xref: /freebsd-src/contrib/llvm-project/libcxx/include/__expected/expected.h (revision b121cb0095c8c1a060f66a8c4b118a54ebaa2551)

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

#include <__assert>
#include <__config>
#include <__expected/bad_expected_access.h>
#include <__expected/unexpect.h>
#include <__expected/unexpected.h>
#include <__functional/invoke.h>
#include <__memory/addressof.h>
#include <__memory/construct_at.h>
#include <__type_traits/conjunction.h>
#include <__type_traits/disjunction.h>
#include <__type_traits/integral_constant.h>
#include <__type_traits/is_assignable.h>
#include <__type_traits/is_constructible.h>
#include <__type_traits/is_convertible.h>
#include <__type_traits/is_copy_assignable.h>
#include <__type_traits/is_copy_constructible.h>
#include <__type_traits/is_default_constructible.h>
#include <__type_traits/is_function.h>
#include <__type_traits/is_move_assignable.h>
#include <__type_traits/is_move_constructible.h>
#include <__type_traits/is_nothrow_constructible.h>
#include <__type_traits/is_nothrow_copy_assignable.h>
#include <__type_traits/is_nothrow_copy_constructible.h>
#include <__type_traits/is_nothrow_default_constructible.h>
#include <__type_traits/is_nothrow_move_assignable.h>
#include <__type_traits/is_nothrow_move_constructible.h>
#include <__type_traits/is_reference.h>
#include <__type_traits/is_same.h>
#include <__type_traits/is_swappable.h>
#include <__type_traits/is_trivially_copy_constructible.h>
#include <__type_traits/is_trivially_destructible.h>
#include <__type_traits/is_trivially_move_constructible.h>
#include <__type_traits/is_void.h>
#include <__type_traits/lazy.h>
#include <__type_traits/negation.h>
#include <__type_traits/remove_cv.h>
#include <__type_traits/remove_cvref.h>
#include <__utility/as_const.h>
#include <__utility/exception_guard.h>
#include <__utility/forward.h>
#include <__utility/in_place.h>
#include <__utility/move.h>
#include <__utility/swap.h>
#include <__verbose_abort>
#include <initializer_list>

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

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

#if _LIBCPP_STD_VER >= 23

_LIBCPP_BEGIN_NAMESPACE_STD

template <class _Tp, class _Err>
class expected;

template <class _Tp>
struct __is_std_expected : false_type {};

template <class _Tp, class _Err>
struct __is_std_expected<expected<_Tp, _Err>> : true_type {};

struct __expected_construct_in_place_from_invoke_tag {};
struct __expected_construct_unexpected_from_invoke_tag {};

template <class _Err, class _Arg>
_LIBCPP_HIDE_FROM_ABI void __throw_bad_expected_access(_Arg&& __arg) {
#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
  throw bad_expected_access<_Err>(std::forward<_Arg>(__arg));
#  else
  (void)__arg;
  _LIBCPP_VERBOSE_ABORT("bad_expected_access was thrown in -fno-exceptions mode");
#  endif
}

template <class _Tp, class _Err>
class expected {
  static_assert(
      !is_reference_v<_Tp> &&
          !is_function_v<_Tp> &&
          !is_same_v<remove_cv_t<_Tp>, in_place_t> &&
          !is_same_v<remove_cv_t<_Tp>, unexpect_t> &&
          !__is_std_unexpected<remove_cv_t<_Tp>>::value &&
          __valid_std_unexpected<_Err>::value
      ,
      "[expected.object.general] A program that instantiates the definition of template expected<T, E> for a "
      "reference type, a function type, or for possibly cv-qualified types in_place_t, unexpect_t, or a "
      "specialization of unexpected for the T parameter is ill-formed. A program that instantiates the "
      "definition of the template expected<T, E> with a type for the E parameter that is not a valid "
      "template argument for unexpected is ill-formed.");

  template <class _Up, class _OtherErr>
  friend class expected;

public:
  using value_type      = _Tp;
  using error_type      = _Err;
  using unexpected_type = unexpected<_Err>;

  template <class _Up>
  using rebind = expected<_Up, error_type>;

  // [expected.object.ctor], constructors
  _LIBCPP_HIDE_FROM_ABI constexpr expected()
    noexcept(is_nothrow_default_constructible_v<_Tp>) // strengthened
    requires is_default_constructible_v<_Tp>
      : __union_(std::in_place), __has_val_(true) {}

  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&) = delete;

  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&)
    requires(is_copy_constructible_v<_Tp> &&
             is_copy_constructible_v<_Err> &&
             is_trivially_copy_constructible_v<_Tp> &&
             is_trivially_copy_constructible_v<_Err>)
  = default;

  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected& __other)
    noexcept(is_nothrow_copy_constructible_v<_Tp> && is_nothrow_copy_constructible_v<_Err>) // strengthened
    requires(is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Err> &&
             !(is_trivially_copy_constructible_v<_Tp> && is_trivially_copy_constructible_v<_Err>))
      : __union_(__other.__has_val_, __other.__union_), __has_val_(__other.__has_val_) { }

  _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&&)
    requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err>
              && is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>)
  = default;

  _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&& __other)
    noexcept(is_nothrow_move_constructible_v<_Tp> && is_nothrow_move_constructible_v<_Err>)
    requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err> &&
             !(is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>))
      : __union_(__other.__has_val_, std::move(__other.__union_)), __has_val_(__other.__has_val_) { }

private:
  template <class _Up, class _OtherErr, class _UfQual, class _OtherErrQual>
  using __can_convert =
      _And< is_constructible<_Tp, _UfQual>,
            is_constructible<_Err, _OtherErrQual>,
            _Not<is_constructible<_Tp, expected<_Up, _OtherErr>&>>,
            _Not<is_constructible<_Tp, expected<_Up, _OtherErr>>>,
            _Not<is_constructible<_Tp, const expected<_Up, _OtherErr>&>>,
            _Not<is_constructible<_Tp, const expected<_Up, _OtherErr>>>,
            _Not<is_convertible<expected<_Up, _OtherErr>&, _Tp>>,
            _Not<is_convertible<expected<_Up, _OtherErr>&&, _Tp>>,
            _Not<is_convertible<const expected<_Up, _OtherErr>&, _Tp>>,
            _Not<is_convertible<const expected<_Up, _OtherErr>&&, _Tp>>,
            _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>&>>,
            _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>>>,
            _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>&>>,
            _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>>> >;

