// <optional> -*- C++ -*-

// Copyright (C) 2013-2018 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 <stdexcept>
#include <new>
#include <initializer_list>
#include <bits/functexcept.h>
#include <bits/functional_hash.h>
#include <bits/enable_special_members.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  /**
   *  @addtogroup utilities
   *  @{
   */

#define __cpp_lib_optional 201603

  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() { }

    virtual const char* what() const noexcept override
    { return "bad optional access"; }

    virtual ~bad_optional_access() noexcept = default;
  };

  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()); }


  // Payload for optionals with non-trivial destructor.
  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
    {
      constexpr _Optional_payload() noexcept : _M_empty() { }

      template <typename... _Args>
	constexpr
	_Optional_payload(in_place_t, _Args&&... __args)
	: _M_payload(std::forward<_Args>(__args)...), _M_engaged(true) { }

      template<typename _Up, typename... _Args>
	constexpr
	_Optional_payload(std::initializer_list<_Up> __il,
			  _Args&&... __args)
	: _M_payload(__il, std::forward<_Args>(__args)...),
	  _M_engaged(true)
	{ }

      constexpr
      _Optional_payload(bool __engaged, const _Optional_payload& __other)
      : _Optional_payload(__other)
      { }

      constexpr
      _Optional_payload(bool __engaged, _Optional_payload&& __other)
      : _Optional_payload(std::move(__other))
      { }

      constexpr
      _Optional_payload(const _Optional_payload& __other)
      {
	if (__other._M_engaged)
	  this->_M_construct(__other._M_payload);
      }

      constexpr
      _Optional_payload(_Optional_payload&& __other)
      {
	if (__other._M_engaged)
	  this->_M_construct(std::move(__other._M_payload));
      }

      constexpr
      _Optional_payload&
      operator=(const _Optional_payload& __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;
      }

      constexpr
      _Optional_payload&
      operator=(_Optional_payload&& __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;
      }

      using _Stored_type = remove_const_t<_Tp>;

      struct _Empty_byte { };

      union {
          _Empty_byte _M_empty;
          _Stored_type _M_payload;
      };
      bool _M_engaged = false;

      ~_Optional_payload()
      {
        if (_M_engaged)
          _M_payload.~_Stored_type();
      }

      template<typename... _Args>
        void
        _M_construct(_Args&&... __args)
        noexcept(is_nothrow_constructible<_Stored_type, _Args...>())
        {
          ::new ((void *) std::__addressof(this->_M_payload))
            _Stored_type(std::forward<_Args>(__args)...);
          this->_M_engaged = true;
        }

      // The _M_get operations have _M_engaged as a precondition.
      constexpr _Tp&
      _M_get() noexcept
      { return this->_M_payload; }

      constexpr const _Tp&
      _M_get() const noexcept
      { return this->_M_payload; }

      // _M_reset is a 'safe' operation with no precondition.
      constexpr
      void
      _M_reset() noexcept
      {
	if (this->_M_engaged)
	  {
	    this->_M_engaged = false;
	    this->_M_payload.~_Stored_type();
	  }
      }
    };

  // Payload for potentially-constexpr optionals.
  template <typename _Tp>
    struct _Optional_payload<_Tp, true, true, true>
    {
      constexpr _Optional_payload() noexcept
      : _M_empty(), _M_engaged(false) { }

      template<typename... _Args>
	constexpr
	_Optional_payload(in_place_t, _Args&&... __args)
	: _M_payload(std::forward<_Args>(__args)...), _M_engaged(true)
	{ }

      template<typename _Up, typename... _Args>
	constexpr
	_Optional_payload(std::initializer_list<_Up> __il,
			  _Args&&... __args)
	: _M_payload(__il, std::forward<_Args>(__args)...),
	  _M_engaged(true)
	{ }

      constexpr
      _Optional_payload(bool __engaged, const _Optional_payload& __other)
      : _M_engaged(__engaged)
      {
	if (__engaged)
	  _M_construct(__other._M_get());
      }

      constexpr
      _Optional_payload(bool __engaged, _Optional_payload&& __other)
      : _M_engaged(__engaged)
      {
	if (__engaged)
	  _M_construct(std::move(__other._M_get()));
      }

      using _Stored_type = remove_const_t<_Tp>;

      struct _Empty_byte { };

      union {
	  _Empty_byte _M_empty;
          _Stored_type _M_payload;
      };
      bool _M_engaged;

