experimental/optional

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// <experimental/optional> -*- C++ -*-
 
// Copyright (C) 2013-2023 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 experimental/optional
 *  This is a TS C++ Library header.
 *  @ingroup libfund-ts
 */
 
#ifndef _GLIBCXX_EXPERIMENTAL_OPTIONAL
#define _GLIBCXX_EXPERIMENTAL_OPTIONAL 1
 
#include <bits/requires_hosted.h> // experimental is currently omitted
 
#if __cplusplus >= 201402L
 
#include <type_traits>
#include <stdexcept>
#include <new>
#include <initializer_list>
#include <bits/functexcept.h>
#include <bits/functional_hash.h>
#include <bits/enable_special_members.h>
#include <bits/move.h>
#include <experimental/bits/lfts_config.h>
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
namespace experimental
{
inline namespace fundamentals_v1
{
  /**
   * @defgroup optional Optional values
   * @ingroup libfund-ts
   *
   * Class template for optional values and surrounding facilities, as
   * described in n3793 "A proposal to add a utility class to represent
   * optional objects (Revision 5)".
   *
   * @{
   */
 
#define __cpp_lib_experimental_optional 201411
 
  // All subsequent [X.Y.n] references are against n3793.
 
  // [X.Y.4]
  template<typename _Tp>
    class optional;
 
  // [X.Y.5]
  /// Tag type for in-place construction.
  struct in_place_t { };
 
  /// Tag for in-place construction.
  constexpr in_place_t in_place { };
 
  // [X.Y.6]
  /// 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) { }
  };
 
  // [X.Y.6]
  /// Tag to disengage optional objects.
  constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };
 
  // [X.Y.7]
  /**
   *  @brief Exception class thrown when a disengaged optional object is
   *  dereferenced.
   *  @ingroup exceptions
   */
  class bad_optional_access : public logic_error
  {
  public:
    bad_optional_access() : logic_error("bad optional access") { }
 
    // XXX This constructor is non-standard. Should not be inline
    explicit bad_optional_access(const char* __arg) : logic_error(__arg) { }
 
    virtual ~bad_optional_access() noexcept = default;
  };
 
  /// @cond undocumented
 
  // XXX Does not belong here.
  [[noreturn]] inline void
  __throw_bad_optional_access(const char* __s)
  { _GLIBCXX_THROW_OR_ABORT(bad_optional_access(__s)); }
 
  /**
    * @brief Class template that holds the necessary state for @ref optional
    * and that has the responsibility for construction and the special members.
    *
    * Such a separate base class template is necessary in order to
    * conditionally enable the special members (e.g. copy/move constructors).
    * Note that this means that @ref _Optional_base implements the
    * functionality for copy and move assignment, but not for converting
    * assignment.
    *
    * @see optional, _Enable_special_members
    */
  template<typename _Tp, bool _ShouldProvideDestructor =
           !is_trivially_destructible<_Tp>::value>
    class _Optional_base
    {
    private:
      // Remove const to avoid prohibition of reusing object storage for
      // const-qualified types in [3.8/9]. This is strictly internal
      // and even optional itself is oblivious to it.
      using _Stored_type = remove_const_t<_Tp>;
 
    public:
      // [X.Y.4.1] Constructors.
 
      // Constructors for disengaged optionals.
      constexpr _Optional_base() noexcept
      : _M_empty{} { }
 
      constexpr _Optional_base(nullopt_t) noexcept
      : _Optional_base{} { }
 
      // Constructors for engaged optionals.
      template<typename... _Args>
        constexpr explicit _Optional_base(in_place_t, _Args&&... __args)
        : _M_payload(std::forward<_Args>(__args)...), _M_engaged(true) { }
 
      template<typename _Up, typename... _Args,
               enable_if_t<is_constructible<_Tp,
                                            initializer_list<_Up>&,
                                            _Args&&...>::value,
                           int>...>
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list<_Up> __il,
                                          _Args&&... __args)
        : _M_payload(__il, std::forward<_Args>(__args)...),
          _M_engaged(true) { }
 
      // Copy and move constructors.
      _Optional_base(const _Optional_base& __other)
      {
        if (__other._M_engaged)
          this->_M_construct(__other._M_get());
      }
 
