if_t<is_default_constructible_v<_Tp>, int> = 0>
+  constexpr variant() noexcept(is_nothrow_default_constructible_v<_Tp>)
+      : __impl(in_place_index<0>) {}
+
+  variant(const variant&) = default;
+  variant(variant&&) = default;
+
+  template <class _Arg,
+            enable_if_t<!is_same_v<decay_t<_Arg>, variant>, int> = 0,
+            class _Tp = __variant_best_match_t<_Arg&&, _Types...>,
+            size_t _Ip = __find_unambiguous_index_sfinae<_Tp, _Types...>::value,
+            enable_if_t<is_constructible_v<_Tp, _Arg&&>, int> = 0>
+  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,
+            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>
+  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>
+  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_unambiguous_index_sfinae<_Tp, _Types...>::value,
+            enable_if_t<is_constructible_v<_Tp, _Args&&...>, int> = 0>
+  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_unambiguous_index_sfinae<_Tp, _Types...>::value,
+      enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args&&...>,
+                  int> = 0>
+  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<decay_t<_Arg>, variant>, int> = 0,
+            class _Tp = __variant_best_match_t<_Arg&&, _Types...>,
+            size_t _Ip = __find_unambiguous_index_sfinae<_Tp, _Types...>::value,
+            enable_if_t<(is_assignable_v<_Tp&, _Arg&&> &&
+                         is_constructible_v<_Tp, _Arg&&>), int> = 0>
+  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>
+  void emplace(_Args&&... __args) {
+    __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>
+  void emplace(initializer_list<_Up> __il, _Args&&... __args) {
+    __impl.template __emplace<_Ip>(__il, _VSTD::forward<_Args>(__args)...);
+  }
+
+  template <
+      class _Tp,
+      class... _Args,
+      size_t _Ip = __find_unambiguous_index_sfinae<_Tp, _Types...>::value,
+      enable_if_t<is_constructible_v<_Tp, _Args&&...>, int> = 0>
+  void emplace(_Args&&... __args) {
+    __impl.template __emplace<_Ip>(_VSTD::forward<_Args>(__args)...);
+  }
+
+  template <
+      class _Tp,
+      class _Up,
+      class... _Args,
+      size_t _Ip = __find_unambiguous_index_sfinae<_Tp, _Types...>::value,
+      enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args&&...>,
+                  int> = 0>
+  void emplace(initializer_list<_Up> __il, _Args&&... __args) {
+    __impl.template __emplace<_Ip>(__il, _VSTD::forward<_Args>(__args)...);
+  }
+
+  constexpr bool valueless_by_exception() const noexcept {
+    return __impl.valueless_by_exception();
+  }
+
+  constexpr size_t index() const noexcept { return __impl.index(); }
+
+  template <
+      bool _Dummy = false,
+      enable_if_t<
+          __all<(
+              __dependent_type<is_move_constructible<_Types>, _Dummy>::value &&
+              __dependent_type<is_swappable<_Types>, _Dummy>::value)...>::value,
+          int> = 0>
+  void swap(variant& __that) noexcept(
+      __all<(is_nothrow_move_constructible_v<_Types> &&
+             is_nothrow_swappable_v<_Types>)...>::value) {
+    __impl.__swap(__that.__impl);
+  }
+
+private:
+  __variant_impl<_Types...> __impl;
+
+  friend struct __variant_access;
+  friend struct __variant_visitation;
+};
+
+template <size_t _Ip, class... _Types>
+constexpr bool __holds_alternative(const variant<_Types...>& __v) noexcept {
+  return __v.index() == _Ip;
+}
+
+template <class _Tp, class... _Types>
+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>
+static constexpr auto&& __generic_get(_Vp&& __v) {
+  return __holds_alternative<_Ip>(__v)
+             ? __variant_access::__get_alt<_Ip>(_VSTD::forward<_Vp>(__v))
+                   .__value
+             : throw bad_variant_access{};
+}
+
+template <size_t _Ip, class... _Types>
+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>
+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>
+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>
+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>
+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>
+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>
+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>
+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>
+constexpr auto* __generic_get_if(_Vp* __v) noexcept {
+  return __v && __holds_alternative<_Ip>(*__v)
+             ? _VSTD::addressof(__variant_access::__get_alt<_Ip>(*__v).__value)
+             : nullptr;
+}
+
+template <size_t _Ip, class... _Types>