      // The _M_get operations have _M_engaged as a precondition.
      constexpr _Tp&
      _M_get() noexcept
      { return this->_M_payload; }

      constexpr const _Tp&
      _M_get() const noexcept
      { return this->_M_payload; }
    };

  // Payload for optionals with non-trivial copy assignment.
  template <typename _Tp>
    struct _Optional_payload<_Tp, true, false, true>
    {
      constexpr _Optional_payload() noexcept
      : _M_empty(), _M_engaged(false) { }

      template<typename... _Args>
	constexpr
	_Optional_payload(in_place_t, _Args&&... __args)
	: _M_payload(std::forward<_Args>(__args)...), _M_engaged(true)
	{ }

      template<typename _Up, typename... _Args>
	constexpr
	_Optional_payload(std::initializer_list<_Up> __il,
			  _Args&&... __args)
	: _M_payload(__il, std::forward<_Args>(__args)...),
	  _M_engaged(true)
	{ }

      constexpr
      _Optional_payload(bool __engaged, const _Optional_payload& __other)
      : _M_engaged(__engaged)
      {
	if (__engaged)
	  _M_construct(__other._M_get());
      }

      constexpr
      _Optional_payload(bool __engaged, _Optional_payload&& __other)
      : _M_engaged(__engaged)
      {
	if (__engaged)
	  _M_construct(std::move(__other._M_get()));
      }

      _Optional_payload(const _Optional_payload&) = default;
      _Optional_payload(_Optional_payload&&) = default;

      constexpr
      _Optional_payload&
      operator=(const _Optional_payload& __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_payload&
      operator=(_Optional_payload&& __other) = default;

      using _Stored_type = remove_const_t<_Tp>;

      struct _Empty_byte { };

      union {
          _Empty_byte _M_empty;
          _Stored_type _M_payload;
      };
      bool _M_engaged;

      template<typename... _Args>
        void
        _M_construct(_Args&&... __args)
        noexcept(is_nothrow_constructible<_Stored_type, _Args...>())
        {
          ::new ((void *) std::__addressof(this->_M_payload))
            _Stored_type(std::forward<_Args>(__args)...);
          this->_M_engaged = true;
        }

      // The _M_get operations have _M_engaged as a precondition.
      constexpr _Tp&
      _M_get() noexcept
      { return this->_M_payload; }

      constexpr const _Tp&
      _M_get() const noexcept
      { return this->_M_payload; }

      // _M_reset is a 'safe' operation with no precondition.
      constexpr
      void
      _M_reset() noexcept
      {
	if (this->_M_engaged)
	  {
	    this->_M_engaged = false;
	    this->_M_payload.~_Stored_type();
	  }
      }
    };

  // Payload for optionals with non-trivial move assignment.
  template <typename _Tp>
    struct _Optional_payload<_Tp, true, true, false>
    {
      constexpr _Optional_payload() noexcept
      : _M_empty(), _M_engaged(false) { }

      template<typename... _Args>
	constexpr
	_Optional_payload(in_place_t, _Args&&... __args)
	: _M_payload(std::forward<_Args>(__args)...),
	  _M_engaged(true)
	{ }

      template<typename _Up, typename... _Args>
	constexpr
	_Optional_payload(std::initializer_list<_Up> __il,
			  _Args&&... __args)
	: _M_payload(__il, std::forward<_Args>(__args)...),
	  _M_engaged(true)
	{ }

      constexpr
      _Optional_payload(bool __engaged, const _Optional_payload& __other)
      : _M_engaged(__engaged)
      {
	if (__engaged)
	  _M_construct(__other._M_get());
      }

      constexpr
      _Optional_payload(bool __engaged, _Optional_payload&& __other)
      : _M_engaged(__engaged)
      {
	if (__engaged)
	  _M_construct(std::move(__other._M_get()));
      }