  template <class _Func, class... _Args>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(
      std::__expected_construct_in_place_from_invoke_tag __tag, _Func&& __f, _Args&&... __args)
      : __union_(__tag, std::forward<_Func>(__f), std::forward<_Args>(__args)...), __has_val_(true) {}

  template <class _Func, class... _Args>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(
      std::__expected_construct_unexpected_from_invoke_tag __tag, _Func&& __f, _Args&&... __args)
      : __union_(__tag, std::forward<_Func>(__f), std::forward<_Args>(__args)...), __has_val_(false) {}

public:
  template <class _Up, class _OtherErr>
    requires __can_convert<_Up, _OtherErr, const _Up&, const _OtherErr&>::value
  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _Up&, _Tp> ||
                                           !is_convertible_v<const _OtherErr&, _Err>)
  expected(const expected<_Up, _OtherErr>& __other)
    noexcept(is_nothrow_constructible_v<_Tp, const _Up&> &&
             is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
      : __union_(__other.__has_val_, __other.__union_), __has_val_(__other.__has_val_) {}

  template <class _Up, class _OtherErr>
    requires __can_convert<_Up, _OtherErr, _Up, _OtherErr>::value
  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp> || !is_convertible_v<_OtherErr, _Err>)
  expected(expected<_Up, _OtherErr>&& __other)
    noexcept(is_nothrow_constructible_v<_Tp, _Up> && is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
      : __union_(__other.__has_val_, std::move(__other.__union_)), __has_val_(__other.__has_val_) {}

  template <class _Up = _Tp>
    requires(!is_same_v<remove_cvref_t<_Up>, in_place_t> && !is_same_v<expected, remove_cvref_t<_Up>> &&
             !__is_std_unexpected<remove_cvref_t<_Up>>::value && is_constructible_v<_Tp, _Up>)
  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp>)
      expected(_Up&& __u) noexcept(is_nothrow_constructible_v<_Tp, _Up>) // strengthened
      : __union_(std::in_place, std::forward<_Up>(__u)), __has_val_(true) {}


  template <class _OtherErr>
    requires is_constructible_v<_Err, const _OtherErr&>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
  expected(const unexpected<_OtherErr>& __unex)
    noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
      : __union_(std::unexpect, __unex.error()), __has_val_(false) {}

  template <class _OtherErr>
    requires is_constructible_v<_Err, _OtherErr>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
  expected(unexpected<_OtherErr>&& __unex)
    noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
      : __union_(std::unexpect, std::move(__unex.error())), __has_val_(false) {}

  template <class... _Args>
    requires is_constructible_v<_Tp, _Args...>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t, _Args&&... __args)
    noexcept(is_nothrow_constructible_v<_Tp, _Args...>) // strengthened
      : __union_(std::in_place, std::forward<_Args>(__args)...), __has_val_(true) {}

  template <class _Up, class... _Args>
    requires is_constructible_v< _Tp, initializer_list<_Up>&, _Args... >
  _LIBCPP_HIDE_FROM_ABI constexpr explicit
  expected(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
    noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _Args...>) // strengthened
      : __union_(std::in_place, __il, std::forward<_Args>(__args)...), __has_val_(true) {}

  template <class... _Args>
    requires is_constructible_v<_Err, _Args...>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, _Args&&... __args)
    noexcept(is_nothrow_constructible_v<_Err, _Args...>) // strengthened
      : __union_(std::unexpect, std::forward<_Args>(__args)...), __has_val_(false) {}

  template <class _Up, class... _Args>
    requires is_constructible_v< _Err, initializer_list<_Up>&, _Args... >
  _LIBCPP_HIDE_FROM_ABI constexpr explicit
  expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args)
    noexcept(is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
      : __union_(std::unexpect, __il, std::forward<_Args>(__args)...), __has_val_(false) {}

  // [expected.object.dtor], destructor

  _LIBCPP_HIDE_FROM_ABI constexpr ~expected()
    requires(is_trivially_destructible_v<_Tp> && is_trivially_destructible_v<_Err>)
  = default;

  _LIBCPP_HIDE_FROM_ABI constexpr ~expected()
    requires(!is_trivially_destructible_v<_Tp> || !is_trivially_destructible_v<_Err>)
  {
    if (__has_val_) {
      std::destroy_at(std::addressof(__union_.__val_));
    } else {
      std::destroy_at(std::addressof(__union_.__unex_));
    }
  }

private:
  template <class _T1, class _T2, class... _Args>
  _LIBCPP_HIDE_FROM_ABI static constexpr void __reinit_expected(_T1& __newval, _T2& __oldval, _Args&&... __args) {
    if constexpr (is_nothrow_constructible_v<_T1, _Args...>) {
      std::destroy_at(std::addressof(__oldval));
      std::construct_at(std::addressof(__newval), std::forward<_Args>(__args)...);
    } else if constexpr (is_nothrow_move_constructible_v<_T1>) {
      _T1 __tmp(std::forward<_Args>(__args)...);
      std::destroy_at(std::addressof(__oldval));
      std::construct_at(std::addressof(__newval), std::move(__tmp));
    } else {
      static_assert(
          is_nothrow_move_constructible_v<_T2>,
          "To provide strong exception guarantee, T2 has to satisfy `is_nothrow_move_constructible_v` so that it can "
          "be reverted to the previous state in case an exception is thrown during the assignment.");
      _T2 __tmp(std::move(__oldval));
      std::destroy_at(std::addressof(__oldval));
      auto __trans =
          std::__make_exception_guard([&] { std::construct_at(std::addressof(__oldval), std::move(__tmp)); });
      std::construct_at(std::addressof(__newval), std::forward<_Args>(__args)...);
      __trans.__complete();
    }
  }

public:
  // [expected.object.assign], assignment
  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected&) = delete;

