expected

Go to the documentation of this file.

// <expected> -*- C++ -*-
 
// Copyright The GNU Toolchain Authors.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/** @file include/expected
 *  This is a Standard C++ Library header.
 */
 
#ifndef _GLIBCXX_EXPECTED
#define _GLIBCXX_EXPECTED
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#define __glibcxx_want_expected
#define __glibcxx_want_freestanding_expected
#define __glibcxx_want_constrained_equality
#define __glibcxx_want_constexpr_exceptions
#define __glibcxx_want_hardened_expected
#include <bits/version.h>
 
#ifdef __cpp_lib_expected // C++ >= 23 && __cpp_concepts >= 202002L
#include <initializer_list>
#include <bits/exception.h>     // exception
#include <bits/invoke.h>        // __invoke
#include <bits/stl_construct.h> // construct_at
#include <bits/utility.h>       // in_place_t
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  /**
   * @defgroup expected_values Expected values
   * @addtogroup utilities
   * @since C++23
   * @{
   */
 
  /// Discriminated union that holds an expected value or an error value.
  /**
   * @since C++23
   */
  template<typename _Tp, typename _Er>
    class expected;
 
  /// Wrapper type used to pass an error value to a `std::expected`.
  /**
   * @since C++23
   */
  template<typename _Er>
    class unexpected;
 
  /// Exception thrown by std::expected when the value() is not present.
  /**
   * @since C++23
   */
  template<typename _Er>
    class bad_expected_access;
 
#if __cpp_lib_constexpr_exceptions >= 202502L
#define _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS constexpr
#else
#define _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS
#endif
 
  template<>
    class bad_expected_access<void> : public exception
    {
    protected:
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS bad_expected_access() noexcept { }
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS
      bad_expected_access(const bad_expected_access&) noexcept = default;
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS
      bad_expected_access(bad_expected_access&&) noexcept = default;
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS
      bad_expected_access& operator=(const bad_expected_access&) noexcept = default;
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS
      bad_expected_access& operator=(bad_expected_access&&) noexcept = default;
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS
      ~bad_expected_access() = default;
 
    public:
 
      [[nodiscard]]
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS const char*
      what() const noexcept override
      { return "bad access to std::expected without expected value"; }
    };
 
  template<typename _Er>
    class bad_expected_access : public bad_expected_access<void> {
    public:
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS explicit
      bad_expected_access(_Er __e) : _M_unex(std::move(__e)) { }
 
      // XXX const char* what() const noexcept override;
 
      [[nodiscard]]
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS _Er&
      error() & noexcept
      { return _M_unex; }
 
      [[nodiscard]]
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS const _Er&
      error() const & noexcept
      { return _M_unex; }
 
      [[nodiscard]]
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS _Er&&
      error() && noexcept
      { return std::move(_M_unex); }
 
      [[nodiscard]]
      _GLIBCXX_CONSTEXPR_BAD_EXPECTED_ACCESS const _Er&&
      error() const && noexcept
      { return std::move(_M_unex); }
 
    private:
      _Er _M_unex;
    };
 
  /// Tag type for constructing unexpected values in a std::expected
  /**
   * @since C++23
   */
  struct unexpect_t
  {
    explicit unexpect_t() = default;
  };
 
  /// Tag for constructing unexpected values in a std::expected
  /**
   * @since C++23
   */
  inline constexpr unexpect_t unexpect{};
 
/// @cond undocumented
namespace __expected
{
  template<typename _Tp>
    constexpr bool __is_expected = false;
  template<typename _Tp, typename _Er>
    constexpr bool __is_expected<expected<_Tp, _Er>> = true;
 
  template<typename _Tp>
    constexpr bool __is_unexpected = false;
  template<typename _Tp>
    constexpr bool __is_unexpected<unexpected<_Tp>> = true;
 
  template<typename _Fn, typename _Tp>
    using __result = remove_cvref_t<invoke_result_t<_Fn&&, _Tp&&>>;
  template<typename _Fn, typename _Tp>
    using __result_xform = remove_cv_t<invoke_result_t<_Fn&&, _Tp&&>>;
  template<typename _Fn>
    using __result0 = remove_cvref_t<invoke_result_t<_Fn&&>>;
  template<typename _Fn>
    using __result0_xform = remove_cv_t<invoke_result_t<_Fn&&>>;
 
  template<typename _Er>
    concept __can_be_unexpected
      = is_object_v<_Er> && (!is_array_v<_Er>)
          && (!__expected::__is_unexpected<_Er>)
          && (!is_const_v<_Er>) && (!is_volatile_v<_Er>);
 
  // Tag types for in-place construction from an invocation result.
  struct __in_place_inv { };
  struct __unexpect_inv { };
}
/// @endcond
 
  template<typename _Er>
    class unexpected
    {
      static_assert( __expected::__can_be_unexpected<_Er> );
 
    public:
      constexpr unexpected(const unexpected&) = default;
      constexpr unexpected(unexpected&&) = default;
 
      template<typename _Err = _Er>
        requires (!is_same_v<remove_cvref_t<_Err>, unexpected>)
          && (!is_same_v<remove_cvref_t<_Err>, in_place_t>)
          && is_constructible_v<_Er, _Err>
        constexpr explicit
        unexpected(_Err&& __e)
        noexcept(is_nothrow_constructible_v<_Er, _Err>)
        : _M_unex(std::forward<_Err>(__e))
        { }
 
      template<typename... _Args>
        requires is_constructible_v<_Er, _Args...>
        constexpr explicit
        unexpected(in_place_t, _Args&&... __args)
        noexcept(is_nothrow_constructible_v<_Er, _Args...>)
        : _M_unex(std::forward<_Args>(__args)...)
        { }
 
      template<typename _Up, typename... _Args>
        requires is_constructible_v<_Er, initializer_list<_Up>&, _Args...>
        constexpr explicit
        unexpected(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
        noexcept(is_nothrow_constructible_v<_Er, initializer_list<_Up>&,
                                            _Args...>)
        : _M_unex(__il, std::forward<_Args>(__args)...)
        { }
 
      constexpr unexpected& operator=(const unexpected&) = default;
      constexpr unexpected& operator=(unexpected&&) = default;
 
 
      [[nodiscard]]
      constexpr const _Er&
      error() const & noexcept { return _M_unex; }
 
      [[nodiscard]]
      constexpr _Er&
      error() & noexcept { return _M_unex; }
 
      [[nodiscard]]
      constexpr const _Er&&
      error() const && noexcept { return std::move(_M_unex); }
 
      [[nodiscard]]
      constexpr _Er&&
      error() && noexcept { return std::move(_M_unex); }
 
      constexpr void
      swap(unexpected& __other) noexcept(is_nothrow_swappable_v<_Er>)
      requires is_swappable_v<_Er>
      {
        using std::swap;
        swap(_M_unex, __other._M_unex);
      }
 
      template<typename _Err>
        [[nodiscard]]
        friend constexpr bool
        operator==(const unexpected& __x, const unexpected<_Err>& __y)
        { return __x._M_unex == __y.error(); }
 
      friend constexpr void
      swap(unexpected& __x, unexpected& __y) noexcept(noexcept(__x.swap(__y)))
      requires is_swappable_v<_Er>
      { __x.swap(__y); }
 
    private:
      _Er _M_unex;
    };
 
  template<typename _Er> unexpected(_Er) -> unexpected<_Er>;
 
/// @cond undocumented
namespace __expected
{
  template<typename _Tp>
    struct _Guard
    {
      static_assert( is_nothrow_move_constructible_v<_Tp> );
 
      constexpr explicit
      _Guard(_Tp& __x)
      : _M_guarded(__builtin_addressof(__x)), _M_tmp(std::move(__x)) // nothrow
      { std::destroy_at(_M_guarded); }
 
      constexpr
      ~_Guard()
      {
        if (_M_guarded) [[unlikely]]
          std::construct_at(_M_guarded, std::move(_M_tmp));
      }
 