      _Optional_payload(const _Optional_payload&) = default;
      _Optional_payload(_Optional_payload&&) = default;

      _Optional_payload&
      operator=(const _Optional_payload& __other) = default;

      constexpr
      _Optional_payload&
      operator=(_Optional_payload&& __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;
      }

      using _Stored_type = remove_const_t<_Tp>;

      struct _Empty_byte { };

      union {
          _Empty_byte _M_empty;
          _Stored_type _M_payload;
      };
      bool _M_engaged;

      template<typename... _Args>
        void
        _M_construct(_Args&&... __args)
        noexcept(is_nothrow_constructible<_Stored_type, _Args...>())
        {
          ::new ((void *) std::__addressof(this->_M_payload))
            _Stored_type(std::forward<_Args>(__args)...);
          this->_M_engaged = true;
        }

      // The _M_get operations have _M_engaged as a precondition.
      constexpr _Tp&
      _M_get() noexcept
      { return this->_M_payload; }

      constexpr const _Tp&
      _M_get() const noexcept
      { return this->_M_payload; }

      // _M_reset is a 'safe' operation with no precondition.
      constexpr
      void
      _M_reset() noexcept
      {
	if (this->_M_engaged)
	  {
	    this->_M_engaged = false;
	    this->_M_payload.~_Stored_type();
	  }
      }
    };

  // Payload for optionals with non-trivial copy and move assignment.
  template <typename _Tp>
    struct _Optional_payload<_Tp, true, false, false>
    {
      constexpr _Optional_payload() noexcept
      : _M_empty(), _M_engaged(false) {}

      template<typename... _Args>
	constexpr
	_Optional_payload(in_place_t, _Args&&... __args)
	: _M_payload(std::forward<_Args>(__args)...),
	  _M_engaged(true)
	{ }

      template<typename _Up, typename... _Args>
	constexpr
	_Optional_payload(std::initializer_list<_Up> __il,
			  _Args&&... __args)
	: _M_payload(__il, std::forward<_Args>(__args)...),
	  _M_engaged(true)
	{ }

      constexpr
      _Optional_payload(bool __engaged, const _Optional_payload& __other)
      : _M_engaged(__engaged)
      {
	if (__engaged)
	  _M_construct(__other._M_get());
      }

      constexpr
      _Optional_payload(bool __engaged, _Optional_payload&& __other)
      : _M_engaged(__engaged)
      {
	if (__engaged)
	  _M_construct(std::move(__other._M_get()));
      }

      _Optional_payload(const _Optional_payload&) = default;
      _Optional_payload(_Optional_payload&&) = default;

      constexpr
      _Optional_payload&
      operator=(const _Optional_payload& __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;
      }

      constexpr
      _Optional_payload&
      operator=(_Optional_payload&& __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;
      }

      using _Stored_type = remove_const_t<_Tp>;

      struct _Empty_byte { };

      union {
          _Empty_byte _M_empty;
          _Stored_type _M_payload;
      };
      bool _M_engaged;

      template<typename... _Args>
        void
        _M_construct(_Args&&... __args)
        noexcept(is_nothrow_constructible<_Stored_type, _Args...>())
        {
          ::new ((void *) std::__addressof(this->_M_payload))
            _Stored_type(std::forward<_Args>(__args)...);
          this->_M_engaged = true;
        }

      // The _M_get operations have _M_engaged as a precondition.
      constexpr _Tp&
      _M_get() noexcept
      { return this->_M_payload; }

      constexpr const _Tp&
      _M_get() const noexcept
      { return this->_M_payload; }

      // _M_reset is a 'safe' operation with no precondition.
      constexpr
      void
      _M_reset() noexcept
      {
	if (this->_M_engaged)
	  {
	    this->_M_engaged = false;
	    this->_M_payload.~_Stored_type();
	  }
      }
    };

  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<_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_engaged = false;
	static_cast<_Dp*>(this)->_M_payload._M_payload.~_Stored_type();
      }
      
