optional

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// <optional> -*- C++ -*-
 
// Copyright (C) 2013-2020 Free Software Foundation, Inc.
//
// 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/optional
 *  This is a Standard C++ Library header.
 */
 
#ifndef _GLIBCXX_OPTIONAL
#define _GLIBCXX_OPTIONAL 1
 
#pragma GCC system_header
 
#if __cplusplus >= 201703L
 
#include <utility>
#include <type_traits>
#include <exception>
#include <new>
#include <initializer_list>
#include <bits/exception_defines.h>
#include <bits/functional_hash.h>
#include <bits/enable_special_members.h>
#if __cplusplus > 201703L
# include <compare>
#endif
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  /**
   *  @addtogroup utilities
   *  @{
   */
 
#define __cpp_lib_optional 201606L
 
  template<typename _Tp>
    class optional;
 
  /// Tag type to disengage optional objects.
  struct nullopt_t
  {
    // Do not user-declare default constructor at all for
    // optional_value = {} syntax to work.
    // nullopt_t() = delete;
 
    // Used for constructing nullopt.
    enum class _Construct { _Token };
 
    // Must be constexpr for nullopt_t to be literal.
    explicit constexpr nullopt_t(_Construct) { }
  };
 
  /// Tag to disengage optional objects.
  inline constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };
 
  /**
   *  @brief Exception class thrown when a disengaged optional object is
   *  dereferenced.
   *  @ingroup exceptions
   */
  class bad_optional_access : public exception
  {
  public:
    bad_optional_access() = default;
    virtual ~bad_optional_access() = default;
 
    const char* what() const noexcept override
    { return "bad optional access"; }
  };
 
  void
  __throw_bad_optional_access()
  __attribute__((__noreturn__));
 
  // XXX Does not belong here.
  inline void
  __throw_bad_optional_access()
  { _GLIBCXX_THROW_OR_ABORT(bad_optional_access()); }
 
  // This class template manages construction/destruction of
  // the contained value for a std::optional.
  template <typename _Tp>
    struct _Optional_payload_base
    {
      using _Stored_type = remove_const_t<_Tp>;
 
      _Optional_payload_base() = default;
      ~_Optional_payload_base() = default;
 
      template<typename... _Args>
        constexpr
        _Optional_payload_base(in_place_t __tag, _Args&&... __args)
        : _M_payload(__tag, std::forward<_Args>(__args)...),
          _M_engaged(true)
        { }
 
      template<typename _Up, typename... _Args>
        constexpr
        _Optional_payload_base(std::initializer_list<_Up> __il,
                               _Args&&... __args)
        : _M_payload(__il, std::forward<_Args>(__args)...),
          _M_engaged(true)
        { }
 
      // Constructor used by _Optional_base copy constructor when the
      // contained value is not trivially copy constructible.
      constexpr
      _Optional_payload_base(bool __engaged,
                             const _Optional_payload_base& __other)
      {
        if (__other._M_engaged)
          this->_M_construct(__other._M_get());
      }
 
      // Constructor used by _Optional_base move constructor when the
      // contained value is not trivially move constructible.
      constexpr
      _Optional_payload_base(bool __engaged,
                             _Optional_payload_base&& __other)
      {
        if (__other._M_engaged)
          this->_M_construct(std::move(__other._M_get()));
      }
 
      // Copy constructor is only used to when the contained value is
      // trivially copy constructible.
      _Optional_payload_base(const _Optional_payload_base&) = default;
 
      // Move constructor is only used to when the contained value is
      // trivially copy constructible.
      _Optional_payload_base(_Optional_payload_base&&) = default;
 
      _Optional_payload_base&
      operator=(const _Optional_payload_base&) = default;
 
      _Optional_payload_base&
      operator=(_Optional_payload_base&&) = default;
 
      // used to perform non-trivial copy assignment.
      constexpr void
      _M_copy_assign(const _Optional_payload_base& __other)
      {
        if (this->_M_engaged && __other._M_engaged)
          this->_M_get() = __other._M_get();
        else
          {
            if (__other._M_engaged)
              this->_M_construct(__other._M_get());
            else
              this->_M_reset();
          }
      }
 
      // used to perform non-trivial move assignment.
      constexpr void
      _M_move_assign(_Optional_payload_base&& __other)
      noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
                       is_nothrow_move_assignable<_Tp>>)
      {
        if (this->_M_engaged && __other._M_engaged)
          this->_M_get() = std::move(__other._M_get());
        else
          {
            if (__other._M_engaged)
              this->_M_construct(std::move(__other._M_get()));
            else
              this->_M_reset();
          }
      }
 
      struct _Empty_byte { };
 
      template<typename _Up, bool = is_trivially_destructible_v<_Up>>
        union _Storage
        {
          constexpr _Storage() noexcept : _M_empty() { }
 
          template<typename... _Args>
            constexpr
            _Storage(in_place_t, _Args&&... __args)
            : _M_value(std::forward<_Args>(__args)...)
            { }
 
          template<typename _Vp, typename... _Args>
            constexpr
            _Storage(std::initializer_list<_Vp> __il, _Args&&... __args)
            : _M_value(__il, std::forward<_Args>(__args)...)
            { }
 