  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected& __rhs)
    noexcept(is_nothrow_copy_assignable_v<_Tp> &&
             is_nothrow_copy_constructible_v<_Tp> &&
             is_nothrow_copy_assignable_v<_Err> &&
             is_nothrow_copy_constructible_v<_Err>) // strengthened
    requires(is_copy_assignable_v<_Tp> &&
             is_copy_constructible_v<_Tp> &&
             is_copy_assignable_v<_Err> &&
             is_copy_constructible_v<_Err> &&
             (is_nothrow_move_constructible_v<_Tp> ||
              is_nothrow_move_constructible_v<_Err>))
  {
    if (__has_val_ && __rhs.__has_val_) {
      __union_.__val_ = __rhs.__union_.__val_;
    } else if (__has_val_) {
      __reinit_expected(__union_.__unex_, __union_.__val_, __rhs.__union_.__unex_);
    } else if (__rhs.__has_val_) {
      __reinit_expected(__union_.__val_, __union_.__unex_, __rhs.__union_.__val_);
    } else {
      __union_.__unex_ = __rhs.__union_.__unex_;
    }
    // note: only reached if no exception+rollback was done inside __reinit_expected
    __has_val_ = __rhs.__has_val_;
    return *this;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(expected&& __rhs)
    noexcept(is_nothrow_move_assignable_v<_Tp> &&
             is_nothrow_move_constructible_v<_Tp> &&
             is_nothrow_move_assignable_v<_Err> &&
             is_nothrow_move_constructible_v<_Err>)
    requires(is_move_constructible_v<_Tp> &&
             is_move_assignable_v<_Tp> &&
             is_move_constructible_v<_Err> &&
             is_move_assignable_v<_Err> &&
             (is_nothrow_move_constructible_v<_Tp> ||
              is_nothrow_move_constructible_v<_Err>))
  {
    if (__has_val_ && __rhs.__has_val_) {
      __union_.__val_ = std::move(__rhs.__union_.__val_);
    } else if (__has_val_) {
      __reinit_expected(__union_.__unex_, __union_.__val_, std::move(__rhs.__union_.__unex_));
    } else if (__rhs.__has_val_) {
      __reinit_expected(__union_.__val_, __union_.__unex_, std::move(__rhs.__union_.__val_));
    } else {
      __union_.__unex_ = std::move(__rhs.__union_.__unex_);
    }
    // note: only reached if no exception+rollback was done inside __reinit_expected
    __has_val_ = __rhs.__has_val_;
    return *this;
  }

  template <class _Up = _Tp>
  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(_Up&& __v)
    requires(!is_same_v<expected, remove_cvref_t<_Up>> &&
             !__is_std_unexpected<remove_cvref_t<_Up>>::value &&
             is_constructible_v<_Tp, _Up> &&
             is_assignable_v<_Tp&, _Up> &&
             (is_nothrow_constructible_v<_Tp, _Up> ||
              is_nothrow_move_constructible_v<_Tp> ||
              is_nothrow_move_constructible_v<_Err>))
  {
    if (__has_val_) {
      __union_.__val_ = std::forward<_Up>(__v);
    } else {
      __reinit_expected(__union_.__val_, __union_.__unex_, std::forward<_Up>(__v));
      __has_val_ = true;
    }
    return *this;
  }

private:
  template <class _OtherErrQual>
  static constexpr bool __can_assign_from_unexpected =
      _And< is_constructible<_Err, _OtherErrQual>,
            is_assignable<_Err&, _OtherErrQual>,
            _Lazy<_Or,
                  is_nothrow_constructible<_Err, _OtherErrQual>,
                  is_nothrow_move_constructible<_Tp>,
                  is_nothrow_move_constructible<_Err>> >::value;

public:
  template <class _OtherErr>
    requires(__can_assign_from_unexpected<const _OtherErr&>)
  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const unexpected<_OtherErr>& __un) {
    if (__has_val_) {
      __reinit_expected(__union_.__unex_, __union_.__val_, __un.error());
      __has_val_ = false;
    } else {
      __union_.__unex_ = __un.error();
    }
    return *this;
  }

  template <class _OtherErr>
    requires(__can_assign_from_unexpected<_OtherErr>)
  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(unexpected<_OtherErr>&& __un) {
    if (__has_val_) {
      __reinit_expected(__union_.__unex_, __union_.__val_, std::move(__un.error()));
      __has_val_ = false;
    } else {
      __union_.__unex_ = std::move(__un.error());
    }
    return *this;
  }

  template <class... _Args>
    requires is_nothrow_constructible_v<_Tp, _Args...>
  _LIBCPP_HIDE_FROM_ABI constexpr _Tp& emplace(_Args&&... __args) noexcept {
    if (__has_val_) {
      std::destroy_at(std::addressof(__union_.__val_));
    } else {
      std::destroy_at(std::addressof(__union_.__unex_));
    }
    std::construct_at(std::addressof(__union_.__val_), std::forward<_Args>(__args)...);
    __has_val_ = true;
    return __union_.__val_;
  }

  template <class _Up, class... _Args>
    requires is_nothrow_constructible_v< _Tp, initializer_list<_Up>&, _Args... >
  _LIBCPP_HIDE_FROM_ABI constexpr _Tp& emplace(initializer_list<_Up> __il, _Args&&... __args) noexcept {
    if (__has_val_) {
      std::destroy_at(std::addressof(__union_.__val_));
    } else {
      std::destroy_at(std::addressof(__union_.__unex_));
    }
    std::construct_at(std::addressof(__union_.__val_), __il, std::forward<_Args>(__args)...);
    __has_val_ = true;
    return __union_.__val_;
  }


public:
  // [expected.object.swap], swap
  _LIBCPP_HIDE_FROM_ABI constexpr void swap(expected& __rhs)
    noexcept(is_nothrow_move_constructible_v<_Tp> &&
             is_nothrow_swappable_v<_Tp> &&
             is_nothrow_move_constructible_v<_Err> &&
             is_nothrow_swappable_v<_Err>)
    requires(is_swappable_v<_Tp> &&
             is_swappable_v<_Err> &&
             is_move_constructible_v<_Tp> &&
             is_move_constructible_v<_Err> &&
             (is_nothrow_move_constructible_v<_Tp> ||
              is_nothrow_move_constructible_v<_Err>))
  {
    auto __swap_val_unex_impl = [&](expected& __with_val, expected& __with_err) {
      if constexpr (is_nothrow_move_constructible_v<_Err>) {
        _Err __tmp(std::move(__with_err.__union_.__unex_));
        std::destroy_at(std::addressof(__with_err.__union_.__unex_));
        auto __trans = std::__make_exception_guard([&] {
          std::construct_at(std::addressof(__with_err.__union_.__unex_), std::move(__tmp));
        });
        std::construct_at(std::addressof(__with_err.__union_.__val_), std::move(__with_val.__union_.__val_));
        __trans.__complete();
        std::destroy_at(std::addressof(__with_val.__union_.__val_));
        std::construct_at(std::addressof(__with_val.__union_.__unex_), std::move(__tmp));
      } else {
        static_assert(is_nothrow_move_constructible_v<_Tp>,
                      "To provide strong exception guarantee, Tp has to satisfy `is_nothrow_move_constructible_v` so "
                      "that it can be reverted to the previous state in case an exception is thrown during swap.");
        _Tp __tmp(std::move(__with_val.__union_.__val_));
        std::destroy_at(std::addressof(__with_val.__union_.__val_));
        auto __trans = std::__make_exception_guard([&] {
          std::construct_at(std::addressof(__with_val.__union_.__val_), std::move(__tmp));
        });
        std::construct_at(std::addressof(__with_val.__union_.__unex_), std::move(__with_err.__union_.__unex_));
        __trans.__complete();
        std::destroy_at(std::addressof(__with_err.__union_.__unex_));
        std::construct_at(std::addressof(__with_err.__union_.__val_), std::move(__tmp));
      }
      __with_val.__has_val_ = false;
      __with_err.__has_val_ = true;
    };