      _Guard(const _Guard&) = delete;
      _Guard& operator=(const _Guard&) = delete;
 
      constexpr _Tp&&
      release() noexcept
      {
        _M_guarded = nullptr;
        return std::move(_M_tmp);
      }
 
    private:
      _Tp* _M_guarded;
      _Tp _M_tmp;
    };
 
  // reinit-expected helper from [expected.object.assign]
  template<typename _Tp, typename _Up, typename _Vp>
    constexpr void
    __reinit(_Tp* __newval, _Up* __oldval, _Vp&& __arg)
    noexcept(is_nothrow_constructible_v<_Tp, _Vp>)
    {
      if constexpr (is_nothrow_constructible_v<_Tp, _Vp>)
        {
          std::destroy_at(__oldval);
          std::construct_at(__newval, std::forward<_Vp>(__arg));
        }
      else if constexpr (is_nothrow_move_constructible_v<_Tp>)
        {
          _Tp __tmp(std::forward<_Vp>(__arg)); // might throw
          std::destroy_at(__oldval);
          std::construct_at(__newval, std::move(__tmp));
        }
      else
        {
          _Guard<_Up> __guard(*__oldval);
          std::construct_at(__newval, std::forward<_Vp>(__arg)); // might throw
          __guard.release();
        }
    }
 
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 3836. std::expected<bool, E1> conversion constructor
  // expected(const expected<U, G>&) should take precedence over
  // expected(U&&) with operator bool
 
  // If T is cv bool, remove_cvref_t<U> is not a specialization of expected.
  template<typename _Tp, typename _Up>
    concept __not_constructing_bool_from_expected
      = ! is_same_v<remove_cv_t<_Tp>, bool>
          || ! __is_expected<remove_cvref_t<_Up>>;
 
  template<typename _Tp, typename _Up = remove_cvref_t<_Tp>>
    concept __trivially_replaceable
      = is_trivially_constructible_v<_Up, _Tp>
          && is_trivially_assignable_v<_Up&, _Tp>
          && is_trivially_destructible_v<_Up>;
 
  template<typename _Tp, typename _Up = remove_cvref_t<_Tp>>
    concept __usable_for_assign
      = is_constructible_v<_Up, _Tp> && is_assignable_v<_Up&, _Tp>;
 
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 4026. Assignment operators of std::expected should propagate triviality
  template<typename _Tp>
    concept __usable_for_trivial_assign
      = __trivially_replaceable<_Tp> && __usable_for_assign<_Tp>;
 
  // For copy/move assignment to replace T with E (or vice versa)
  // we require at least one of them to be nothrow move constructible.
  template<typename _Tp, typename _Er>
    concept __can_reassign_type
      = is_nothrow_move_constructible_v<_Tp>
          || is_nothrow_move_constructible_v<_Er>;
}
/// @endcond
 
  template<typename _Tp, typename _Er>
    class [[nodiscard]] expected
    {
      static_assert( ! is_reference_v<_Tp> );
      static_assert( ! is_function_v<_Tp> );
      static_assert( ! is_same_v<remove_cv_t<_Tp>, in_place_t> );
      static_assert( ! is_same_v<remove_cv_t<_Tp>, unexpect_t> );
      static_assert( ! __expected::__is_unexpected<remove_cv_t<_Tp>> );
      static_assert( __expected::__can_be_unexpected<_Er> );
 
      // If T is not cv bool, converts-from-any-cvref<T, expected<U, G>> and
      // is_constructible<unexpected<E>, cv expected<U, G> ref-qual> are false.
      template<typename _Up, typename _Gr, typename _Unex = unexpected<_Er>,
               typename = remove_cv_t<_Tp>>
        static constexpr bool __cons_from_expected
          = __or_v<is_constructible<_Tp, expected<_Up, _Gr>&>,
                   is_constructible<_Tp, expected<_Up, _Gr>>,
                   is_constructible<_Tp, const expected<_Up, _Gr>&>,
                   is_constructible<_Tp, const expected<_Up, _Gr>>,
                   is_convertible<expected<_Up, _Gr>&, _Tp>,
                   is_convertible<expected<_Up, _Gr>, _Tp>,
                   is_convertible<const expected<_Up, _Gr>&, _Tp>,
                   is_convertible<const expected<_Up, _Gr>, _Tp>,
                   is_constructible<_Unex, expected<_Up, _Gr>&>,
                   is_constructible<_Unex, expected<_Up, _Gr>>,
                   is_constructible<_Unex, const expected<_Up, _Gr>&>,
                   is_constructible<_Unex, const expected<_Up, _Gr>>
                  >;
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // If t is cv bool, we know it can be constructed from expected<U, G>,
      // but we don't want to cause the expected(U&&) constructor to be used,
      // so we only check the is_constructible<unexpected<E>, ...> cases.
      template<typename _Up, typename _Gr, typename _Unex>
        static constexpr bool __cons_from_expected<_Up, _Gr, _Unex, bool>
          = __or_v<is_constructible<_Unex, expected<_Up, _Gr>&>,
                   is_constructible<_Unex, expected<_Up, _Gr>>,
                   is_constructible<_Unex, const expected<_Up, _Gr>&>,
                   is_constructible<_Unex, const expected<_Up, _Gr>>
                  >;
 
      template<typename _Up, typename _Gr>
        constexpr static bool __explicit_conv
          = __or_v<__not_<is_convertible<_Up, _Tp>>,
                   __not_<is_convertible<_Gr, _Er>>
                  >;
 
      template<typename _Up>
        static constexpr bool __same_val
          = is_same_v<typename _Up::value_type, _Tp>;
 
      template<typename _Up>
        static constexpr bool __same_err
          = is_same_v<typename _Up::error_type, _Er>;
 
    public:
      using value_type = _Tp;
      using error_type = _Er;
      using unexpected_type = unexpected<_Er>;
 
      template<typename _Up>
        using rebind = expected<_Up, error_type>;
 
      constexpr
      expected()
      noexcept(is_nothrow_default_constructible_v<_Tp>)
      requires is_default_constructible_v<_Tp>
      : _M_val(), _M_has_value(true)
      { }
 
      expected(const expected&) = default;
 
      constexpr
      expected(const expected& __x)
      noexcept(__and_v<is_nothrow_copy_constructible<_Tp>,
                       is_nothrow_copy_constructible<_Er>>)
      requires is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Er>
      && (!is_trivially_copy_constructible_v<_Tp>
          || !is_trivially_copy_constructible_v<_Er>)
      : _M_has_value(__x._M_has_value)
      {
        if (_M_has_value)
          std::construct_at(__builtin_addressof(_M_val), __x._M_val);
        else
          std::construct_at(__builtin_addressof(_M_unex), __x._M_unex);
      }
 
      expected(expected&&) = default;
 
      constexpr
      expected(expected&& __x)
      noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
                       is_nothrow_move_constructible<_Er>>)
      requires is_move_constructible_v<_Tp> && is_move_constructible_v<_Er>
      && (!is_trivially_move_constructible_v<_Tp>
          || !is_trivially_move_constructible_v<_Er>)
      : _M_has_value(__x._M_has_value)
      {
        if (_M_has_value)
          std::construct_at(__builtin_addressof(_M_val),
                            std::move(__x)._M_val);
        else
          std::construct_at(__builtin_addressof(_M_unex),
                            std::move(__x)._M_unex);
      }
 
      template<typename _Up, typename _Gr>
        requires is_constructible_v<_Tp, const _Up&>
              && is_constructible_v<_Er, const _Gr&>
              && (!__cons_from_expected<_Up, _Gr>)
        constexpr explicit(__explicit_conv<const _Up&, const _Gr&>)
        expected(const expected<_Up, _Gr>& __x)
        noexcept(__and_v<is_nothrow_constructible<_Tp, const _Up&>,
                         is_nothrow_constructible<_Er, const _Gr&>>)
        : _M_has_value(__x._M_has_value)
        {
          if (_M_has_value)
            std::construct_at(__builtin_addressof(_M_val), __x._M_val);
          else
            std::construct_at(__builtin_addressof(_M_unex), __x._M_unex);
        }
 