+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>
+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>
+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>
+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... _Types>
+constexpr bool operator==(const variant<_Types...>& __lhs,
+                          const variant<_Types...>& __rhs) {
+  if (__lhs.index() != __rhs.index()) return false;
+  if (__lhs.valueless_by_exception()) return true;
+  return __variant_visitation::__visit_value_at(
+      __lhs.index(), equal_to<>{}, __lhs, __rhs);
+}
+
+template <class... _Types>
+constexpr bool operator!=(const variant<_Types...>& __lhs,
+                          const variant<_Types...>& __rhs) {
+  if (__lhs.index() != __rhs.index()) return true;
+  if (__lhs.valueless_by_exception()) return false;
+  return __variant_visitation::__visit_value_at(
+      __lhs.index(), not_equal_to<>{}, __lhs, __rhs);
+}
+
+template <class... _Types>
+constexpr bool operator<(const variant<_Types...>& __lhs,
+                         const variant<_Types...>& __rhs) {
+  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_visitation::__visit_value_at(
+      __lhs.index(), less<>{}, __lhs, __rhs);
+}
+
+template <class... _Types>
+constexpr bool operator>(const variant<_Types...>& __lhs,
+                         const variant<_Types...>& __rhs) {
+  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_visitation::__visit_value_at(
+      __lhs.index(), greater<>{}, __lhs, __rhs);
+}
+
+template <class... _Types>
+constexpr bool operator<=(const variant<_Types...>& __lhs,
+                          const variant<_Types...>& __rhs) {
+  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_visitation::__visit_value_at(
+      __lhs.index(), less_equal<>{}, __lhs, __rhs);
+}
+
+template <class... _Types>
+constexpr bool operator>=(const variant<_Types...>& __lhs,
+                          const variant<_Types...>& __rhs) {
+  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_visitation::__visit_value_at(
+      __lhs.index(), greater_equal<>{}, __lhs, __rhs);
+}
+
+template <class _Visitor, class... _Vs>
+constexpr decltype(auto) visit(_Visitor&& __visitor, _Vs&&... __vs) {
+  bool __results[] = {__vs.valueless_by_exception()...};
+  for (bool __result : __results) {
+    if (__result) {
+      throw bad_variant_access{};
+    }
+  }
+  return __variant_visitation::__visit_value(
+      _VSTD::forward<_Visitor>(__visitor), _VSTD::forward<_Vs>(__vs)...);
+}
+
+struct _LIBCPP_TYPE_VIS_ONLY monostate {};
+
+constexpr bool operator<(monostate, monostate) noexcept { return false; }
+constexpr bool operator>(monostate, monostate) noexcept { return false; }
+constexpr bool operator<=(monostate, monostate) noexcept { return true; }
+constexpr bool operator>=(monostate, monostate) noexcept { return true; }
+constexpr bool operator==(monostate, monostate) noexcept { return true; }
+constexpr bool operator!=(monostate, monostate) noexcept { return false; }
+
+template <class... _Types>
+void swap(variant<_Types...> &__lhs,
+          variant<_Types...> &__rhs) noexcept(noexcept(__lhs.swap(__rhs))) {
+  __lhs.swap(__rhs);
+}
+
+template <class... _Types>
+struct _LIBCPP_TYPE_VIS_ONLY hash<variant<_Types...>> {
+  using argument_type = variant<_Types...>;
+  using result_type = size_t;
+
+  result_type operator()(const argument_type& __v) const {
+    return __v.valueless_by_exception()
+               ? variant_npos
+               : __variant_visitation::__visit_alt(
+                     [](const auto& __alt) {
+                       using __alt_type = decay_t<decltype(__alt)>;
+                       using __value_type = typename __alt_type::__value_type;
+                       return hash<__value_type>{}(__alt.__value);
+                     },
+                     __v);
+  }
+};
+
+template <>
+struct _LIBCPP_TYPE_VIS_ONLY hash<monostate> {
+  using argument_type = monostate;
+  using result_type = size_t;
+
+  result_type operator()(const argument_type&) const { return 0; }
+};
+
+#endif  // _LIBCPP_STD_VER > 14
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif  // _LIBCPP_VARIANT
Index: src/variant.cpp
===================================================================
--- /dev/null
+++ src/variant.cpp
@@ -0,0 +1,18 @@
+//===------------------------ variant.cpp ---------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "variant"
+
+namespace std {
+
+const char* bad_variant_access::what() const noexcept {
+  return "bad_variant_access";
+}
+
+}  // namespace std