    if (__has_val_) {
      if (__rhs.__has_val_) {
        using std::swap;
        swap(__union_.__val_, __rhs.__union_.__val_);
      } else {
        __swap_val_unex_impl(*this, __rhs);
      }
    } else {
      if (__rhs.__has_val_) {
        __swap_val_unex_impl(__rhs, *this);
      } else {
        using std::swap;
        swap(__union_.__unex_, __rhs.__union_.__unex_);
      }
    }
  }

  _LIBCPP_HIDE_FROM_ABI friend constexpr void swap(expected& __x, expected& __y)
    noexcept(noexcept(__x.swap(__y)))
    requires requires { __x.swap(__y); }
  {
    __x.swap(__y);
  }

  // [expected.object.obs], observers
  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* operator->() const noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator-> requires the expected to contain a value");
    return std::addressof(__union_.__val_);
  }

  _LIBCPP_HIDE_FROM_ABI constexpr _Tp* operator->() noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator-> requires the expected to contain a value");
    return std::addressof(__union_.__val_);
  }

  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& operator*() const& noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator* requires the expected to contain a value");
    return __union_.__val_;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() & noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator* requires the expected to contain a value");
    return __union_.__val_;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& operator*() const&& noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator* requires the expected to contain a value");
    return std::move(__union_.__val_);
  }

  _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator*() && noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator* requires the expected to contain a value");
    return std::move(__union_.__val_);
  }

  _LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return __has_val_; }

  _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return __has_val_; }

  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& value() const& {
    static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
    if (!__has_val_) {
      std::__throw_bad_expected_access<_Err>(std::as_const(error()));
    }
    return __union_.__val_;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr _Tp& value() & {
    static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
    if (!__has_val_) {
      std::__throw_bad_expected_access<_Err>(std::as_const(error()));
    }
    return __union_.__val_;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& value() const&& {
    static_assert(is_copy_constructible_v<_Err> && is_constructible_v<_Err, decltype(std::move(error()))>,
                  "error_type has to be both copy constructible and constructible from decltype(std::move(error()))");
    if (!__has_val_) {
      std::__throw_bad_expected_access<_Err>(std::move(error()));
    }
    return std::move(__union_.__val_);
  }

  _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& value() && {
    static_assert(is_copy_constructible_v<_Err> && is_constructible_v<_Err, decltype(std::move(error()))>,
                  "error_type has to be both copy constructible and constructible from decltype(std::move(error()))");
    if (!__has_val_) {
      std::__throw_bad_expected_access<_Err>(std::move(error()));
    }
    return std::move(__union_.__val_);
  }

  _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
    return __union_.__unex_;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
    return __union_.__unex_;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
    return std::move(__union_.__unex_);
  }

  _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
    return std::move(__union_.__unex_);
  }

  template <class _Up>
  _LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) const& {
    static_assert(is_copy_constructible_v<_Tp>, "value_type has to be copy constructible");
    static_assert(is_convertible_v<_Up, _Tp>, "argument has to be convertible to value_type");
    return __has_val_ ? __union_.__val_ : static_cast<_Tp>(std::forward<_Up>(__v));
  }

  template <class _Up>
  _LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) && {
    static_assert(is_move_constructible_v<_Tp>, "value_type has to be move constructible");
    static_assert(is_convertible_v<_Up, _Tp>, "argument has to be convertible to value_type");
    return __has_val_ ? std::move(__union_.__val_) : static_cast<_Tp>(std::forward<_Up>(__v));
  }

  template <class _Up = _Err>
  _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) const& {
    static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
    static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
    if (has_value())
      return std::forward<_Up>(__error);
    return error();
  }

  template <class _Up = _Err>
  _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) && {
    static_assert(is_move_constructible_v<_Err>, "error_type has to be move constructible");
    static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
    if (has_value())
      return std::forward<_Up>(__error);
    return std::move(error());
  }

  // [expected.void.monadic], monadic
  template <class _Func>
    requires is_constructible_v<_Err, _Err&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) & {
    using _Up = remove_cvref_t<invoke_result_t<_Func, _Tp&>>;
    static_assert(__is_std_expected<_Up>::value, "The result of f(value()) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Up::error_type, _Err>,
                  "The result of f(value()) must have the same error_type as this expected");
    if (has_value()) {
      return std::invoke(std::forward<_Func>(__f), value());
    }
    return _Up(unexpect, error());
  }

  template <class _Func>
    requires is_constructible_v<_Err, const _Err&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const& {
    using _Up = remove_cvref_t<invoke_result_t<_Func, const _Tp&>>;
    static_assert(__is_std_expected<_Up>::value, "The result of f(value()) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Up::error_type, _Err>,
                  "The result of f(value()) must have the same error_type as this expected");
    if (has_value()) {
      return std::invoke(std::forward<_Func>(__f), value());
    }
    return _Up(unexpect, error());
  }

  template <class _Func>
    requires is_constructible_v<_Err, _Err&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) && {
    using _Up = remove_cvref_t<invoke_result_t<_Func, _Tp&&>>;
    static_assert(
        __is_std_expected<_Up>::value, "The result of f(std::move(value())) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Up::error_type, _Err>,
                  "The result of f(std::move(value())) must have the same error_type as this expected");
    if (has_value()) {
      return std::invoke(std::forward<_Func>(__f), std::move(value()));
    }
    return _Up(unexpect, std::move(error()));
  }

  template <class _Func>
    requires is_constructible_v<_Err, const _Err&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const&& {
    using _Up = remove_cvref_t<invoke_result_t<_Func, const _Tp&&>>;
    static_assert(
        __is_std_expected<_Up>::value, "The result of f(std::move(value())) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Up::error_type, _Err>,
                  "The result of f(std::move(value())) must have the same error_type as this expected");
    if (has_value()) {
      return std::invoke(std::forward<_Func>(__f), std::move(value()));
    }
    return _Up(unexpect, std::move(error()));
  }