      template<typename _Up, typename _Gr>
        requires is_constructible_v<_Tp, _Up>
              && is_constructible_v<_Er, _Gr>
              && (!__cons_from_expected<_Up, _Gr>)
        constexpr explicit(__explicit_conv<_Up, _Gr>)
        expected(expected<_Up, _Gr>&& __x)
        noexcept(__and_v<is_nothrow_constructible<_Tp, _Up>,
                         is_nothrow_constructible<_Er, _Gr>>)
        : _M_has_value(__x._M_has_value)
        {
          if (_M_has_value)
            std::construct_at(__builtin_addressof(_M_val),
                              std::move(__x)._M_val);
          else
            std::construct_at(__builtin_addressof(_M_unex),
                              std::move(__x)._M_unex);
        }
 
      template<typename _Up = remove_cv_t<_Tp>>
        requires (!is_same_v<remove_cvref_t<_Up>, expected>)
          && (!is_same_v<remove_cvref_t<_Up>, in_place_t>)
          && (!is_same_v<remove_cvref_t<_Up>, unexpect_t>)
          && is_constructible_v<_Tp, _Up>
          && (!__expected::__is_unexpected<remove_cvref_t<_Up>>)
          && __expected::__not_constructing_bool_from_expected<_Tp, _Up>
        constexpr explicit(!is_convertible_v<_Up, _Tp>)
        expected(_Up&& __v)
        noexcept(is_nothrow_constructible_v<_Tp, _Up>)
        : _M_val(std::forward<_Up>(__v)), _M_has_value(true)
        { }
 
      template<typename _Gr = _Er>
        requires is_constructible_v<_Er, const _Gr&>
        constexpr explicit(!is_convertible_v<const _Gr&, _Er>)
        expected(const unexpected<_Gr>& __u)
        noexcept(is_nothrow_constructible_v<_Er, const _Gr&>)
        : _M_unex(__u.error()), _M_has_value(false)
        { }
 
      template<typename _Gr = _Er>
        requires is_constructible_v<_Er, _Gr>
        constexpr explicit(!is_convertible_v<_Gr, _Er>)
        expected(unexpected<_Gr>&& __u)
        noexcept(is_nothrow_constructible_v<_Er, _Gr>)
        : _M_unex(std::move(__u).error()), _M_has_value(false)
        { }
 
      template<typename... _Args>
        requires is_constructible_v<_Tp, _Args...>
        constexpr explicit
        expected(in_place_t, _Args&&... __args)
        noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
        : _M_val(std::forward<_Args>(__args)...), _M_has_value(true)
        { }
 
      template<typename _Up, typename... _Args>
        requires is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>
        constexpr explicit
        expected(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
        noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&,
                                            _Args...>)
        : _M_val(__il, std::forward<_Args>(__args)...), _M_has_value(true)
        { }
 
      template<typename... _Args>
        requires is_constructible_v<_Er, _Args...>
        constexpr explicit
        expected(unexpect_t, _Args&&... __args)
        noexcept(is_nothrow_constructible_v<_Er, _Args...>)
        : _M_unex(std::forward<_Args>(__args)...), _M_has_value(false)
        { }
 
      template<typename _Up, typename... _Args>
        requires is_constructible_v<_Er, initializer_list<_Up>&, _Args...>
        constexpr explicit
        expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args)
        noexcept(is_nothrow_constructible_v<_Er, initializer_list<_Up>&,
                                            _Args...>)
        : _M_unex(__il, std::forward<_Args>(__args)...), _M_has_value(false)
        { }
 
      constexpr ~expected() = default;
 
      constexpr ~expected()
      requires (!is_trivially_destructible_v<_Tp>)
            || (!is_trivially_destructible_v<_Er>)
      {
        if (_M_has_value)
          std::destroy_at(__builtin_addressof(_M_val));
        else
          std::destroy_at(__builtin_addressof(_M_unex));
      }
 
      // assignment
 
      // Deleted copy assignment, when constraints not met for other overloads
      expected& operator=(const expected&) = delete;
 
      // Trivial copy assignment
      expected&
      operator=(const expected&)
      noexcept(__and_v<is_nothrow_copy_constructible<_Tp>,
                       is_nothrow_copy_constructible<_Er>,
                       is_nothrow_copy_assignable<_Tp>,
                       is_nothrow_copy_assignable<_Er>>)
      requires __expected::__usable_for_trivial_assign<const _Tp&>
            && __expected::__usable_for_trivial_assign<const _Er&>
            && __expected::__can_reassign_type<_Tp, _Er>
        = default;
 
      // Non-trivial copy assignment
      constexpr expected&
      operator=(const expected& __x)
      noexcept(__and_v<is_nothrow_copy_constructible<_Tp>,
                       is_nothrow_copy_constructible<_Er>,
                       is_nothrow_copy_assignable<_Tp>,
                       is_nothrow_copy_assignable<_Er>>)
      requires __expected::__usable_for_assign<const _Tp&>
            && __expected::__usable_for_assign<const _Er&>
            && __expected::__can_reassign_type<_Tp, _Er>
      {
        if (__x._M_has_value)
          this->_M_assign_val(__x._M_val);
        else
          this->_M_assign_unex(__x._M_unex);
        return *this;
      }
 
      // Trivial move assignment
      expected&
      operator=(expected&&)
      noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
                       is_nothrow_move_constructible<_Er>,
                       is_nothrow_move_assignable<_Tp>,
                       is_nothrow_move_assignable<_Er>>)
      requires __expected::__usable_for_trivial_assign<_Tp&&>
            && __expected::__usable_for_trivial_assign<_Er&&>
            && __expected::__can_reassign_type<_Tp, _Er>
        = default;
 
      // Non-trivial move assignment
      constexpr expected&
      operator=(expected&& __x)
      noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
                       is_nothrow_move_constructible<_Er>,
                       is_nothrow_move_assignable<_Tp>,
                       is_nothrow_move_assignable<_Er>>)
      requires __expected::__usable_for_assign<_Tp&&>
            && __expected::__usable_for_assign<_Er&&>
            && __expected::__can_reassign_type<_Tp, _Er>
      {
        if (__x._M_has_value)
          _M_assign_val(std::move(__x._M_val));
        else
          _M_assign_unex(std::move(__x._M_unex));
        return *this;
      }
 
      template<typename _Up = remove_cv_t<_Tp>>
        requires (!is_same_v<expected, remove_cvref_t<_Up>>)
              && (!__expected::__is_unexpected<remove_cvref_t<_Up>>)
              && 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<_Er>)
        constexpr expected&
        operator=(_Up&& __v)
        {
          _M_assign_val(std::forward<_Up>(__v));
          return *this;
        }
 
      template<typename _Gr>
        requires is_constructible_v<_Er, const _Gr&>
              && is_assignable_v<_Er&, const _Gr&>
              && (is_nothrow_constructible_v<_Er, const _Gr&>
                  || is_nothrow_move_constructible_v<_Tp>
                  || is_nothrow_move_constructible_v<_Er>)
        constexpr expected&
        operator=(const unexpected<_Gr>& __e)
        {
          _M_assign_unex(__e.error());
          return *this;
        }
 
      template<typename _Gr>
        requires is_constructible_v<_Er, _Gr>
              && is_assignable_v<_Er&, _Gr>
              && (is_nothrow_constructible_v<_Er, _Gr>
                  || is_nothrow_move_constructible_v<_Tp>
                  || is_nothrow_move_constructible_v<_Er>)
        constexpr expected&
        operator=(unexpected<_Gr>&& __e)
        {
          _M_assign_unex(std::move(__e).error());
          return *this;
        }
 