          _Empty_byte _M_empty;
          _Up _M_value;
        };
 
      template<typename _Up>
        union _Storage<_Up, false>
        {
          constexpr _Storage() noexcept : _M_empty() { }
 
          template<typename... _Args>
            constexpr
            _Storage(in_place_t, _Args&&... __args)
            : _M_value(std::forward<_Args>(__args)...)
            { }
 
          template<typename _Vp, typename... _Args>
            constexpr
            _Storage(std::initializer_list<_Vp> __il, _Args&&... __args)
            : _M_value(__il, std::forward<_Args>(__args)...)
            { }
 
          // User-provided destructor is needed when _Up has non-trivial dtor.
          ~_Storage() { }
 
          _Empty_byte _M_empty;
          _Up _M_value;
        };
 
      _Storage<_Stored_type> _M_payload;
 
      bool _M_engaged = false;
 
      template<typename... _Args>
        void
        _M_construct(_Args&&... __args)
        noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)
        {
          ::new ((void *) std::__addressof(this->_M_payload))
            _Stored_type(std::forward<_Args>(__args)...);
          this->_M_engaged = true;
        }
 
      constexpr void
      _M_destroy() noexcept
      {
        _M_engaged = false;
        _M_payload._M_value.~_Stored_type();
      }
 
      // The _M_get() operations have _M_engaged as a precondition.
      // They exist to access the contained value with the appropriate
      // const-qualification, because _M_payload has had the const removed.
 
      constexpr _Tp&
      _M_get() noexcept
      { return this->_M_payload._M_value; }
 
      constexpr const _Tp&
      _M_get() const noexcept
      { return this->_M_payload._M_value; }
 
      // _M_reset is a 'safe' operation with no precondition.
      constexpr void
      _M_reset() noexcept
      {
        if (this->_M_engaged)
          _M_destroy();
      }
    };
 
  // Class template that manages the payload for optionals.
  template <typename _Tp,
            bool /*_HasTrivialDestructor*/ =
              is_trivially_destructible_v<_Tp>,
            bool /*_HasTrivialCopy */ =
              is_trivially_copy_assignable_v<_Tp>
              && is_trivially_copy_constructible_v<_Tp>,
            bool /*_HasTrivialMove */ =
              is_trivially_move_assignable_v<_Tp>
              && is_trivially_move_constructible_v<_Tp>>
    struct _Optional_payload;
 
  // Payload for potentially-constexpr optionals (trivial copy/move/destroy).
  template <typename _Tp>
    struct _Optional_payload<_Tp, true, true, true>
    : _Optional_payload_base<_Tp>
    {
      using _Optional_payload_base<_Tp>::_Optional_payload_base;
 
      _Optional_payload() = default;
    };
 
  // Payload for optionals with non-trivial copy construction/assignment.
  template <typename _Tp>
    struct _Optional_payload<_Tp, true, false, true>
    : _Optional_payload_base<_Tp>
    {
      using _Optional_payload_base<_Tp>::_Optional_payload_base;
 
      _Optional_payload() = default;
      ~_Optional_payload() = default;
      _Optional_payload(const _Optional_payload&) = default;
      _Optional_payload(_Optional_payload&&) = default;
      _Optional_payload& operator=(_Optional_payload&&) = default;
 
      // Non-trivial copy assignment.
      constexpr
      _Optional_payload&
      operator=(const _Optional_payload& __other)
      {
        this->_M_copy_assign(__other);
        return *this;
      }
    };
 
  // Payload for optionals with non-trivial move construction/assignment.
  template <typename _Tp>
    struct _Optional_payload<_Tp, true, true, false>
    : _Optional_payload_base<_Tp>
    {
      using _Optional_payload_base<_Tp>::_Optional_payload_base;
 
      _Optional_payload() = default;
      ~_Optional_payload() = default;
      _Optional_payload(const _Optional_payload&) = default;
      _Optional_payload(_Optional_payload&&) = default;
      _Optional_payload& operator=(const _Optional_payload&) = default;
 
      // Non-trivial move assignment.
      constexpr
      _Optional_payload&
      operator=(_Optional_payload&& __other)
      noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
                       is_nothrow_move_assignable<_Tp>>)
      {
        this->_M_move_assign(std::move(__other));
        return *this;
      }
    };
 
  // Payload for optionals with non-trivial copy and move assignment.
  template <typename _Tp>
    struct _Optional_payload<_Tp, true, false, false>
    : _Optional_payload_base<_Tp>
    {
      using _Optional_payload_base<_Tp>::_Optional_payload_base;
 
      _Optional_payload() = default;
      ~_Optional_payload() = default;
      _Optional_payload(const _Optional_payload&) = default;
      _Optional_payload(_Optional_payload&&) = default;
 