  template <class _Func>
    requires is_constructible_v<_Tp, _Tp&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) & {
    using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&>>;
    static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
                  "The result of f(error()) must have the same value_type as this expected");
    if (has_value()) {
      return _Gp(in_place, value());
    }
    return std::invoke(std::forward<_Func>(__f), error());
  }

  template <class _Func>
    requires is_constructible_v<_Tp, const _Tp&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const& {
    using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&>>;
    static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
                  "The result of f(error()) must have the same value_type as this expected");
    if (has_value()) {
      return _Gp(in_place, value());
    }
    return std::invoke(std::forward<_Func>(__f), error());
  }

  template <class _Func>
    requires is_constructible_v<_Tp, _Tp&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) && {
    using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&&>>;
    static_assert(
        __is_std_expected<_Gp>::value, "The result of f(std::move(error())) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
                  "The result of f(std::move(error())) must have the same value_type as this expected");
    if (has_value()) {
      return _Gp(in_place, std::move(value()));
    }
    return std::invoke(std::forward<_Func>(__f), std::move(error()));
  }

  template <class _Func>
    requires is_constructible_v<_Tp, const _Tp&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const&& {
    using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&&>>;
    static_assert(
        __is_std_expected<_Gp>::value, "The result of f(std::move(error())) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
                  "The result of f(std::move(error())) must have the same value_type as this expected");
    if (has_value()) {
      return _Gp(in_place, std::move(value()));
    }
    return std::invoke(std::forward<_Func>(__f), std::move(error()));
  }

  template <class _Func>
    requires is_constructible_v<_Err, _Err&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) & {
    using _Up = remove_cv_t<invoke_result_t<_Func, _Tp&>>;
    if (!has_value()) {
      return expected<_Up, _Err>(unexpect, error());
    }
    if constexpr (!is_void_v<_Up>) {
      return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), value());
    } else {
      std::invoke(std::forward<_Func>(__f), value());
      return expected<_Up, _Err>();
    }
  }

  template <class _Func>
    requires is_constructible_v<_Err, const _Err&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const& {
    using _Up = remove_cv_t<invoke_result_t<_Func, const _Tp&>>;
    if (!has_value()) {
      return expected<_Up, _Err>(unexpect, error());
    }
    if constexpr (!is_void_v<_Up>) {
      return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), value());
    } else {
      std::invoke(std::forward<_Func>(__f), value());
      return expected<_Up, _Err>();
    }
  }

  template <class _Func>
    requires is_constructible_v<_Err, _Err&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) && {
    using _Up = remove_cv_t<invoke_result_t<_Func, _Tp&&>>;
    if (!has_value()) {
      return expected<_Up, _Err>(unexpect, std::move(error()));
    }
    if constexpr (!is_void_v<_Up>) {
      return expected<_Up, _Err>(
          __expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), std::move(value()));
    } else {
      std::invoke(std::forward<_Func>(__f), std::move(value()));
      return expected<_Up, _Err>();
    }
  }

  template <class _Func>
    requires is_constructible_v<_Err, const _Err&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const&& {
    using _Up = remove_cv_t<invoke_result_t<_Func, const _Tp&&>>;
    if (!has_value()) {
      return expected<_Up, _Err>(unexpect, std::move(error()));
    }
    if constexpr (!is_void_v<_Up>) {
      return expected<_Up, _Err>(
          __expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), std::move(value()));
    } else {
      std::invoke(std::forward<_Func>(__f), std::move(value()));
      return expected<_Up, _Err>();
    }
  }

  template <class _Func>
    requires is_constructible_v<_Tp, _Tp&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) & {
    using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&>>;
    static_assert(__valid_std_unexpected<_Gp>::value,
                  "The result of f(error()) must be a valid template argument for unexpected");
    if (has_value()) {
      return expected<_Tp, _Gp>(in_place, value());
    }
    return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
  }

  template <class _Func>
    requires is_constructible_v<_Tp, const _Tp&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const& {
    using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&>>;
    static_assert(__valid_std_unexpected<_Gp>::value,
                  "The result of f(error()) must be a valid template argument for unexpected");
    if (has_value()) {
      return expected<_Tp, _Gp>(in_place, value());
    }
    return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
  }

  template <class _Func>
    requires is_constructible_v<_Tp, _Tp&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) && {
    using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&&>>;
    static_assert(__valid_std_unexpected<_Gp>::value,
                  "The result of f(std::move(error())) must be a valid template argument for unexpected");
    if (has_value()) {
      return expected<_Tp, _Gp>(in_place, std::move(value()));
    }
    return expected<_Tp, _Gp>(
        __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
  }

  template <class _Func>
    requires is_constructible_v<_Tp, const _Tp&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const&& {
    using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&&>>;
    static_assert(__valid_std_unexpected<_Gp>::value,
                  "The result of f(std::move(error())) must be a valid template argument for unexpected");
    if (has_value()) {
      return expected<_Tp, _Gp>(in_place, std::move(value()));
    }
    return expected<_Tp, _Gp>(
        __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
  }

  // [expected.object.eq], equality operators
  template <class _T2, class _E2>
    requires(!is_void_v<_T2>)
  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
    if (__x.__has_val_ != __y.__has_val_) {
      return false;
    } else {
      if (__x.__has_val_) {
        return __x.__union_.__val_ == __y.__union_.__val_;
      } else {
        return __x.__union_.__unex_ == __y.__union_.__unex_;
      }
    }
  }

  template <class _T2>
  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const _T2& __v) {
    return __x.__has_val_ && static_cast<bool>(__x.__union_.__val_ == __v);
  }

  template <class _E2>
  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __e) {
    return !__x.__has_val_ && static_cast<bool>(__x.__union_.__unex_ == __e.error());
  }

private:
  template <class _ValueType, class _ErrorType>
  union __union_t {
    template <class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::in_place_t, _Args&&... __args)
        : __val_(std::forward<_Args>(__args)...) {}

    template <class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
        : __unex_(std::forward<_Args>(__args)...) {}

    template <class _Func, class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
        std::__expected_construct_in_place_from_invoke_tag, _Func&& __f, _Args&&... __args)
        : __val_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}

    template <class _Func, class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
        std::__expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
        : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}

    template <class _Union>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
      if (__has_val)
        std::construct_at(std::addressof(__val_), std::forward<_Union>(__other).__val_);
      else
        std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
    }