      // modifiers
 
      template<typename... _Args>
        requires is_nothrow_constructible_v<_Tp, _Args...>
        constexpr _Tp&
        emplace(_Args&&... __args) noexcept
        {
          if (_M_has_value)
            std::destroy_at(__builtin_addressof(_M_val));
          else
            {
              std::destroy_at(__builtin_addressof(_M_unex));
              _M_has_value = true;
            }
          std::construct_at(__builtin_addressof(_M_val),
                            std::forward<_Args>(__args)...);
          return _M_val;
        }
 
      template<typename _Up, typename... _Args>
        requires is_nothrow_constructible_v<_Tp, initializer_list<_Up>&,
                                            _Args...>
        constexpr _Tp&
        emplace(initializer_list<_Up> __il, _Args&&... __args) noexcept
        {
          if (_M_has_value)
            std::destroy_at(__builtin_addressof(_M_val));
          else
            {
              std::destroy_at(__builtin_addressof(_M_unex));
              _M_has_value = true;
            }
          std::construct_at(__builtin_addressof(_M_val),
                            __il, std::forward<_Args>(__args)...);
          return _M_val;
        }
 
      // swap
      constexpr void
      swap(expected& __x)
      noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
                       is_nothrow_move_constructible<_Er>,
                       is_nothrow_swappable<_Tp&>,
                       is_nothrow_swappable<_Er&>>)
      requires is_swappable_v<_Tp> && is_swappable_v<_Er>
            && is_move_constructible_v<_Tp>
            && is_move_constructible_v<_Er>
            && (is_nothrow_move_constructible_v<_Tp>
                || is_nothrow_move_constructible_v<_Er>)
      {
        if (_M_has_value)
          {
            if (__x._M_has_value)
              {
                using std::swap;
                swap(_M_val, __x._M_val);
              }
            else
              this->_M_swap_val_unex(__x);
          }
        else
          {
            if (__x._M_has_value)
              __x._M_swap_val_unex(*this);
            else
              {
                using std::swap;
                swap(_M_unex, __x._M_unex);
              }
          }
      }
 
      // observers
 
      [[nodiscard]]
      constexpr const _Tp*
      operator->() const noexcept
      {
        __glibcxx_assert(_M_has_value);
        return __builtin_addressof(_M_val);
      }
 
      [[nodiscard]]
      constexpr _Tp*
      operator->() noexcept
      {
        __glibcxx_assert(_M_has_value);
        return __builtin_addressof(_M_val);
      }
 
      [[nodiscard]]
      constexpr const _Tp&
      operator*() const & noexcept
      {
        __glibcxx_assert(_M_has_value);
        return _M_val;
      }
 
      [[nodiscard]]
      constexpr _Tp&
      operator*() & noexcept
      {
        __glibcxx_assert(_M_has_value);
        return _M_val;
      }
 
      [[nodiscard]]
      constexpr const _Tp&&
      operator*() const && noexcept
      {
        __glibcxx_assert(_M_has_value);
        return std::move(_M_val);
      }
 
      [[nodiscard]]
      constexpr _Tp&&
      operator*() && noexcept
      {
        __glibcxx_assert(_M_has_value);
        return std::move(_M_val);
      }
 
      [[nodiscard]]
      constexpr explicit
      operator bool() const noexcept { return _M_has_value; }
 
      [[nodiscard]]
      constexpr bool has_value() const noexcept { return _M_has_value; }
 
      constexpr const _Tp&
      value() const &
      {
        static_assert( is_copy_constructible_v<_Er> );
        if (_M_has_value) [[likely]]
          return _M_val;
        _GLIBCXX_THROW_OR_ABORT(bad_expected_access<_Er>(_M_unex));
      }
 
      constexpr _Tp&
      value() &
      {
        static_assert( is_copy_constructible_v<_Er> );
        if (_M_has_value) [[likely]]
          return _M_val;
        const auto& __unex = _M_unex;
        _GLIBCXX_THROW_OR_ABORT(bad_expected_access<_Er>(__unex));
      }
 
      constexpr const _Tp&&
      value() const &&
      {
        static_assert( is_copy_constructible_v<_Er> );
        static_assert( is_constructible_v<_Er, const _Er&&> );
        if (_M_has_value) [[likely]]
          return std::move(_M_val);
        _GLIBCXX_THROW_OR_ABORT(bad_expected_access<_Er>(std::move(_M_unex)));
      }
 
      constexpr _Tp&&
      value() &&
      {
        static_assert( is_copy_constructible_v<_Er> );
        static_assert( is_constructible_v<_Er, _Er&&> );
        if (_M_has_value) [[likely]]
          return std::move(_M_val);
        _GLIBCXX_THROW_OR_ABORT(bad_expected_access<_Er>(std::move(_M_unex)));
      }
 
      constexpr const _Er&
      error() const & noexcept
      {
        __glibcxx_assert(!_M_has_value);
        return _M_unex;
      }
 
      constexpr _Er&
      error() & noexcept
      {
        __glibcxx_assert(!_M_has_value);
        return _M_unex;
      }
 
      constexpr const _Er&&
      error() const && noexcept
      {
        __glibcxx_assert(!_M_has_value);
        return std::move(_M_unex);
      }
 
      constexpr _Er&&
      error() && noexcept
      {
        __glibcxx_assert(!_M_has_value);
        return std::move(_M_unex);
      }
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 4406. value_or return statement is inconsistent with Mandates
      template<typename _Up = remove_cv_t<_Tp>>
        constexpr remove_cv_t<_Tp>
        value_or(_Up&& __v) const &
        noexcept(__and_v<is_nothrow_copy_constructible<_Tp>,
                         is_nothrow_convertible<_Up, _Tp>>)
        {
          using _Xp = remove_cv_t<_Tp>;
          static_assert( is_convertible_v<const _Tp&, _Xp> );
          static_assert( is_convertible_v<_Up, _Tp> );
 
          if (_M_has_value)
            return _M_val;
          return std::forward<_Up>(__v);
        }
 
      template<typename _Up = remove_cv_t<_Tp>>
        constexpr remove_cv_t<_Tp>
        value_or(_Up&& __v) &&
        noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
                         is_nothrow_convertible<_Up, _Tp>>)
        {
          using _Xp = remove_cv_t<_Tp>;
          static_assert( is_convertible_v<_Tp, _Xp> );
          static_assert( is_convertible_v<_Up, _Xp> );
 
          if (_M_has_value)
            return std::move(_M_val);
          return std::forward<_Up>(__v);
        }
 
      template<typename _Gr = _Er>
        constexpr _Er
        error_or(_Gr&& __e) const&
        {
          static_assert( is_copy_constructible_v<_Er> );
          static_assert( is_convertible_v<_Gr, _Er> );
 
          if (_M_has_value)
            return std::forward<_Gr>(__e);
          return _M_unex;
        }
 
      template<typename _Gr = _Er>
        constexpr _Er
        error_or(_Gr&& __e) &&
        {
          static_assert( is_move_constructible_v<_Er> );
          static_assert( is_convertible_v<_Gr, _Er> );
 
          if (_M_has_value)
            return std::forward<_Gr>(__e);
          return std::move(_M_unex);
        }
 