      _Optional_base(_Optional_base&& __other)
      noexcept(is_nothrow_move_constructible<_Tp>())
      {
        if (__other._M_engaged)
          this->_M_construct(std::move(__other._M_get()));
      }
 
      // [X.Y.4.3] (partly) Assignment.
      _Optional_base&
      operator=(const _Optional_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();
          }
 
        return *this;
      }
 
      _Optional_base&
      operator=(_Optional_base&& __other)
      noexcept(__and_<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();
          }
        return *this;
      }
 
      // [X.Y.4.2] Destructor.
      ~_Optional_base()
      {
        if (this->_M_engaged)
          this->_M_payload.~_Stored_type();
      }
 
      // The following functionality is also needed by optional, hence the
      // protected accessibility.
    protected:
      constexpr bool _M_is_engaged() const noexcept
      { return this->_M_engaged; }
 
      // The _M_get operations have _M_engaged as a precondition.
      constexpr _Tp&
      _M_get() noexcept
      { return _M_payload; }
 
      constexpr const _Tp&
      _M_get() const noexcept
      { return _M_payload; }
 
      // 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<_Stored_type, _Args...>())
        {
          ::new (std::__addressof(this->_M_payload))
            _Stored_type(std::forward<_Args>(__args)...);
          this->_M_engaged = true;
        }
 
      void
      _M_destruct()
      {
        this->_M_engaged = false;
        this->_M_payload.~_Stored_type();
      }
 
      // _M_reset is a 'safe' operation with no precondition.
      void
      _M_reset()
      {
        if (this->_M_engaged)
          this->_M_destruct();
      }
 
    private:
      struct _Empty_byte { };
      union {
          _Empty_byte _M_empty;
          _Stored_type _M_payload;
      };
      bool _M_engaged = false;
    };
 
  /// Partial specialization that is exactly identical to the primary template
  /// save for not providing a destructor, to fulfill triviality requirements.
  template<typename _Tp>
    class _Optional_base<_Tp, false>
    {
    private:
      using _Stored_type = remove_const_t<_Tp>;
 
    public:
      constexpr _Optional_base() noexcept
      : _M_empty{} { }
 
      constexpr _Optional_base(nullopt_t) noexcept
      : _Optional_base{} { }
 
      template<typename... _Args>
        constexpr explicit _Optional_base(in_place_t, _Args&&... __args)
        : _M_payload(std::forward<_Args>(__args)...), _M_engaged(true) { }
 
      template<typename _Up, typename... _Args,
               enable_if_t<is_constructible<_Tp,
                                            initializer_list<_Up>&,
                                            _Args&&...>::value,
                           int>...>
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list<_Up> __il,
                                          _Args&&... __args)
        : _M_payload(__il, std::forward<_Args>(__args)...),
          _M_engaged(true) { }
 
      _Optional_base(const _Optional_base& __other)
      {
        if (__other._M_engaged)
          this->_M_construct(__other._M_get());
      }
 
      _Optional_base(_Optional_base&& __other)
      noexcept(is_nothrow_move_constructible<_Tp>())
      {
        if (__other._M_engaged)
          this->_M_construct(std::move(__other._M_get()));
      }
 
      _Optional_base&
      operator=(const _Optional_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();
          }
        return *this;
      }
 
      _Optional_base&
      operator=(_Optional_base&& __other)
      noexcept(__and_<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();
          }
        return *this;
      }
 
      // Sole difference
      // ~_Optional_base() noexcept = default;
 
    protected:
      constexpr bool _M_is_engaged() const noexcept
      { return this->_M_engaged; }
 
      _Tp&
      _M_get() noexcept
      { return _M_payload; }
 
      constexpr const _Tp&
      _M_get() const noexcept
      { return _M_payload; }
 
      template<typename... _Args>
        void
        _M_construct(_Args&&... __args)
        noexcept(is_nothrow_constructible<_Stored_type, _Args...>())
        {
          ::new (std::__addressof(this->_M_payload))
            _Stored_type(std::forward<_Args>(__args)...);
          this->_M_engaged = true;
        }
 
      void
      _M_destruct()
      {
        this->_M_engaged = false;
        this->_M_payload.~_Stored_type();
      }
 
      void
      _M_reset()
      {
        if (this->_M_engaged)
          this->_M_destruct();
      }
 
    private:
      struct _Empty_byte { };
      union
      {
        _Empty_byte _M_empty;
        _Stored_type _M_payload;
      };
      bool _M_engaged = false;
    };
 