    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
      requires(is_trivially_destructible_v<_ValueType> && is_trivially_destructible_v<_ErrorType>)
    = default;

    // the expected's destructor handles this
    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t() {}

    _ValueType __val_;
    _ErrorType __unex_;
  };

  // use named union because [[no_unique_address]] cannot be applied to an unnamed union,
  // also guaranteed elision into a potentially-overlapping subobject is unsettled (and
  // it's not clear that it's implementable, given that the function is allowed to clobber
  // the tail padding) - see https://github.com/itanium-cxx-abi/cxx-abi/issues/107.
  template <class _ValueType, class _ErrorType>
    requires(is_trivially_move_constructible_v<_ValueType> && is_trivially_move_constructible_v<_ErrorType>)
  union __union_t<_ValueType, _ErrorType> {
    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&) = default;
    _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(const __union_t&) = default;

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

    template <class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
        : __unex_(std::forward<_Args>(__args)...) {}

    template <class _Func, class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
        std::__expected_construct_in_place_from_invoke_tag, _Func&& __f, _Args&&... __args)
        : __val_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}

    template <class _Func, class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
        std::__expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
        : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}

    template <class _Union>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
      if (__has_val)
        std::construct_at(std::addressof(__val_), std::forward<_Union>(__other).__val_);
      else
        std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
    }

    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
      requires(is_trivially_destructible_v<_ValueType> && is_trivially_destructible_v<_ErrorType>)
    = default;

    // the expected's destructor handles this
    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
      requires(!is_trivially_destructible_v<_ValueType> || !is_trivially_destructible_v<_ErrorType>)
    {}

    _LIBCPP_NO_UNIQUE_ADDRESS _ValueType __val_;
    _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
  };

  _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
  bool __has_val_;
};

template <class _Tp, class _Err>
  requires is_void_v<_Tp>
class expected<_Tp, _Err> {
  static_assert(__valid_std_unexpected<_Err>::value,
                "[expected.void.general] A program that instantiates expected<T, E> with a E that is not a "
                "valid argument for unexpected<E> is ill-formed");

  template <class, class>
  friend class expected;

  template <class _Up, class _OtherErr, class _OtherErrQual>
  using __can_convert =
      _And< is_void<_Up>,
            is_constructible<_Err, _OtherErrQual>,
            _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>&>>,
            _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>>>,
            _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>&>>,
            _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>>>>;

public:
  using value_type      = _Tp;
  using error_type      = _Err;
  using unexpected_type = unexpected<_Err>;

  template <class _Up>
  using rebind = expected<_Up, error_type>;

  // [expected.void.ctor], constructors
  _LIBCPP_HIDE_FROM_ABI constexpr expected() noexcept : __has_val_(true) {}

  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&) = delete;

  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&)
    requires(is_copy_constructible_v<_Err> && is_trivially_copy_constructible_v<_Err>)
  = default;

  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected& __rhs)
    noexcept(is_nothrow_copy_constructible_v<_Err>) // strengthened
    requires(is_copy_constructible_v<_Err> && !is_trivially_copy_constructible_v<_Err>)
      : __union_(__rhs.__has_val_, __rhs.__union_), __has_val_(__rhs.__has_val_) {}

  _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&&)
    requires(is_move_constructible_v<_Err> && is_trivially_move_constructible_v<_Err>)
  = default;

  _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&& __rhs)
    noexcept(is_nothrow_move_constructible_v<_Err>)
    requires(is_move_constructible_v<_Err> && !is_trivially_move_constructible_v<_Err>)
      : __union_(__rhs.__has_val_, std::move(__rhs.__union_)), __has_val_(__rhs.__has_val_) {}

  template <class _Up, class _OtherErr>
    requires __can_convert<_Up, _OtherErr, const _OtherErr&>::value
  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
  expected(const expected<_Up, _OtherErr>& __rhs)
    noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
      : __union_(__rhs.__has_val_, __rhs.__union_), __has_val_(__rhs.__has_val_) {}

  template <class _Up, class _OtherErr>
    requires __can_convert<_Up, _OtherErr, _OtherErr>::value
  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
  expected(expected<_Up, _OtherErr>&& __rhs)
    noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
      : __union_(__rhs.__has_val_, std::move(__rhs.__union_)), __has_val_(__rhs.__has_val_) {}

  template <class _OtherErr>
    requires is_constructible_v<_Err, const _OtherErr&>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
  expected(const unexpected<_OtherErr>& __unex)
    noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
      : __union_(std::unexpect, __unex.error()), __has_val_(false) {}

  template <class _OtherErr>
    requires is_constructible_v<_Err, _OtherErr>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
  expected(unexpected<_OtherErr>&& __unex)
    noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
      : __union_(std::unexpect, std::move(__unex.error())), __has_val_(false) {}

  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t) noexcept : __has_val_(true) {}

  template <class... _Args>
    requires is_constructible_v<_Err, _Args...>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, _Args&&... __args)
    noexcept(is_nothrow_constructible_v<_Err, _Args...>) // strengthened
      : __union_(std::unexpect, std::forward<_Args>(__args)...), __has_val_(false) {}

  template <class _Up, class... _Args>
    requires is_constructible_v< _Err, initializer_list<_Up>&, _Args... >
  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args)
    noexcept(is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
      : __union_(std::unexpect, __il, std::forward<_Args>(__args)...), __has_val_(false) {}

private:
  template <class _Func>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(__expected_construct_in_place_from_invoke_tag, _Func&& __f)
      : __has_val_(true) {
    std::invoke(std::forward<_Func>(__f));
  }

  template <class _Func, class... _Args>
  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(
      __expected_construct_unexpected_from_invoke_tag __tag, _Func&& __f, _Args&&... __args)
      : __union_(__tag, std::forward<_Func>(__f), std::forward<_Args>(__args)...), __has_val_(false) {}

public:
  // [expected.void.dtor], destructor

  _LIBCPP_HIDE_FROM_ABI constexpr ~expected()
    requires is_trivially_destructible_v<_Err>
  = default;

  _LIBCPP_HIDE_FROM_ABI constexpr ~expected()
    requires(!is_trivially_destructible_v<_Err>)
  {
    if (!__has_val_) {
      std::destroy_at(std::addressof(__union_.__unex_));
    }
  }

  // [expected.void.assign], assignment

  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected&) = delete;