      // Non-trivial copy assignment.
      constexpr
      _Optional_payload&
      operator=(const _Optional_payload& __other)
      {
        this->_M_copy_assign(__other);
        return *this;
      }
 
      // Non-trivial move assignment.
      constexpr
      _Optional_payload&
      operator=(_Optional_payload&& __other)
      noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
                       is_nothrow_move_assignable<_Tp>>)
      {
        this->_M_move_assign(std::move(__other));
        return *this;
      }
    };
 
  // Payload for optionals with non-trivial destructors.
  template <typename _Tp, bool _Copy, bool _Move>
    struct _Optional_payload<_Tp, false, _Copy, _Move>
    : _Optional_payload<_Tp, true, false, false>
    {
      // Base class implements all the constructors and assignment operators:
      using _Optional_payload<_Tp, true, false, false>::_Optional_payload;
      _Optional_payload() = default;
      _Optional_payload(const _Optional_payload&) = default;
      _Optional_payload(_Optional_payload&&) = default;
      _Optional_payload& operator=(const _Optional_payload&) = default;
      _Optional_payload& operator=(_Optional_payload&&) = default;
 
      // Destructor needs to destroy the contained value:
      ~_Optional_payload() { this->_M_reset(); }
    };
 
  // Common base class for _Optional_base<T> to avoid repeating these
  // member functions in each specialization.
  template<typename _Tp, typename _Dp>
    class _Optional_base_impl
    {
    protected:
      using _Stored_type = remove_const_t<_Tp>;
 
      // The _M_construct operation has !_M_engaged as a precondition
      // while _M_destruct has _M_engaged as a precondition.
      template<typename... _Args>
        void
        _M_construct(_Args&&... __args)
        noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)
        {
          ::new
            (std::__addressof(static_cast<_Dp*>(this)->_M_payload._M_payload))
            _Stored_type(std::forward<_Args>(__args)...);
          static_cast<_Dp*>(this)->_M_payload._M_engaged = true;
        }
 
      void
      _M_destruct() noexcept
      { static_cast<_Dp*>(this)->_M_payload._M_destroy(); }
 
      // _M_reset is a 'safe' operation with no precondition.
      constexpr void
      _M_reset() noexcept
      { static_cast<_Dp*>(this)->_M_payload._M_reset(); }
 
      constexpr bool _M_is_engaged() const noexcept
      { return static_cast<const _Dp*>(this)->_M_payload._M_engaged; }
 
      // The _M_get operations have _M_engaged as a precondition.
      constexpr _Tp&
      _M_get() noexcept
      {
        __glibcxx_assert(this->_M_is_engaged());
        return static_cast<_Dp*>(this)->_M_payload._M_get();
      }
 
      constexpr const _Tp&
      _M_get() const noexcept
      {
        __glibcxx_assert(this->_M_is_engaged());
        return static_cast<const _Dp*>(this)->_M_payload._M_get();
      }
    };
 
  /**
    * @brief Class template that provides copy/move constructors of optional.
    *
    * Such a separate base class template is necessary in order to
    * conditionally make copy/move constructors trivial.
    *
    * When the contained value is trivially copy/move constructible,
    * the copy/move constructors of _Optional_base will invoke the
    * trivial copy/move constructor of _Optional_payload. Otherwise,
    * they will invoke _Optional_payload(bool, const _Optional_payload&)
    * or _Optional_payload(bool, _Optional_payload&&) to initialize
    * the contained value, if copying/moving an engaged optional.
    *
    * Whether the other special members are trivial is determined by the
    * _Optional_payload<_Tp> specialization used for the _M_payload member.
    *
    * @see optional, _Enable_special_members
    */
  template<typename _Tp,
           bool = is_trivially_copy_constructible_v<_Tp>,
           bool = is_trivially_move_constructible_v<_Tp>>
    struct _Optional_base
      : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
    {
      // Constructors for disengaged optionals.
      constexpr _Optional_base() = default;
 
      // Constructors for engaged optionals.
      template<typename... _Args,
               enable_if_t<is_constructible_v<_Tp, _Args&&...>, bool> = false>
        constexpr explicit _Optional_base(in_place_t, _Args&&... __args)
        : _M_payload(in_place,
                     std::forward<_Args>(__args)...) { }
 
      template<typename _Up, typename... _Args,
               enable_if_t<is_constructible_v<_Tp,
                                              initializer_list<_Up>&,
                                              _Args&&...>, bool> = false>
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list<_Up> __il,
                                          _Args&&... __args)
        : _M_payload(in_place,
                     __il, std::forward<_Args>(__args)...)
        { }
 
      // Copy and move constructors.
      constexpr _Optional_base(const _Optional_base& __other)
        : _M_payload(__other._M_payload._M_engaged,
                     __other._M_payload)
      { }
 
      constexpr _Optional_base(_Optional_base&& __other)
      noexcept(is_nothrow_move_constructible_v<_Tp>)
        : _M_payload(__other._M_payload._M_engaged,
                     std::move(__other._M_payload))
      { }
 