      // monadic operations
 
      template<typename _Fn> requires is_constructible_v<_Er, _Er&>
        constexpr auto
        and_then(_Fn&& __f) &
        {
          using _Up = __expected::__result<_Fn, _Tp&>;
          static_assert(__expected::__is_expected<_Up>,
                        "the function passed to std::expected<T, E>::and_then "
                        "must return a std::expected");
          static_assert(is_same_v<typename _Up::error_type, _Er>,
                        "the function passed to std::expected<T, E>::and_then "
                        "must return a std::expected with the same error_type");
 
          if (has_value())
            return std::__invoke(std::forward<_Fn>(__f), _M_val);
          else
            return _Up(unexpect, _M_unex);
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, const _Er&>
        constexpr auto
        and_then(_Fn&& __f) const &
        {
          using _Up = __expected::__result<_Fn, const _Tp&>;
          static_assert(__expected::__is_expected<_Up>,
                        "the function passed to std::expected<T, E>::and_then "
                        "must return a std::expected");
          static_assert(is_same_v<typename _Up::error_type, _Er>,
                        "the function passed to std::expected<T, E>::and_then "
                        "must return a std::expected with the same error_type");
 
          if (has_value())
            return std::__invoke(std::forward<_Fn>(__f), _M_val);
          else
            return _Up(unexpect, _M_unex);
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, _Er>
        constexpr auto
        and_then(_Fn&& __f) &&
        {
          using _Up = __expected::__result<_Fn, _Tp&&>;
          static_assert(__expected::__is_expected<_Up>,
                        "the function passed to std::expected<T, E>::and_then "
                        "must return a std::expected");
          static_assert(is_same_v<typename _Up::error_type, _Er>,
                        "the function passed to std::expected<T, E>::and_then "
                        "must return a std::expected with the same error_type");
 
          if (has_value())
            return std::__invoke(std::forward<_Fn>(__f), std::move(_M_val));
          else
            return _Up(unexpect, std::move(_M_unex));
        }
 
 
      template<typename _Fn> requires is_constructible_v<_Er, const _Er>
        constexpr auto
        and_then(_Fn&& __f) const &&
        {
          using _Up = __expected::__result<_Fn, const _Tp&&>;
          static_assert(__expected::__is_expected<_Up>,
                        "the function passed to std::expected<T, E>::and_then "
                        "must return a std::expected");
          static_assert(is_same_v<typename _Up::error_type, _Er>,
                        "the function passed to std::expected<T, E>::and_then "
                        "must return a std::expected with the same error_type");
 
          if (has_value())
            return std::__invoke(std::forward<_Fn>(__f), std::move(_M_val));
          else
            return _Up(unexpect, std::move(_M_unex));
        }
 
      template<typename _Fn> requires is_constructible_v<_Tp, _Tp&>
        constexpr auto
        or_else(_Fn&& __f) &
        {
          using _Gr = __expected::__result<_Fn, _Er&>;
          static_assert(__expected::__is_expected<_Gr>,
                        "the function passed to std::expected<T, E>::or_else "
                        "must return a std::expected");
          static_assert(is_same_v<typename _Gr::value_type, _Tp>,
                        "the function passed to std::expected<T, E>::or_else "
                        "must return a std::expected with the same value_type");
 
          if (has_value())
            return _Gr(in_place, _M_val);
          else
            return std::__invoke(std::forward<_Fn>(__f), _M_unex);
        }
 
      template<typename _Fn> requires is_constructible_v<_Tp, const _Tp&>
        constexpr auto
        or_else(_Fn&& __f) const &
        {
          using _Gr = __expected::__result<_Fn, const _Er&>;
          static_assert(__expected::__is_expected<_Gr>,
                        "the function passed to std::expected<T, E>::or_else "
                        "must return a std::expected");
          static_assert(is_same_v<typename _Gr::value_type, _Tp>,
                        "the function passed to std::expected<T, E>::or_else "
                        "must return a std::expected with the same value_type");
 
          if (has_value())
            return _Gr(in_place, _M_val);
          else
            return std::__invoke(std::forward<_Fn>(__f), _M_unex);
        }
 
 
      template<typename _Fn> requires is_constructible_v<_Tp, _Tp>
        constexpr auto
        or_else(_Fn&& __f) &&
        {
          using _Gr = __expected::__result<_Fn, _Er&&>;
          static_assert(__expected::__is_expected<_Gr>,
                        "the function passed to std::expected<T, E>::or_else "
                        "must return a std::expected");
          static_assert(is_same_v<typename _Gr::value_type, _Tp>,
                        "the function passed to std::expected<T, E>::or_else "
                        "must return a std::expected with the same value_type");
 
          if (has_value())
            return _Gr(in_place, std::move(_M_val));
          else
            return std::__invoke(std::forward<_Fn>(__f), std::move(_M_unex));
        }
 
      template<typename _Fn> requires is_constructible_v<_Tp, const _Tp>
        constexpr auto
        or_else(_Fn&& __f) const &&
        {
          using _Gr = __expected::__result<_Fn, const _Er&&>;
          static_assert(__expected::__is_expected<_Gr>,
                        "the function passed to std::expected<T, E>::or_else "
                        "must return a std::expected");
          static_assert(is_same_v<typename _Gr::value_type, _Tp>,
                        "the function passed to std::expected<T, E>::or_else "
                        "must return a std::expected with the same value_type");
 
          if (has_value())
            return _Gr(in_place, std::move(_M_val));
          else
            return std::__invoke(std::forward<_Fn>(__f), std::move(_M_unex));
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, _Er&>
        constexpr auto
        transform(_Fn&& __f) &
        {
          using _Up = __expected::__result_xform<_Fn, _Tp&>;
          using _Res = expected<_Up, _Er>;
 
          if (has_value())
            return _Res(__in_place_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               _M_val);
                        });
          else
            return _Res(unexpect, _M_unex);
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, const _Er&>
        constexpr auto
        transform(_Fn&& __f) const &
        {
          using _Up = __expected::__result_xform<_Fn, const _Tp&>;
          using _Res = expected<_Up, _Er>;
 
          if (has_value())
            return _Res(__in_place_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               _M_val);
                        });
          else
            return _Res(unexpect, _M_unex);
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, _Er>
        constexpr auto
        transform(_Fn&& __f) &&
        {
          using _Up = __expected::__result_xform<_Fn, _Tp>;
          using _Res = expected<_Up, _Er>;
 
          if (has_value())
            return _Res(__in_place_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               std::move(_M_val));
                        });
          else
            return _Res(unexpect, std::move(_M_unex));
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, const _Er>
        constexpr auto
        transform(_Fn&& __f) const &&
        {
          using _Up = __expected::__result_xform<_Fn, const _Tp>;
          using _Res = expected<_Up, _Er>;
 
          if (has_value())
            return _Res(__in_place_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               std::move(_M_val));
                        });
          else
            return _Res(unexpect, std::move(_M_unex));
        }
 
      template<typename _Fn> requires is_constructible_v<_Tp, _Tp&>
        constexpr auto
        transform_error(_Fn&& __f) &
        {
          using _Gr = __expected::__result_xform<_Fn, _Er&>;
          using _Res = expected<_Tp, _Gr>;
 
          if (has_value())
            return _Res(in_place, _M_val);
          else
            return _Res(__unexpect_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               _M_unex);
                        });
        }
 
      template<typename _Fn> requires is_constructible_v<_Tp, const _Tp&>
        constexpr auto
        transform_error(_Fn&& __f) const &
        {
          using _Gr = __expected::__result_xform<_Fn, const _Er&>;
          using _Res = expected<_Tp, _Gr>;
 
          if (has_value())
            return _Res(in_place, _M_val);
          else
            return _Res(__unexpect_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               _M_unex);
                        });
        }
 
      template<typename _Fn> requires is_constructible_v<_Tp, _Tp>
        constexpr auto
        transform_error(_Fn&& __f) &&
        {
          using _Gr = __expected::__result_xform<_Fn, _Er&&>;
          using _Res = expected<_Tp, _Gr>;
 
          if (has_value())
            return _Res(in_place, std::move(_M_val));
          else
            return _Res(__unexpect_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               std::move(_M_unex));
                        });
        }
 
      template<typename _Fn> requires is_constructible_v<_Tp, const _Tp>
        constexpr auto
        transform_error(_Fn&& __f) const &&
        {
          using _Gr = __expected::__result_xform<_Fn, const _Er&&>;
          using _Res = expected<_Tp, _Gr>;
 
          if (has_value())
            return _Res(in_place, std::move(_M_val));
          else
            return _Res(__unexpect_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               std::move(_M_unex));
                        });
        }
 