  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected& __rhs)
    noexcept(is_nothrow_copy_assignable_v<_Err> && is_nothrow_copy_constructible_v<_Err>) // strengthened
    requires(is_copy_assignable_v<_Err> && is_copy_constructible_v<_Err>)
  {
    if (__has_val_) {
      if (!__rhs.__has_val_) {
        std::construct_at(std::addressof(__union_.__unex_), __rhs.__union_.__unex_);
        __has_val_ = false;
      }
    } else {
      if (__rhs.__has_val_) {
        std::destroy_at(std::addressof(__union_.__unex_));
        __has_val_ = true;
      } else {
        __union_.__unex_ = __rhs.__union_.__unex_;
      }
    }
    return *this;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(expected&&) = delete;

  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(expected&& __rhs)
    noexcept(is_nothrow_move_assignable_v<_Err> &&
             is_nothrow_move_constructible_v<_Err>)
    requires(is_move_assignable_v<_Err> &&
             is_move_constructible_v<_Err>)
  {
    if (__has_val_) {
      if (!__rhs.__has_val_) {
        std::construct_at(std::addressof(__union_.__unex_), std::move(__rhs.__union_.__unex_));
        __has_val_ = false;
      }
    } else {
      if (__rhs.__has_val_) {
        std::destroy_at(std::addressof(__union_.__unex_));
        __has_val_ = true;
      } else {
        __union_.__unex_ = std::move(__rhs.__union_.__unex_);
      }
    }
    return *this;
  }

  template <class _OtherErr>
    requires(is_constructible_v<_Err, const _OtherErr&> && is_assignable_v<_Err&, const _OtherErr&>)
  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const unexpected<_OtherErr>& __un) {
    if (__has_val_) {
      std::construct_at(std::addressof(__union_.__unex_), __un.error());
      __has_val_ = false;
    } else {
      __union_.__unex_ = __un.error();
    }
    return *this;
  }

  template <class _OtherErr>
    requires(is_constructible_v<_Err, _OtherErr> && is_assignable_v<_Err&, _OtherErr>)
  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(unexpected<_OtherErr>&& __un) {
    if (__has_val_) {
      std::construct_at(std::addressof(__union_.__unex_), std::move(__un.error()));
      __has_val_ = false;
    } else {
      __union_.__unex_ = std::move(__un.error());
    }
    return *this;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr void emplace() noexcept {
    if (!__has_val_) {
      std::destroy_at(std::addressof(__union_.__unex_));
      __has_val_ = true;
    }
  }

  // [expected.void.swap], swap
  _LIBCPP_HIDE_FROM_ABI constexpr void swap(expected& __rhs)
    noexcept(is_nothrow_move_constructible_v<_Err> && is_nothrow_swappable_v<_Err>)
    requires(is_swappable_v<_Err> && is_move_constructible_v<_Err>)
  {
    auto __swap_val_unex_impl = [&](expected& __with_val, expected& __with_err) {
      std::construct_at(std::addressof(__with_val.__union_.__unex_), std::move(__with_err.__union_.__unex_));
      std::destroy_at(std::addressof(__with_err.__union_.__unex_));
      __with_val.__has_val_ = false;
      __with_err.__has_val_ = true;
    };

    if (__has_val_) {
      if (!__rhs.__has_val_) {
        __swap_val_unex_impl(*this, __rhs);
      }
    } else {
      if (__rhs.__has_val_) {
        __swap_val_unex_impl(__rhs, *this);
      } else {
        using std::swap;
        swap(__union_.__unex_, __rhs.__union_.__unex_);
      }
    }
  }

  _LIBCPP_HIDE_FROM_ABI friend constexpr void swap(expected& __x, expected& __y)
    noexcept(noexcept(__x.swap(__y)))
    requires requires { __x.swap(__y); }
  {
    __x.swap(__y);
  }

  // [expected.void.obs], observers
  _LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return __has_val_; }

  _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return __has_val_; }

  _LIBCPP_HIDE_FROM_ABI constexpr void operator*() const noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator* requires the expected to contain a value");
  }

  _LIBCPP_HIDE_FROM_ABI constexpr void value() const& {
    if (!__has_val_) {
      std::__throw_bad_expected_access<_Err>(__union_.__unex_);
    }
  }

  _LIBCPP_HIDE_FROM_ABI constexpr void value() && {
    if (!__has_val_) {
      std::__throw_bad_expected_access<_Err>(std::move(__union_.__unex_));
    }
  }

  _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
    return __union_.__unex_;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
    return __union_.__unex_;
  }

  _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
    return std::move(__union_.__unex_);
  }

  _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept {
    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
    return std::move(__union_.__unex_);
  }

  template <class _Up = _Err>
  _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) const& {
    static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
    static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
    if (has_value()) {
      return std::forward<_Up>(__error);
    }
    return error();
  }

  template <class _Up = _Err>
  _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) && {
    static_assert(is_move_constructible_v<_Err>, "error_type has to be move constructible");
    static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
    if (has_value()) {
      return std::forward<_Up>(__error);
    }
    return std::move(error());
  }

  // [expected.void.monadic], monadic
  template <class _Func>
    requires is_constructible_v<_Err, _Err&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) & {
    using _Up = remove_cvref_t<invoke_result_t<_Func>>;
    static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
    static_assert(
        is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
    if (has_value()) {
      return std::invoke(std::forward<_Func>(__f));
    }
    return _Up(unexpect, error());
  }

  template <class _Func>
    requires is_constructible_v<_Err, const _Err&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const& {
    using _Up = remove_cvref_t<invoke_result_t<_Func>>;
    static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
    static_assert(
        is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
    if (has_value()) {
      return std::invoke(std::forward<_Func>(__f));
    }
    return _Up(unexpect, error());
  }

  template <class _Func>
    requires is_constructible_v<_Err, _Err&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) && {
    using _Up = remove_cvref_t<invoke_result_t<_Func>>;
    static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
    static_assert(
        is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
    if (has_value()) {
      return std::invoke(std::forward<_Func>(__f));
    }
    return _Up(unexpect, std::move(error()));
  }

  template <class _Func>
    requires is_constructible_v<_Err, const _Err&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const&& {
    using _Up = remove_cvref_t<invoke_result_t<_Func>>;
    static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
    static_assert(
        is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
    if (has_value()) {
      return std::invoke(std::forward<_Func>(__f));
    }
    return _Up(unexpect, std::move(error()));
  }

  template <class _Func>
  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) & {
    using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&>>;
    static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
                  "The result of f(error()) must have the same value_type as this expected");
    if (has_value()) {
      return _Gp();
    }
    return std::invoke(std::forward<_Func>(__f), error());
  }