  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>&&>>;
 
  /// @endcond
 
  /**
    * @brief Class template for optional values.
    */
  template<typename _Tp>
    class optional
    : private _Optional_base<_Tp>,
      private _Enable_copy_move<
        // Copy constructor.
        is_copy_constructible<_Tp>::value,
        // Copy assignment.
        __and_<is_copy_constructible<_Tp>, is_copy_assignable<_Tp>>::value,
        // Move constructor.
        is_move_constructible<_Tp>::value,
        // Move assignment.
        __and_<is_move_constructible<_Tp>, is_move_assignable<_Tp>>::value,
        // Unique tag type.
        optional<_Tp>>
    {
      static_assert(__and_<__not_<is_same<remove_cv_t<_Tp>, nullopt_t>>,
                           __not_<is_same<remove_cv_t<_Tp>, in_place_t>>,
                           __not_<is_reference<_Tp>>>(),
                    "Invalid instantiation of optional<T>");
 
    private:
      using _Base = _Optional_base<_Tp>;
 
    public:
      using value_type = _Tp;
 
      // _Optional_base has the responsibility for construction.
      using _Base::_Base;
 
      constexpr optional() = default;
      // Converting constructors for engaged optionals.
      template <typename _Up = _Tp,
                enable_if_t<__and_<
                              __not_<is_same<optional<_Tp>, decay_t<_Up>>>,
                              is_constructible<_Tp, _Up&&>,
                              is_convertible<_Up&&, _Tp>
                              >::value, bool> = true>
      constexpr optional(_Up&& __t)
        : _Base(in_place, std::forward<_Up>(__t)) { }
 
      template <typename _Up = _Tp,
                enable_if_t<__and_<
                              __not_<is_same<optional<_Tp>, decay_t<_Up>>>,
                              is_constructible<_Tp, _Up&&>,
                              __not_<is_convertible<_Up&&, _Tp>>
                              >::value, bool> = false>
      explicit constexpr optional(_Up&& __t)
        : _Base(in_place, std::forward<_Up>(__t)) { }
 
      template <typename _Up,
                enable_if_t<__and_<
                            __not_<is_same<_Tp, _Up>>,
                            is_constructible<_Tp, const _Up&>,
                            is_convertible<const _Up&, _Tp>,
                            __not_<__converts_from_optional<_Tp, _Up>>
                            >::value, bool> = true>
      constexpr optional(const optional<_Up>& __t)
      {
        if (__t)
          emplace(*__t);
      }
 
      template <typename _Up,
                 enable_if_t<__and_<
                               __not_<is_same<_Tp, _Up>>,
                               is_constructible<_Tp, const _Up&>,
                               __not_<is_convertible<const _Up&, _Tp>>,
                               __not_<__converts_from_optional<_Tp, _Up>>
                               >::value, bool> = false>
      explicit constexpr optional(const optional<_Up>& __t)
      {
        if (__t)
          emplace(*__t);
      }
 
      template <typename _Up,
                enable_if_t<__and_<
                              __not_<is_same<_Tp, _Up>>,
                              is_constructible<_Tp, _Up&&>,
                              is_convertible<_Up&&, _Tp>,
                              __not_<__converts_from_optional<_Tp, _Up>>
                              >::value, bool> = true>
      constexpr optional(optional<_Up>&& __t)
      {
        if (__t)
          emplace(std::move(*__t));
      }
 
      template <typename _Up,
                enable_if_t<__and_<
                            __not_<is_same<_Tp, _Up>>,
                            is_constructible<_Tp, _Up&&>,
                            __not_<is_convertible<_Up&&, _Tp>>,
                            __not_<__converts_from_optional<_Tp, _Up>>
                            >::value, bool> = false>
      explicit constexpr optional(optional<_Up>&& __t)
      {
        if (__t)
          emplace(std::move(*__t));
      }
 