      // equality operators
 
      template<typename _Up, typename _Er2>
        requires (!is_void_v<_Up>)
          && requires (const _Tp& __t, const _Up& __u,
                       const _Er& __e, const _Er2& __e2) {
            { __t == __u } -> convertible_to<bool>;
            { __e == __e2 } -> convertible_to<bool>;
          }
        [[nodiscard]]
        friend constexpr bool
        operator==(const expected& __x, const expected<_Up, _Er2>& __y)
        noexcept(noexcept(bool(*__x == *__y))
                  && noexcept(bool(__x.error() == __y.error())))
        {
          if (__x.has_value() != __y.has_value())
            return false;
          if (__x.has_value())
            return *__x == *__y;
          return __x.error() == __y.error();
        }
 
      template<typename _Up, same_as<_Tp> _Vp>
        requires (!__expected::__is_expected<_Up>)
          && requires (const _Tp& __t, const _Up& __u) {
            { __t == __u } -> convertible_to<bool>;
          }
        [[nodiscard]]
        friend constexpr bool
        operator==(const expected<_Vp, _Er>& __x, const _Up& __v)
        noexcept(noexcept(bool(*__x == __v)))
        {
          if (__x.has_value())
            return *__x == __v;
          return false;
        }
 
      template<typename _Er2>
        requires requires (const _Er& __e, const _Er2& __e2) {
          { __e == __e2 } -> convertible_to<bool>;
        }
        [[nodiscard]]
        friend constexpr bool
        operator==(const expected& __x, const unexpected<_Er2>& __e)
        noexcept(noexcept(bool(__x.error() == __e.error())))
        {
          if (!__x.has_value())
            return __x.error() == __e.error();
          return false;
        }
 
      friend constexpr void
      swap(expected& __x, expected& __y)
      noexcept(noexcept(__x.swap(__y)))
      requires requires {__x.swap(__y);}
      { __x.swap(__y); }
 
    private:
      template<typename, typename> friend class expected;
 
      template<typename _Vp>
        constexpr void
        _M_assign_val(_Vp&& __v)
        {
          if (_M_has_value)
            _M_val = std::forward<_Vp>(__v);
          else
            {
              __expected::__reinit(__builtin_addressof(_M_val),
                                   __builtin_addressof(_M_unex),
                                   std::forward<_Vp>(__v));
              _M_has_value = true;
            }
        }
 
      template<typename _Vp>
        constexpr void
        _M_assign_unex(_Vp&& __v)
        {
          if (_M_has_value)
            {
              __expected::__reinit(__builtin_addressof(_M_unex),
                                   __builtin_addressof(_M_val),
                                   std::forward<_Vp>(__v));
              _M_has_value = false;
            }
          else
            _M_unex = std::forward<_Vp>(__v);
        }
 
      // Swap two expected objects when only one has a value.
      // Precondition: this->_M_has_value && !__rhs._M_has_value
      constexpr void
      _M_swap_val_unex(expected& __rhs)
      noexcept(__and_v<is_nothrow_move_constructible<_Er>,
                       is_nothrow_move_constructible<_Tp>>)
      {
        if constexpr (is_nothrow_move_constructible_v<_Er>)
          {
            __expected::_Guard<_Er> __guard(__rhs._M_unex);
            std::construct_at(__builtin_addressof(__rhs._M_val),
                              std::move(_M_val)); // might throw
            __rhs._M_has_value = true;
            std::destroy_at(__builtin_addressof(_M_val));
            std::construct_at(__builtin_addressof(_M_unex),
                              __guard.release());
            _M_has_value = false;
          }
        else
          {
            __expected::_Guard<_Tp> __guard(_M_val);
            std::construct_at(__builtin_addressof(_M_unex),
                              std::move(__rhs._M_unex)); // might throw
            _M_has_value = false;
            std::destroy_at(__builtin_addressof(__rhs._M_unex));
            std::construct_at(__builtin_addressof(__rhs._M_val),
                              __guard.release());
            __rhs._M_has_value = true;
          }
      }
 
      using __in_place_inv = __expected::__in_place_inv;
      using __unexpect_inv = __expected::__unexpect_inv;
 
      template<typename _Fn>
        explicit constexpr
        expected(__in_place_inv, _Fn&& __fn)
        : _M_val(std::forward<_Fn>(__fn)()), _M_has_value(true)
        { }
 
      template<typename _Fn>
        explicit constexpr
        expected(__unexpect_inv, _Fn&& __fn)
        : _M_unex(std::forward<_Fn>(__fn)()), _M_has_value(false)
        { }
 
      union {
        remove_cv_t<_Tp> _M_val;
        _Er _M_unex;
      };
 
      bool _M_has_value;
    };
 
  // Partial specialization for std::expected<cv void, E>
  template<typename _Tp, typename _Er> requires is_void_v<_Tp>
    class [[nodiscard]] expected<_Tp, _Er>
    {
      static_assert( __expected::__can_be_unexpected<_Er> );
 
      template<typename _Up, typename _Err, typename _Unex = unexpected<_Er>>
        static constexpr bool __cons_from_expected
          = __or_v<is_constructible<_Unex, expected<_Up, _Err>&>,
                   is_constructible<_Unex, expected<_Up, _Err>>,
                   is_constructible<_Unex, const expected<_Up, _Err>&>,
                   is_constructible<_Unex, const expected<_Up, _Err>>
                  >;
 
      template<typename _Up>
        static constexpr bool __same_val
          = is_same_v<typename _Up::value_type, _Tp>;
 
      template<typename _Up>
        static constexpr bool __same_err
          = is_same_v<typename _Up::error_type, _Er>;
 
    public:
      using value_type = _Tp;
      using error_type = _Er;
      using unexpected_type = unexpected<_Er>;
 
      template<typename _Up>
        using rebind = expected<_Up, error_type>;
 
      constexpr
      expected() noexcept
      : _M_void(), _M_has_value(true)
      { }
 
      expected(const expected&) = default;
 
      constexpr
      expected(const expected& __x)
      noexcept(is_nothrow_copy_constructible_v<_Er>)
      requires is_copy_constructible_v<_Er>
            && (!is_trivially_copy_constructible_v<_Er>)
      : _M_void(), _M_has_value(__x._M_has_value)
      {
        if (!_M_has_value)
          std::construct_at(__builtin_addressof(_M_unex), __x._M_unex);
      }
 
      expected(expected&&) = default;
 
      constexpr
      expected(expected&& __x)
      noexcept(is_nothrow_move_constructible_v<_Er>)
      requires is_move_constructible_v<_Er>
            && (!is_trivially_move_constructible_v<_Er>)
      : _M_void(), _M_has_value(__x._M_has_value)
      {
        if (!_M_has_value)
          std::construct_at(__builtin_addressof(_M_unex),
                            std::move(__x)._M_unex);
      }
 
      template<typename _Up, typename _Gr>
        requires is_void_v<_Up>
              && is_constructible_v<_Er, const _Gr&>
              && (!__cons_from_expected<_Up, _Gr>)
        constexpr explicit(!is_convertible_v<const _Gr&, _Er>)
        expected(const expected<_Up, _Gr>& __x)
        noexcept(is_nothrow_constructible_v<_Er, const _Gr&>)
        : _M_void(), _M_has_value(__x._M_has_value)
        {
          if (!_M_has_value)
            std::construct_at(__builtin_addressof(_M_unex), __x._M_unex);
        }
 
      template<typename _Up, typename _Gr>
        requires is_void_v<_Up>
              && is_constructible_v<_Er, _Gr>
              && (!__cons_from_expected<_Up, _Gr>)
        constexpr explicit(!is_convertible_v<_Gr, _Er>)
        expected(expected<_Up, _Gr>&& __x)
        noexcept(is_nothrow_constructible_v<_Er, _Gr>)
        : _M_void(), _M_has_value(__x._M_has_value)
        {
          if (!_M_has_value)
            std::construct_at(__builtin_addressof(_M_unex),
                              std::move(__x)._M_unex);
        }
 
      template<typename _Gr = _Er>
        requires is_constructible_v<_Er, const _Gr&>
        constexpr explicit(!is_convertible_v<const _Gr&, _Er>)
        expected(const unexpected<_Gr>& __u)
        noexcept(is_nothrow_constructible_v<_Er, const _Gr&>)
        : _M_unex(__u.error()), _M_has_value(false)
        { }
 