      // Assignment operators.
      _Optional_base& operator=(const _Optional_base&) = default;
      _Optional_base& operator=(_Optional_base&&) = default;
 
      _Optional_payload<_Tp> _M_payload;
    };
 
  template<typename _Tp>
    struct _Optional_base<_Tp, false, true>
      : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
    {
      // Constructors for disengaged optionals.
      constexpr _Optional_base() = default;
 
      // Constructors for engaged optionals.
      template<typename... _Args,
               enable_if_t<is_constructible_v<_Tp, _Args&&...>, bool> = false>
        constexpr explicit _Optional_base(in_place_t, _Args&&... __args)
        : _M_payload(in_place,
                     std::forward<_Args>(__args)...) { }
 
      template<typename _Up, typename... _Args,
               enable_if_t<is_constructible_v<_Tp,
                                              initializer_list<_Up>&,
                                              _Args&&...>, bool> = false>
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list<_Up> __il,
                                          _Args&&... __args)
        : _M_payload(in_place,
                     __il, std::forward<_Args>(__args)...)
        { }
 
      // Copy and move constructors.
      constexpr _Optional_base(const _Optional_base& __other)
        : _M_payload(__other._M_payload._M_engaged,
                     __other._M_payload)
      { }
 
      constexpr _Optional_base(_Optional_base&& __other) = default;
 
      // Assignment operators.
      _Optional_base& operator=(const _Optional_base&) = default;
      _Optional_base& operator=(_Optional_base&&) = default;
 
      _Optional_payload<_Tp> _M_payload;
    };
 
  template<typename _Tp>
    struct _Optional_base<_Tp, true, false>
      : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
    {
      // Constructors for disengaged optionals.
      constexpr _Optional_base() = default;
 
      // Constructors for engaged optionals.
      template<typename... _Args,
               enable_if_t<is_constructible_v<_Tp, _Args&&...>, bool> = false>
        constexpr explicit _Optional_base(in_place_t, _Args&&... __args)
        : _M_payload(in_place,
                     std::forward<_Args>(__args)...) { }
 
      template<typename _Up, typename... _Args,
               enable_if_t<is_constructible_v<_Tp,
                                              initializer_list<_Up>&,
                                              _Args&&...>, bool> = false>
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list<_Up> __il,
                                          _Args&&... __args)
        : _M_payload(in_place,
                     __il, std::forward<_Args>(__args)...)
        { }
 
      // Copy and move constructors.
      constexpr _Optional_base(const _Optional_base& __other) = default;
 
      constexpr _Optional_base(_Optional_base&& __other)
      noexcept(is_nothrow_move_constructible_v<_Tp>)
        : _M_payload(__other._M_payload._M_engaged,
                     std::move(__other._M_payload))
      { }
 
      // Assignment operators.
      _Optional_base& operator=(const _Optional_base&) = default;
      _Optional_base& operator=(_Optional_base&&) = default;
 
      _Optional_payload<_Tp> _M_payload;
    };
 
  template<typename _Tp>
    struct _Optional_base<_Tp, true, true>
      : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
    {
      // Constructors for disengaged optionals.
      constexpr _Optional_base() = default;
 
      // Constructors for engaged optionals.
      template<typename... _Args,
               enable_if_t<is_constructible_v<_Tp, _Args&&...>, bool> = false>
        constexpr explicit _Optional_base(in_place_t, _Args&&... __args)
        : _M_payload(in_place,
                     std::forward<_Args>(__args)...) { }
 
      template<typename _Up, typename... _Args,
               enable_if_t<is_constructible_v<_Tp,
                                              initializer_list<_Up>&,
                                              _Args&&...>, bool> = false>
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list<_Up> __il,
                                          _Args&&... __args)
        : _M_payload(in_place,
                     __il, std::forward<_Args>(__args)...)
        { }
 
      // Copy and move constructors.
      constexpr _Optional_base(const _Optional_base& __other) = default;
      constexpr _Optional_base(_Optional_base&& __other) = default;
 
      // Assignment operators.
      _Optional_base& operator=(const _Optional_base&) = default;
      _Optional_base& operator=(_Optional_base&&) = default;
 
      _Optional_payload<_Tp> _M_payload;
    };
 
  template<typename _Tp>
  class optional;
 
  template<typename _Tp, typename _Up>
    using __converts_from_optional =
      __or_<is_constructible<_Tp, const optional<_Up>&>,
            is_constructible<_Tp, optional<_Up>&>,
            is_constructible<_Tp, const optional<_Up>&&>,
            is_constructible<_Tp, optional<_Up>&&>,
            is_convertible<const optional<_Up>&, _Tp>,
            is_convertible<optional<_Up>&, _Tp>,
            is_convertible<const optional<_Up>&&, _Tp>,
            is_convertible<optional<_Up>&&, _Tp>>;
 
  template<typename _Tp, typename _Up>
    using __assigns_from_optional =
      __or_<is_assignable<_Tp&, const optional<_Up>&>,
            is_assignable<_Tp&, optional<_Up>&>,
            is_assignable<_Tp&, const optional<_Up>&&>,
            is_assignable<_Tp&, optional<_Up>&&>>;
 