      // [X.Y.4.3] (partly) Assignment.
      optional&
      operator=(nullopt_t) noexcept
      {
        this->_M_reset();
        return *this;
      }
 
      template<typename _Up = _Tp>
        enable_if_t<__and_<
                      __not_<is_same<optional<_Tp>, decay_t<_Up>>>,
                      is_constructible<_Tp, _Up>,
                      __not_<__and_<is_scalar<_Tp>,
                                    is_same<_Tp, decay_t<_Up>>>>,
                      is_assignable<_Tp&, _Up>>::value,
                    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_<
                      __not_<is_same<_Tp, _Up>>,
                      is_constructible<_Tp, const _Up&>,
                      is_assignable<_Tp&, _Up>,
                      __not_<__converts_from_optional<_Tp, _Up>>,
                      __not_<__assigns_from_optional<_Tp, _Up>>
                      >::value,
                    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_<
                      __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>>
                      >::value,
                    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<_Tp, _Args&&...>::value>
        emplace(_Args&&... __args)
        {
          this->_M_reset();
          this->_M_construct(std::forward<_Args>(__args)...);
        }
 
      template<typename _Up, typename... _Args>
        enable_if_t<is_constructible<_Tp, initializer_list<_Up>&,
                                     _Args&&...>::value>
        emplace(initializer_list<_Up> __il, _Args&&... __args)
        {
          this->_M_reset();
          this->_M_construct(__il, std::forward<_Args>(__args)...);
        }
 
      // [X.Y.4.2] Destructor is implicit, implemented in _Optional_base.
 
      // [X.Y.4.4] Swap.
      void
      swap(optional& __other)
      noexcept(is_nothrow_move_constructible<_Tp>()
               && __is_nothrow_swappable<_Tp>::value)
      {
        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();
          }
      }
 
      // [X.Y.4.5] Observers.
      constexpr const _Tp*
      operator->() const
      { return std::__addressof(this->_M_get()); }
 
      _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 const _Tp&
      value() const&
      {
        if (this->_M_is_engaged())
          return this->_M_get();
        __throw_bad_optional_access("Attempt to access value of a "
                                    "disengaged optional object");
      }
 
      constexpr _Tp&
      value()&
      {
        if (this->_M_is_engaged())
          return this->_M_get();
        __throw_bad_optional_access("Attempt to access value of a "
                                    "disengaged optional object");
      }
 
      constexpr _Tp&&
      value()&&
      {
        if (this->_M_is_engaged())
          return std::move(this->_M_get());
        __throw_bad_optional_access("Attempt to access value of a "
                                    "disengaged optional object");
      }
 
      constexpr const _Tp&&
      value() const&&
      {
        if (this->_M_is_engaged())
          return std::move(this->_M_get());
        __throw_bad_optional_access("Attempt to access value of a "
                                    "disengaged optional object");
      }
 
      template<typename _Up>
        constexpr _Tp
        value_or(_Up&& __u) const&
        {
          static_assert(__and_<is_copy_constructible<_Tp>,
                               is_convertible<_Up&&, _Tp>>(),
                        "Cannot return value");
 
          if (this->_M_is_engaged())
            return this->_M_get();
          else
            return static_cast<_Tp>(std::forward<_Up>(__u));
        }
 
      template<typename _Up>
        _Tp
        value_or(_Up&& __u) &&
        {
          static_assert(__and_<is_move_constructible<_Tp>,
                               is_convertible<_Up&&, _Tp>>(),
                        "Cannot return value" );
 
          if (this->_M_is_engaged())
            return std::move(this->_M_get());
          else
            return static_cast<_Tp>(std::forward<_Up>(__u));
        }
    };
 