      template<typename _Gr = _Er>
        requires is_constructible_v<_Er, _Gr>
        constexpr explicit(!is_convertible_v<_Gr, _Er>)
        expected(unexpected<_Gr>&& __u)
        noexcept(is_nothrow_constructible_v<_Er, _Gr>)
        : _M_unex(std::move(__u).error()), _M_has_value(false)
        { }
 
      constexpr explicit
      expected(in_place_t) noexcept
      : expected()
      { }
 
      template<typename... _Args>
        requires is_constructible_v<_Er, _Args...>
        constexpr explicit
        expected(unexpect_t, _Args&&... __args)
        noexcept(is_nothrow_constructible_v<_Er, _Args...>)
        : _M_unex(std::forward<_Args>(__args)...), _M_has_value(false)
        { }
 
      template<typename _Up, typename... _Args>
        requires is_constructible_v<_Er, initializer_list<_Up>&, _Args...>
        constexpr explicit
        expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args)
        noexcept(is_nothrow_constructible_v<_Er, initializer_list<_Up>&,
                                            _Args...>)
        : _M_unex(__il, std::forward<_Args>(__args)...), _M_has_value(false)
        { }
 
      constexpr ~expected() = default;
 
      constexpr ~expected() requires (!is_trivially_destructible_v<_Er>)
      {
        if (!_M_has_value)
          std::destroy_at(__builtin_addressof(_M_unex));
      }
 
      // assignment
 
      // Deleted copy assignment, when constraints not met for other overloads
      expected& operator=(const expected&) = delete;
 
      // Trivial copy assignment
      expected&
      operator=(const expected&)
      noexcept(__and_v<is_nothrow_copy_constructible<_Er>,
                       is_nothrow_copy_assignable<_Er>>)
      requires __expected::__usable_for_trivial_assign<const _Er&>
        = default;
 
      // Non-trivial copy assignment
      constexpr expected&
      operator=(const expected& __x)
      noexcept(__and_v<is_nothrow_copy_constructible<_Er>,
                       is_nothrow_copy_assignable<_Er>>)
      requires __expected::__usable_for_assign<const _Er&>
      {
        if (__x._M_has_value)
          emplace();
        else
          _M_assign_unex(__x._M_unex);
        return *this;
      }
 
      // Trivial move assignment
      expected&
      operator=(expected&&)
      noexcept(__and_v<is_nothrow_move_constructible<_Er>,
                       is_nothrow_move_assignable<_Er>>)
      requires __expected::__usable_for_trivial_assign<_Er&&>
        = default;
 
      // Non-trivial move assignment
      constexpr expected&
      operator=(expected&& __x)
      noexcept(__and_v<is_nothrow_move_constructible<_Er>,
                       is_nothrow_move_assignable<_Er>>)
      requires __expected::__usable_for_assign<_Er&&>
      {
        if (__x._M_has_value)
          emplace();
        else
          _M_assign_unex(std::move(__x._M_unex));
        return *this;
      }
 
      template<typename _Gr>
        requires is_constructible_v<_Er, const _Gr&>
              && is_assignable_v<_Er&, const _Gr&>
        constexpr expected&
        operator=(const unexpected<_Gr>& __e)
        {
          _M_assign_unex(__e.error());
          return *this;
        }
 
      template<typename _Gr>
        requires is_constructible_v<_Er, _Gr>
              && is_assignable_v<_Er&, _Gr>
        constexpr expected&
        operator=(unexpected<_Gr>&& __e)
        {
          _M_assign_unex(std::move(__e.error()));
          return *this;
        }
 
      // modifiers
 
      constexpr void
      emplace() noexcept
      {
        if (!_M_has_value)
          {
            std::destroy_at(__builtin_addressof(_M_unex));
            _M_has_value = true;
          }
      }
 
      // swap
      constexpr void
      swap(expected& __x)
      noexcept(__and_v<is_nothrow_swappable<_Er&>,
                       is_nothrow_move_constructible<_Er>>)
      requires is_swappable_v<_Er> && is_move_constructible_v<_Er>
      {
        if (_M_has_value)
          {
            if (!__x._M_has_value)
              {
                std::construct_at(__builtin_addressof(_M_unex),
                                  std::move(__x._M_unex)); // might throw
                std::destroy_at(__builtin_addressof(__x._M_unex));
                _M_has_value = false;
                __x._M_has_value = true;
              }
          }
        else
          {
            if (__x._M_has_value)
              {
                std::construct_at(__builtin_addressof(__x._M_unex),
                                  std::move(_M_unex)); // might throw
                std::destroy_at(__builtin_addressof(_M_unex));
                _M_has_value = true;
                __x._M_has_value = false;
              }
            else
              {
                using std::swap;
                swap(_M_unex, __x._M_unex);
              }
          }
      }
 
      // observers
 
      [[nodiscard]]
      constexpr explicit
      operator bool() const noexcept { return _M_has_value; }
 
      [[nodiscard]]
      constexpr bool has_value() const noexcept { return _M_has_value; }
 
      constexpr void
      operator*() const noexcept { __glibcxx_assert(_M_has_value); }
 
      constexpr void
      value() const&
      {
        static_assert( is_copy_constructible_v<_Er> );
        if (_M_has_value) [[likely]]
          return;
        _GLIBCXX_THROW_OR_ABORT(bad_expected_access<_Er>(_M_unex));
      }
 
      constexpr void
      value() &&
      {
        static_assert( is_copy_constructible_v<_Er> );
        static_assert( is_move_constructible_v<_Er> );
        if (_M_has_value) [[likely]]
          return;
        _GLIBCXX_THROW_OR_ABORT(bad_expected_access<_Er>(std::move(_M_unex)));
      }
 
      constexpr const _Er&
      error() const & noexcept
      {
        __glibcxx_assert(!_M_has_value);
        return _M_unex;
      }
 
      constexpr _Er&
      error() & noexcept
      {
        __glibcxx_assert(!_M_has_value);
        return _M_unex;
      }
 
      constexpr const _Er&&
      error() const && noexcept
      {
        __glibcxx_assert(!_M_has_value);
        return std::move(_M_unex);
      }
 
      constexpr _Er&&
      error() && noexcept
      {
        __glibcxx_assert(!_M_has_value);
        return std::move(_M_unex);
      }
 
      template<typename _Gr = _Er>
        constexpr _Er
        error_or(_Gr&& __e) const&
        {
          static_assert( is_copy_constructible_v<_Er> );
          static_assert( is_convertible_v<_Gr, _Er> );
 
          if (_M_has_value)
            return std::forward<_Gr>(__e);
          return _M_unex;
        }
 
      template<typename _Gr = _Er>
        constexpr _Er
        error_or(_Gr&& __e) &&
        {
          static_assert( is_move_constructible_v<_Er> );
          static_assert( is_convertible_v<_Gr, _Er> );
 
          if (_M_has_value)
            return std::forward<_Gr>(__e);
          return std::move(_M_unex);
        }
 