  /**
    * @brief Class template for optional values.
    */
  template<typename _Tp>
    class optional
    : private _Optional_base<_Tp>,
      private _Enable_copy_move<
        // Copy constructor.
        is_copy_constructible_v<_Tp>,
        // Copy assignment.
        __and_v<is_copy_constructible<_Tp>, is_copy_assignable<_Tp>>,
        // Move constructor.
        is_move_constructible_v<_Tp>,
        // Move assignment.
        __and_v<is_move_constructible<_Tp>, is_move_assignable<_Tp>>,
        // Unique tag type.
        optional<_Tp>>
    {
      static_assert(!is_same_v<remove_cv_t<_Tp>, nullopt_t>);
      static_assert(!is_same_v<remove_cv_t<_Tp>, in_place_t>);
      static_assert(!is_reference_v<_Tp>);
 
    private:
      using _Base = _Optional_base<_Tp>;
 
      // SFINAE helpers
      template<typename _Up>
        using __not_self = __not_<is_same<optional, __remove_cvref_t<_Up>>>;
      template<typename _Up>
        using __not_tag = __not_<is_same<in_place_t, __remove_cvref_t<_Up>>>;
      template<typename... _Cond>
        using _Requires = enable_if_t<__and_v<_Cond...>, bool>;
 
    public:
      using value_type = _Tp;
 
      constexpr optional() = default;
 
      constexpr optional(nullopt_t) noexcept { }
 
      // Converting constructors for engaged optionals.
      template<typename _Up = _Tp,
               _Requires<__not_self<_Up>, __not_tag<_Up>,
                         is_constructible<_Tp, _Up&&>,
                         is_convertible<_Up&&, _Tp>> = true>
        constexpr
        optional(_Up&& __t)
        : _Base(std::in_place, std::forward<_Up>(__t)) { }
 
      template<typename _Up = _Tp,
               _Requires<__not_self<_Up>, __not_tag<_Up>,
                         is_constructible<_Tp, _Up&&>,
                         __not_<is_convertible<_Up&&, _Tp>>> = false>
        explicit constexpr
        optional(_Up&& __t)
        : _Base(std::in_place, std::forward<_Up>(__t)) { }
 
      template<typename _Up,
               _Requires<__not_<is_same<_Tp, _Up>>,
                         is_constructible<_Tp, const _Up&>,
                         is_convertible<const _Up&, _Tp>,
                         __not_<__converts_from_optional<_Tp, _Up>>> = true>
        constexpr
        optional(const optional<_Up>& __t)
        {
          if (__t)
            emplace(*__t);
        }
 
      template<typename _Up,
               _Requires<__not_<is_same<_Tp, _Up>>,
                         is_constructible<_Tp, const _Up&>,
                         __not_<is_convertible<const _Up&, _Tp>>,
                         __not_<__converts_from_optional<_Tp, _Up>>> = false>
        explicit constexpr
        optional(const optional<_Up>& __t)
        {
          if (__t)
            emplace(*__t);
        }
 
      template <typename _Up,
                _Requires<__not_<is_same<_Tp, _Up>>,
                          is_constructible<_Tp, _Up&&>,
                          is_convertible<_Up&&, _Tp>,
                          __not_<__converts_from_optional<_Tp, _Up>>> = true>
        constexpr
        optional(optional<_Up>&& __t)
        {
          if (__t)
            emplace(std::move(*__t));
        }
 
      template <typename _Up,
                _Requires<__not_<is_same<_Tp, _Up>>,
                          is_constructible<_Tp, _Up&&>,
                          __not_<is_convertible<_Up&&, _Tp>>,
                          __not_<__converts_from_optional<_Tp, _Up>>> = false>
        explicit constexpr
        optional(optional<_Up>&& __t)
        {
          if (__t)
            emplace(std::move(*__t));
        }
 
      template<typename... _Args,
               _Requires<is_constructible<_Tp, _Args&&...>> = false>
        explicit constexpr
        optional(in_place_t, _Args&&... __args)
        : _Base(std::in_place, std::forward<_Args>(__args)...) { }
 
      template<typename _Up, typename... _Args,
               _Requires<is_constructible<_Tp,
                                          initializer_list<_Up>&,
                                          _Args&&...>> = false>
        explicit constexpr
        optional(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
        : _Base(std::in_place, __il, std::forward<_Args>(__args)...) { }
 