  /// @relates experimental::optional @{
 
  // [X.Y.8] Comparisons between optional values.
  template<typename _Tp>
    constexpr bool
    operator==(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
    {
      return static_cast<bool>(__lhs) == static_cast<bool>(__rhs)
             && (!__lhs || *__lhs == *__rhs);
    }
 
  template<typename _Tp>
    constexpr bool
    operator!=(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
    { return !(__lhs == __rhs); }
 
  template<typename _Tp>
    constexpr bool
    operator<(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
    {
      return static_cast<bool>(__rhs) && (!__lhs || *__lhs < *__rhs);
    }
 
  template<typename _Tp>
    constexpr bool
    operator>(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
    { return __rhs < __lhs; }
 
  template<typename _Tp>
    constexpr bool
    operator<=(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
    { return !(__rhs < __lhs); }
 
  template<typename _Tp>
    constexpr bool
    operator>=(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
    { return !(__lhs < __rhs); }
 
  // [X.Y.9] Comparisons with nullopt.
  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 !__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; }
 
  // [X.Y.10] Comparisons with value type.
  template<typename _Tp>
    constexpr bool
    operator==(const optional<_Tp>& __lhs, const _Tp& __rhs)
    { return __lhs && *__lhs == __rhs; }
 
  template<typename _Tp>
    constexpr bool
    operator==(const _Tp& __lhs, const optional<_Tp>& __rhs)
    { return __rhs && __lhs == *__rhs; }
 
  template<typename _Tp>
    constexpr bool
    operator!=(const optional<_Tp>& __lhs, _Tp const& __rhs)
    { return !__lhs || !(*__lhs == __rhs); }
 
  template<typename _Tp>
    constexpr bool
    operator!=(const _Tp& __lhs, const optional<_Tp>& __rhs)
    { return !__rhs || !(__lhs == *__rhs); }
 
  template<typename _Tp>
    constexpr bool
    operator<(const optional<_Tp>& __lhs, const _Tp& __rhs)
    { return !__lhs || *__lhs < __rhs; }
 
  template<typename _Tp>
    constexpr bool
    operator<(const _Tp& __lhs, const optional<_Tp>& __rhs)
    { return __rhs && __lhs < *__rhs; }
 
  template<typename _Tp>
    constexpr bool
    operator>(const optional<_Tp>& __lhs, const _Tp& __rhs)
    { return __lhs && __rhs < *__lhs; }
 
  template<typename _Tp>
    constexpr bool
    operator>(const _Tp& __lhs, const optional<_Tp>& __rhs)
    { return !__rhs || *__rhs < __lhs; }
 
  template<typename _Tp>
    constexpr bool
    operator<=(const optional<_Tp>& __lhs, const _Tp& __rhs)
    { return !__lhs || !(__rhs < *__lhs); }
 
  template<typename _Tp>
    constexpr bool
    operator<=(const _Tp& __lhs, const optional<_Tp>& __rhs)
    { return __rhs && !(*__rhs < __lhs); }
 
  template<typename _Tp>
    constexpr bool
    operator>=(const optional<_Tp>& __lhs, const _Tp& __rhs)
    { return __lhs && !(*__lhs < __rhs); }
 
  template<typename _Tp>
    constexpr bool
    operator>=(const _Tp& __lhs, const optional<_Tp>& __rhs)
    { return !__rhs || !(__lhs < *__rhs); }
 
  // [X.Y.11]
  template<typename _Tp>
    inline void
    swap(optional<_Tp>& __lhs, optional<_Tp>& __rhs)
    noexcept(noexcept(__lhs.swap(__rhs)))
    { __lhs.swap(__rhs); }
 
  template<typename _Tp>
    constexpr optional<decay_t<_Tp>>
    make_optional(_Tp&& __t)
    { return optional<decay_t<_Tp>> { std::forward<_Tp>(__t) }; }
 
  /// @} relates experimental::optional
  /// @} group optional
} // namespace fundamentals_v1
} // namespace experimental
 
  // [X.Y.12]
  /// std::hash partial specialization for experimental::optional
  /// @relates experimental::optional
  template<typename _Tp>
    struct hash<experimental::optional<_Tp>>
    {
      using result_type = size_t;
      using argument_type = experimental::optional<_Tp>;
 
      size_t
      operator()(const experimental::optional<_Tp>& __t) const
      noexcept(noexcept(hash<_Tp> {}(*__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<_Tp> {}(*__t) : __magic_disengaged_hash;
      }
    };
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
 
#endif // C++14
 
#endif // _GLIBCXX_EXPERIMENTAL_OPTIONAL