      // monadic operations
 
      template<typename _Fn> requires is_constructible_v<_Er, _Er&>
        constexpr auto
        and_then(_Fn&& __f) &
        {
          using _Up = __expected::__result0<_Fn>;
          static_assert(__expected::__is_expected<_Up>);
          static_assert(is_same_v<typename _Up::error_type, _Er>);
 
          if (has_value())
            return std::__invoke(std::forward<_Fn>(__f));
          else
            return _Up(unexpect, _M_unex);
        }
 
     template<typename _Fn> requires is_constructible_v<_Er, const _Er&>
        constexpr auto
        and_then(_Fn&& __f) const &
        {
          using _Up = __expected::__result0<_Fn>;
          static_assert(__expected::__is_expected<_Up>);
          static_assert(is_same_v<typename _Up::error_type, _Er>);
 
          if (has_value())
            return std::__invoke(std::forward<_Fn>(__f));
          else
            return _Up(unexpect, _M_unex);
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, _Er>
        constexpr auto
        and_then(_Fn&& __f) &&
        {
          using _Up = __expected::__result0<_Fn>;
          static_assert(__expected::__is_expected<_Up>);
          static_assert(is_same_v<typename _Up::error_type, _Er>);
 
          if (has_value())
            return std::__invoke(std::forward<_Fn>(__f));
          else
            return _Up(unexpect, std::move(_M_unex));
        }
 
       template<typename _Fn> requires is_constructible_v<_Er, const _Er>
        constexpr auto
        and_then(_Fn&& __f) const &&
        {
          using _Up = __expected::__result0<_Fn>;
          static_assert(__expected::__is_expected<_Up>);
          static_assert(is_same_v<typename _Up::error_type, _Er>);
 
          if (has_value())
            return std::__invoke(std::forward<_Fn>(__f));
          else
            return _Up(unexpect, std::move(_M_unex));
        }
 
      template<typename _Fn>
        constexpr auto
        or_else(_Fn&& __f) &
        {
          using _Gr = __expected::__result<_Fn, _Er&>;
          static_assert(__expected::__is_expected<_Gr>);
          static_assert(is_same_v<typename _Gr::value_type, _Tp>);
 
          if (has_value())
            return _Gr();
          else
            return std::__invoke(std::forward<_Fn>(__f), _M_unex);
        }
 
      template<typename _Fn>
        constexpr auto
        or_else(_Fn&& __f) const &
        {
          using _Gr = __expected::__result<_Fn, const _Er&>;
          static_assert(__expected::__is_expected<_Gr>);
          static_assert(is_same_v<typename _Gr::value_type, _Tp>);
 
          if (has_value())
            return _Gr();
          else
            return std::__invoke(std::forward<_Fn>(__f), _M_unex);
        }
 
      template<typename _Fn>
        constexpr auto
        or_else(_Fn&& __f) &&
        {
          using _Gr = __expected::__result<_Fn, _Er&&>;
          static_assert(__expected::__is_expected<_Gr>);
          static_assert(is_same_v<typename _Gr::value_type, _Tp>);
 
          if (has_value())
            return _Gr();
          else
            return std::__invoke(std::forward<_Fn>(__f), std::move(_M_unex));
        }
 
      template<typename _Fn>
        constexpr auto
        or_else(_Fn&& __f) const &&
        {
          using _Gr = __expected::__result<_Fn, const _Er&&>;
          static_assert(__expected::__is_expected<_Gr>);
          static_assert(is_same_v<typename _Gr::value_type, _Tp>);
 
          if (has_value())
            return _Gr();
          else
            return std::__invoke(std::forward<_Fn>(__f), std::move(_M_unex));
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, _Er&>
        constexpr auto
        transform(_Fn&& __f) &
        {
          using _Up = __expected::__result0_xform<_Fn>;
          using _Res = expected<_Up, _Er>;
 
          if (has_value())
            return _Res(__in_place_inv{}, std::forward<_Fn>(__f));
          else
            return _Res(unexpect, _M_unex);
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, const _Er&>
        constexpr auto
        transform(_Fn&& __f) const &
        {
          using _Up = __expected::__result0_xform<_Fn>;
          using _Res = expected<_Up, _Er>;
 
          if (has_value())
            return _Res(__in_place_inv{}, std::forward<_Fn>(__f));
          else
            return _Res(unexpect, _M_unex);
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, _Er>
        constexpr auto
        transform(_Fn&& __f) &&
        {
          using _Up = __expected::__result0_xform<_Fn>;
          using _Res = expected<_Up, _Er>;
 
          if (has_value())
            return _Res(__in_place_inv{}, std::forward<_Fn>(__f));
          else
            return _Res(unexpect, std::move(_M_unex));
        }
 
      template<typename _Fn> requires is_constructible_v<_Er, const _Er>
        constexpr auto
        transform(_Fn&& __f) const &&
        {
          using _Up = __expected::__result0_xform<_Fn>;
          using _Res = expected<_Up, _Er>;
 
          if (has_value())
            return _Res(__in_place_inv{}, std::forward<_Fn>(__f));
          else
            return _Res(unexpect, std::move(_M_unex));
        }
 
      template<typename _Fn>
        constexpr auto
        transform_error(_Fn&& __f) &
        {
          using _Gr = __expected::__result_xform<_Fn, _Er&>;
          using _Res = expected<_Tp, _Gr>;
 
          if (has_value())
            return _Res();
          else
            return _Res(__unexpect_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               _M_unex);
                        });
        }
 
      template<typename _Fn>
        constexpr auto
        transform_error(_Fn&& __f) const &
        {
          using _Gr = __expected::__result_xform<_Fn, const _Er&>;
          using _Res = expected<_Tp, _Gr>;
 
          if (has_value())
            return _Res();
          else
            return _Res(__unexpect_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               _M_unex);
                        });
        }
 
      template<typename _Fn>
        constexpr auto
        transform_error(_Fn&& __f) &&
        {
          using _Gr = __expected::__result_xform<_Fn, _Er&&>;
          using _Res = expected<_Tp, _Gr>;
 
          if (has_value())
            return _Res();
          else
            return _Res(__unexpect_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               std::move(_M_unex));
                        });
        }
 
      template<typename _Fn>
        constexpr auto
        transform_error(_Fn&& __f) const &&
        {
          using _Gr = __expected::__result_xform<_Fn, const _Er&&>;
          using _Res = expected<_Tp, _Gr>;
 
          if (has_value())
            return _Res();
          else
            return _Res(__unexpect_inv{}, [&]() {
                          return std::__invoke(std::forward<_Fn>(__f),
                                               std::move(_M_unex));
                        });
        }
 
      // equality operators
 
      template<typename _Up, typename _Er2>
        requires is_void_v<_Up>
          && requires (const _Er& __e, const _Er2& __e2) {
            { __e == __e2 } -> convertible_to<bool>;
          }
        [[nodiscard]]
        friend constexpr bool
        operator==(const expected& __x, const expected<_Up, _Er2>& __y)
        noexcept(noexcept(bool(__x.error() == __y.error())))
        {
          if (__x.has_value() != __y.has_value())
            return false;
          if (__x.has_value())
            return true;
          return __x.error() == __y.error();
        }
 
      template<typename _Er2>
        requires requires (const _Er& __e, const _Er2& __e2) {
          { __e == __e2 } -> convertible_to<bool>;
        }
        [[nodiscard]]
        friend constexpr bool
        operator==(const expected& __x, const unexpected<_Er2>& __e)
        noexcept(noexcept(bool(__x.error() == __e.error())))
        {
          if (!__x.has_value())
            return __x.error() == __e.error();
          return false;
        }
 
      friend constexpr void
      swap(expected& __x, expected& __y)
      noexcept(noexcept(__x.swap(__y)))
      requires requires { __x.swap(__y); }
      { __x.swap(__y); }
 
    private:
      template<typename, typename> friend class expected;
 
      template<typename _Vp>
        constexpr void
        _M_assign_unex(_Vp&& __v)
        {
          if (_M_has_value)
            {
              std::construct_at(__builtin_addressof(_M_unex),
                                std::forward<_Vp>(__v));
              _M_has_value = false;
            }
          else
            _M_unex = std::forward<_Vp>(__v);
        }
 
      using __in_place_inv = __expected::__in_place_inv;
      using __unexpect_inv = __expected::__unexpect_inv;
 
      template<typename _Fn>
        explicit constexpr
        expected(__in_place_inv, _Fn&& __fn)
        : _M_void(), _M_has_value(true)
        { std::forward<_Fn>(__fn)(); }
 
      template<typename _Fn>
        explicit constexpr
        expected(__unexpect_inv, _Fn&& __fn)
        : _M_unex(std::forward<_Fn>(__fn)()), _M_has_value(false)
        { }
 
      union {
        struct { } _M_void;
        _Er _M_unex;
      };
 
      bool _M_has_value;
    };
  /// @}
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
 
#endif // __cpp_lib_expected
#endif // _GLIBCXX_EXPECTED