      // Assignment operators.
      optional&
      operator=(nullopt_t) noexcept
      {
        this->_M_reset();
        return *this;
      }
 
      template<typename _Up = _Tp>
        enable_if_t<__and_v<__not_self<_Up>,
                            __not_<__and_<is_scalar<_Tp>,
                                          is_same<_Tp, decay_t<_Up>>>>,
                            is_constructible<_Tp, _Up>,
                            is_assignable<_Tp&, _Up>>,
                    optional&>
        operator=(_Up&& __u)
        {
          if (this->_M_is_engaged())
            this->_M_get() = std::forward<_Up>(__u);
          else
            this->_M_construct(std::forward<_Up>(__u));
 
          return *this;
        }
 
      template<typename _Up>
        enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
                            is_constructible<_Tp, const _Up&>,
                            is_assignable<_Tp&, const _Up&>,
                            __not_<__converts_from_optional<_Tp, _Up>>,
                            __not_<__assigns_from_optional<_Tp, _Up>>>,
                    optional&>
        operator=(const optional<_Up>& __u)
        {
          if (__u)
            {
              if (this->_M_is_engaged())
                this->_M_get() = *__u;
              else
                this->_M_construct(*__u);
            }
          else
            {
              this->_M_reset();
            }
          return *this;
        }
 
      template<typename _Up>
        enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
                            is_constructible<_Tp, _Up>,
                            is_assignable<_Tp&, _Up>,
                            __not_<__converts_from_optional<_Tp, _Up>>,
                            __not_<__assigns_from_optional<_Tp, _Up>>>,
                    optional&>
        operator=(optional<_Up>&& __u)
        {
          if (__u)
            {
              if (this->_M_is_engaged())
                this->_M_get() = std::move(*__u);
              else
                this->_M_construct(std::move(*__u));
            }
          else
            {
              this->_M_reset();
            }
 
          return *this;
        }
 
      template<typename... _Args>
        enable_if_t<is_constructible_v<_Tp, _Args&&...>, _Tp&>
        emplace(_Args&&... __args)
        {
          this->_M_reset();
          this->_M_construct(std::forward<_Args>(__args)...);
          return this->_M_get();
        }
 
      template<typename _Up, typename... _Args>
        enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&,
                                       _Args&&...>, _Tp&>
        emplace(initializer_list<_Up> __il, _Args&&... __args)
        {
          this->_M_reset();
          this->_M_construct(__il, std::forward<_Args>(__args)...);
          return this->_M_get();
        }
 
      // Destructor is implicit, implemented in _Optional_base.
 
      // Swap.
      void
      swap(optional& __other)
      noexcept(is_nothrow_move_constructible_v<_Tp>
               && is_nothrow_swappable_v<_Tp>)
      {
        using std::swap;
 
        if (this->_M_is_engaged() && __other._M_is_engaged())
          swap(this->_M_get(), __other._M_get());
        else if (this->_M_is_engaged())
          {
            __other._M_construct(std::move(this->_M_get()));
            this->_M_destruct();
          }
        else if (__other._M_is_engaged())
          {
            this->_M_construct(std::move(__other._M_get()));
            __other._M_destruct();
          }
      }
 
      // Observers.
      constexpr const _Tp*
      operator->() const
      { return std::__addressof(this->_M_get()); }
 
      constexpr _Tp*
      operator->()
      { return std::__addressof(this->_M_get()); }
 
      constexpr const _Tp&
      operator*() const&
      { return this->_M_get(); }
 
      constexpr _Tp&
      operator*()&
      { return this->_M_get(); }
 
      constexpr _Tp&&
      operator*()&&
      { return std::move(this->_M_get()); }
 
      constexpr const _Tp&&
      operator*() const&&
      { return std::move(this->_M_get()); }
 
      constexpr explicit operator bool() const noexcept
      { return this->_M_is_engaged(); }
 
      constexpr bool has_value() const noexcept
      { return this->_M_is_engaged(); }
 
      constexpr const _Tp&
      value() const&
      {
        return this->_M_is_engaged()
          ? this->_M_get()
          : (__throw_bad_optional_access(), this->_M_get());
      }
 
      constexpr _Tp&
      value()&
      {
        return this->_M_is_engaged()
          ? this->_M_get()
          : (__throw_bad_optional_access(), this->_M_get());
      }
 
      constexpr _Tp&&
      value()&&
      {
        return this->_M_is_engaged()
          ? std::move(this->_M_get())
          : (__throw_bad_optional_access(), std::move(this->_M_get()));
      }
 
      constexpr const _Tp&&
      value() const&&
      {
        return this->_M_is_engaged()
          ? std::move(this->_M_get())
          : (__throw_bad_optional_access(), std::move(this->_M_get()));
      }
 
      template<typename _Up>
        constexpr _Tp
        value_or(_Up&& __u) const&
        {
          static_assert(is_copy_constructible_v<_Tp>);
          static_assert(is_convertible_v<_Up&&, _Tp>);
 
          return this->_M_is_engaged()
            ? this->_M_get() : static_cast<_Tp>(std::forward<_Up>(__u));
        }
 
      template<typename _Up>
        constexpr _Tp
        value_or(_Up&& __u) &&
        {
          static_assert(is_move_constructible_v<_Tp>);
          static_assert(is_convertible_v<_Up&&, _Tp>);
 