  template <class _Func>
  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const& {
    using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&>>;
    static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
                  "The result of f(error()) must have the same value_type as this expected");
    if (has_value()) {
      return _Gp();
    }
    return std::invoke(std::forward<_Func>(__f), error());
  }

  template <class _Func>
  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) && {
    using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&&>>;
    static_assert(__is_std_expected<_Gp>::value,
                  "The result of f(std::move(error())) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
                  "The result of f(std::move(error())) must have the same value_type as this expected");
    if (has_value()) {
      return _Gp();
    }
    return std::invoke(std::forward<_Func>(__f), std::move(error()));
  }

  template <class _Func>
  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const&& {
    using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&&>>;
    static_assert(__is_std_expected<_Gp>::value,
                  "The result of f(std::move(error())) must be a specialization of std::expected");
    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
                  "The result of f(std::move(error())) must have the same value_type as this expected");
    if (has_value()) {
      return _Gp();
    }
    return std::invoke(std::forward<_Func>(__f), std::move(error()));
  }

  template <class _Func>
    requires is_constructible_v<_Err, _Err&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) & {
    using _Up = remove_cv_t<invoke_result_t<_Func>>;
    if (!has_value()) {
      return expected<_Up, _Err>(unexpect, error());
    }
    if constexpr (!is_void_v<_Up>) {
      return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
    } else {
      std::invoke(std::forward<_Func>(__f));
      return expected<_Up, _Err>();
    }
  }

  template <class _Func>
    requires is_constructible_v<_Err, const _Err&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const& {
    using _Up = remove_cv_t<invoke_result_t<_Func>>;
    if (!has_value()) {
      return expected<_Up, _Err>(unexpect, error());
    }
    if constexpr (!is_void_v<_Up>) {
      return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
    } else {
      std::invoke(std::forward<_Func>(__f));
      return expected<_Up, _Err>();
    }
  }

  template <class _Func>
    requires is_constructible_v<_Err, _Err&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) && {
    using _Up = remove_cv_t<invoke_result_t<_Func>>;
    if (!has_value()) {
      return expected<_Up, _Err>(unexpect, std::move(error()));
    }
    if constexpr (!is_void_v<_Up>) {
      return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
    } else {
      std::invoke(std::forward<_Func>(__f));
      return expected<_Up, _Err>();
    }
  }

  template <class _Func>
    requires is_constructible_v<_Err, const _Err&&>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const&& {
    using _Up = remove_cv_t<invoke_result_t<_Func>>;
    if (!has_value()) {
      return expected<_Up, _Err>(unexpect, std::move(error()));
    }
    if constexpr (!is_void_v<_Up>) {
      return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
    } else {
      std::invoke(std::forward<_Func>(__f));
      return expected<_Up, _Err>();
    }
  }

  template <class _Func>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) & {
    using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&>>;
    static_assert(__valid_std_unexpected<_Gp>::value,
                  "The result of f(error()) must be a valid template argument for unexpected");
    if (has_value()) {
      return expected<_Tp, _Gp>();
    }
    return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
  }

  template <class _Func>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const& {
    using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&>>;
    static_assert(__valid_std_unexpected<_Gp>::value,
                  "The result of f(error()) must be a valid template argument for unexpected");
    if (has_value()) {
      return expected<_Tp, _Gp>();
    }
    return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
  }

  template <class _Func>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) && {
    using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&&>>;
    static_assert(__valid_std_unexpected<_Gp>::value,
                  "The result of f(std::move(error())) must be a valid template argument for unexpected");
    if (has_value()) {
      return expected<_Tp, _Gp>();
    }
    return expected<_Tp, _Gp>(
        __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
  }

  template <class _Func>
  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const&& {
    using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&&>>;
    static_assert(__valid_std_unexpected<_Gp>::value,
                  "The result of f(std::move(error())) must be a valid template argument for unexpected");
    if (has_value()) {
      return expected<_Tp, _Gp>();
    }
    return expected<_Tp, _Gp>(
        __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
  }

  // [expected.void.eq], equality operators
  template <class _T2, class _E2>
    requires is_void_v<_T2>
  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
    if (__x.__has_val_ != __y.__has_val_) {
      return false;
    } else {
      return __x.__has_val_ || static_cast<bool>(__x.__union_.__unex_ == __y.__union_.__unex_);
    }
  }

  template <class _E2>
  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __y) {
    return !__x.__has_val_ && static_cast<bool>(__x.__union_.__unex_ == __y.error());
  }

private:
  struct __empty_t {};

  template <class _ErrorType>
  union __union_t {
    _LIBCPP_HIDE_FROM_ABI constexpr __union_t() : __empty_() {}

    template <class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
        : __unex_(std::forward<_Args>(__args)...) {}

    template <class _Func, class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
        __expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
        : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}

    template <class _Union>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
      if (__has_val)
        std::construct_at(std::addressof(__empty_));
      else
        std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
    }

    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
      requires(is_trivially_destructible_v<_ErrorType>)
    = default;

    // the expected's destructor handles this
    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t() {}

    __empty_t __empty_;
    _ErrorType __unex_;
  };

  // use named union because [[no_unique_address]] cannot be applied to an unnamed union,
  // also guaranteed elision into a potentially-overlapping subobject is unsettled (and
  // it's not clear that it's implementable, given that the function is allowed to clobber
  // the tail padding) - see https://github.com/itanium-cxx-abi/cxx-abi/issues/107.
  template <class _ErrorType>
    requires is_trivially_move_constructible_v<_ErrorType>
  union __union_t<_ErrorType> {
    _LIBCPP_HIDE_FROM_ABI constexpr __union_t() : __empty_() {}

    template <class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
        : __unex_(std::forward<_Args>(__args)...) {}

    template <class _Func, class... _Args>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
        __expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
        : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}

    template <class _Union>
    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
      if (__has_val)
        std::construct_at(std::addressof(__empty_));
      else
        std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
    }

    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
      requires(is_trivially_destructible_v<_ErrorType>)
    = default;

    // the expected's destructor handles this
    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
      requires(!is_trivially_destructible_v<_ErrorType>)
    {}

    _LIBCPP_NO_UNIQUE_ADDRESS __empty_t __empty_;
    _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
  };

  _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Err> __union_;
  bool __has_val_;
};

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP_STD_VER >= 23

_LIBCPP_POP_MACROS

#endif // _LIBCPP___EXPECTED_EXPECTED_H