          return this->_M_is_engaged()
            ? std::move(this->_M_get())
            : static_cast<_Tp>(std::forward<_Up>(__u));
        }
 
      void reset() noexcept { this->_M_reset(); }
    };
 
  template<typename _Tp>
    using __optional_relop_t =
      enable_if_t<is_convertible<_Tp, bool>::value, bool>;
 
  template<typename _Tp, typename _Up>
    using __optional_eq_t = __optional_relop_t<
      decltype(std::declval<const _Tp&>() == std::declval<const _Up&>())
      >;
 
  template<typename _Tp, typename _Up>
    using __optional_ne_t = __optional_relop_t<
      decltype(std::declval<const _Tp&>() != std::declval<const _Up&>())
      >;
 
  template<typename _Tp, typename _Up>
    using __optional_lt_t = __optional_relop_t<
      decltype(std::declval<const _Tp&>() < std::declval<const _Up&>())
      >;
 
  template<typename _Tp, typename _Up>
    using __optional_gt_t = __optional_relop_t<
      decltype(std::declval<const _Tp&>() > std::declval<const _Up&>())
      >;
 
  template<typename _Tp, typename _Up>
    using __optional_le_t = __optional_relop_t<
      decltype(std::declval<const _Tp&>() <= std::declval<const _Up&>())
      >;
 
  template<typename _Tp, typename _Up>
    using __optional_ge_t = __optional_relop_t<
      decltype(std::declval<const _Tp&>() >= std::declval<const _Up&>())
      >;
 
  // Comparisons between optional values.
  template<typename _Tp, typename _Up>
    constexpr auto
    operator==(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
    -> __optional_eq_t<_Tp, _Up>
    {
      return static_cast<bool>(__lhs) == static_cast<bool>(__rhs)
             && (!__lhs || *__lhs == *__rhs);
    }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator!=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
    -> __optional_ne_t<_Tp, _Up>
    {
      return static_cast<bool>(__lhs) != static_cast<bool>(__rhs)
        || (static_cast<bool>(__lhs) && *__lhs != *__rhs);
    }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator<(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
    -> __optional_lt_t<_Tp, _Up>
    {
      return static_cast<bool>(__rhs) && (!__lhs || *__lhs < *__rhs);
    }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator>(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
    -> __optional_gt_t<_Tp, _Up>
    {
      return static_cast<bool>(__lhs) && (!__rhs || *__lhs > *__rhs);
    }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator<=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
    -> __optional_le_t<_Tp, _Up>
    {
      return !__lhs || (static_cast<bool>(__rhs) && *__lhs <= *__rhs);
    }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator>=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
    -> __optional_ge_t<_Tp, _Up>
    {
      return !__rhs || (static_cast<bool>(__lhs) && *__lhs >= *__rhs);
    }
 
#ifdef __cpp_lib_three_way_comparison
  template<typename _Tp, three_way_comparable_with<_Tp> _Up>
    constexpr compare_three_way_result_t<_Tp, _Up>
    operator<=>(const optional<_Tp>& __x, const optional<_Up>& __y)
    {
      return __x && __y ? *__x <=> *__y : bool(__x) <=> bool(__y);
    }
#endif
 
  // Comparisons with nullopt.
  template<typename _Tp>
    constexpr bool
    operator==(const optional<_Tp>& __lhs, nullopt_t) noexcept
    { return !__lhs; }
 
#ifdef __cpp_lib_three_way_comparison
  template<typename _Tp>
    constexpr strong_ordering
    operator<=>(const optional<_Tp>& __x, nullopt_t) noexcept
    { return bool(__x) <=> false; }
#else
  template<typename _Tp>
    constexpr bool
    operator==(nullopt_t, const optional<_Tp>& __rhs) noexcept
    { return !__rhs; }
 
  template<typename _Tp>
    constexpr bool
    operator!=(const optional<_Tp>& __lhs, nullopt_t) noexcept
    { return static_cast<bool>(__lhs); }
 
  template<typename _Tp>
    constexpr bool
    operator!=(nullopt_t, const optional<_Tp>& __rhs) noexcept
    { return static_cast<bool>(__rhs); }
 
  template<typename _Tp>
    constexpr bool
    operator<(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept
    { return false; }
 
  template<typename _Tp>
    constexpr bool
    operator<(nullopt_t, const optional<_Tp>& __rhs) noexcept
    { return static_cast<bool>(__rhs); }
 
  template<typename _Tp>
    constexpr bool
    operator>(const optional<_Tp>& __lhs, nullopt_t) noexcept
    { return static_cast<bool>(__lhs); }
 
  template<typename _Tp>
    constexpr bool
    operator>(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept
    { return false; }
 
  template<typename _Tp>
    constexpr bool
    operator<=(const optional<_Tp>& __lhs, nullopt_t) noexcept
    { return !__lhs; }
 
  template<typename _Tp>
    constexpr bool
    operator<=(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept
    { return true; }
 
  template<typename _Tp>
    constexpr bool
    operator>=(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept
    { return true; }
 
  template<typename _Tp>
    constexpr bool
    operator>=(nullopt_t, const optional<_Tp>& __rhs) noexcept
    { return !__rhs; }
#endif // three-way-comparison
 
  // Comparisons with value type.
  template<typename _Tp, typename _Up>
    constexpr auto
    operator==(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_eq_t<_Tp, _Up>
    { return __lhs && *__lhs == __rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator==(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_eq_t<_Up, _Tp>
    { return __rhs && __lhs == *__rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator!=(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_ne_t<_Tp, _Up>
    { return !__lhs || *__lhs != __rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator!=(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_ne_t<_Up, _Tp>
    { return !__rhs || __lhs != *__rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator<(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_lt_t<_Tp, _Up>
    { return !__lhs || *__lhs < __rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator<(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_lt_t<_Up, _Tp>
    { return __rhs && __lhs < *__rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator>(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_gt_t<_Tp, _Up>
    { return __lhs && *__lhs > __rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator>(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_gt_t<_Up, _Tp>
    { return !__rhs || __lhs > *__rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator<=(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_le_t<_Tp, _Up>
    { return !__lhs || *__lhs <= __rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator<=(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_le_t<_Up, _Tp>
    { return __rhs && __lhs <= *__rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator>=(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_ge_t<_Tp, _Up>
    { return __lhs && *__lhs >= __rhs; }
 
  template<typename _Tp, typename _Up>
    constexpr auto
    operator>=(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_ge_t<_Up, _Tp>
    { return !__rhs || __lhs >= *__rhs; }
 
#ifdef __cpp_lib_three_way_comparison
  template<typename _Tp>
    inline constexpr bool __is_optional_v = false;
  template<typename _Tp>
    inline constexpr bool __is_optional_v<optional<_Tp>> = true;
 
  template<typename _Tp, typename _Up>
    requires (!__is_optional_v<_Up>)
      && three_way_comparable_with<_Tp, _Up>
    constexpr compare_three_way_result_t<_Tp, _Up>
    operator<=>(const optional<_Tp>& __x, const _Up& __v)
    { return bool(__x) ? *__x <=> __v : strong_ordering::less; }
#endif
 
  // Swap and creation functions.
 
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2748. swappable traits for optionals
  template<typename _Tp>
    inline enable_if_t<is_move_constructible_v<_Tp> && is_swappable_v<_Tp>>
    swap(optional<_Tp>& __lhs, optional<_Tp>& __rhs)
    noexcept(noexcept(__lhs.swap(__rhs)))
    { __lhs.swap(__rhs); }
 
  template<typename _Tp>
    enable_if_t<!(is_move_constructible_v<_Tp> && is_swappable_v<_Tp>)>
    swap(optional<_Tp>&, optional<_Tp>&) = delete;
 
  template<typename _Tp>
    constexpr optional<decay_t<_Tp>>
    make_optional(_Tp&& __t)
    { return optional<decay_t<_Tp>> { std::forward<_Tp>(__t) }; }
 
  template<typename _Tp, typename ..._Args>
    constexpr optional<_Tp>
    make_optional(_Args&&... __args)
    { return optional<_Tp> { in_place, std::forward<_Args>(__args)... }; }
 
  template<typename _Tp, typename _Up, typename ..._Args>
    constexpr optional<_Tp>
    make_optional(initializer_list<_Up> __il, _Args&&... __args)
    { return optional<_Tp> { in_place, __il, std::forward<_Args>(__args)... }; }
 
  // Hash.
 
  template<typename _Tp, typename _Up = remove_const_t<_Tp>,
           bool = __poison_hash<_Up>::__enable_hash_call>
    struct __optional_hash_call_base
    {
      size_t
      operator()(const optional<_Tp>& __t) const
      noexcept(noexcept(hash<_Up>{}(*__t)))
      {
        // We pick an arbitrary hash for disengaged optionals which hopefully
        // usual values of _Tp won't typically hash to.
        constexpr size_t __magic_disengaged_hash = static_cast<size_t>(-3333);
        return __t ? hash<_Up>{}(*__t) : __magic_disengaged_hash;
      }
    };
 
  template<typename _Tp, typename _Up>
    struct __optional_hash_call_base<_Tp, _Up, false> {};
 
  template<typename _Tp>
    struct hash<optional<_Tp>>
    : private __poison_hash<remove_const_t<_Tp>>,
      public __optional_hash_call_base<_Tp>
    {
      using result_type [[__deprecated__]] = size_t;
      using argument_type [[__deprecated__]] = optional<_Tp>;
    };
 
  template<typename _Tp>
    struct __is_fast_hash<hash<optional<_Tp>>> : __is_fast_hash<hash<_Tp>>
    { };
 
  /// @}
 
#if __cpp_deduction_guides >= 201606
  template <typename _Tp> optional(_Tp) -> optional<_Tp>;
#endif
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
 
#endif // C++17
 
#endif // _GLIBCXX_OPTIONAL