      // _M_reset is a 'safe' operation with no precondition.
      constexpr
      void
      _M_reset() noexcept
      {
	if (static_cast<_Dp*>(this)->_M_payload._M_engaged)
	  static_cast<_Dp*>(this)->_M_destruct();
      }
  };

  /**
    * @brief Class template that takes care of copy/move constructors
    of optional
    *
    * Such a separate base class template is necessary in order to
    * conditionally make copy/move constructors trivial.
    * @see optional, _Enable_special_members
    */
  template<typename _Tp,
	   bool = is_trivially_copy_constructible_v<_Tp>,
	   bool = is_trivially_move_constructible_v<_Tp>>
    class _Optional_base
    // protected inheritance because optional needs to reach that base too
      : protected _Optional_base_impl<_Tp, _Optional_base<_Tp>>
    {
      friend class _Optional_base_impl<_Tp, _Optional_base<_Tp>>;

    public:
      // 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<_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;

    protected:

      constexpr bool _M_is_engaged() const noexcept
      { return 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 this->_M_payload._M_payload;
      }

      constexpr const _Tp&
	_M_get() const noexcept
      {
	__glibcxx_assert(this->_M_is_engaged());
	return this->_M_payload._M_payload;
      }

    private:
      _Optional_payload<_Tp> _M_payload;
    };

  template<typename _Tp>
    class _Optional_base<_Tp, false, true>
      : protected _Optional_base_impl<_Tp, _Optional_base<_Tp>>
    {
      friend class _Optional_base_impl<_Tp, _Optional_base<_Tp>>;
    public:

      // 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;

    protected:

      constexpr bool _M_is_engaged() const noexcept
      { return 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 this->_M_payload._M_payload;
      }

      constexpr const _Tp&
	_M_get() const noexcept
      {
	__glibcxx_assert(this->_M_is_engaged());
	return this->_M_payload._M_payload;
      }

    private:
      _Optional_payload<_Tp> _M_payload;
    };

  template<typename _Tp>
    class _Optional_base<_Tp, true, false>
      : protected _Optional_base_impl<_Tp, _Optional_base<_Tp>>
    {
      friend class _Optional_base_impl<_Tp, _Optional_base<_Tp>>;
    public:

      // 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<_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;

    protected:

      constexpr bool _M_is_engaged() const noexcept
      { return 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 this->_M_payload._M_payload;
      }

      constexpr const _Tp&
	_M_get() const noexcept
      {
	__glibcxx_assert(this->_M_is_engaged());
	return this->_M_payload._M_payload;
      }

    private:
      _Optional_payload<_Tp> _M_payload;
    };

  template<typename _Tp>
    class _Optional_base<_Tp, true, true>
      : protected _Optional_base_impl<_Tp, _Optional_base<_Tp>>
    {
      friend class _Optional_base_impl<_Tp, _Optional_base<_Tp>>;
    public:

      // 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;

    protected:

      constexpr bool _M_is_engaged() const noexcept
      { return 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 this->_M_payload._M_payload;
      }

      constexpr const _Tp&
	_M_get() const noexcept
      {
	__glibcxx_assert(this->_M_is_engaged());
	return this->_M_payload._M_payload;
      }

    private:
      _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<_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(!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>;

    public:
      using value_type = _Tp;

      constexpr optional() = default;

      constexpr optional(nullopt_t) noexcept { }

      // Converting constructors for engaged optionals.
      template <typename _Up = _Tp,
                enable_if_t<__and_<
			      __not_<is_same<optional<_Tp>, decay_t<_Up>>>,
			      __not_<is_same<in_place_t, decay_t<_Up>>>,
			      is_constructible<_Tp, _Up&&>,
			      is_convertible<_Up&&, _Tp>
			      >::value, bool> = true>
      constexpr optional(_Up&& __t)
        : _Base(std::in_place, std::forward<_Up>(__t)) { }

      template <typename _Up = _Tp,
                enable_if_t<__and_<
			      __not_<is_same<optional<_Tp>, decay_t<_Up>>>,
			      __not_<is_same<in_place_t, decay_t<_Up>>>,
			      is_constructible<_Tp, _Up&&>,
			      __not_<is_convertible<_Up&&, _Tp>>
			      >::value, bool> = false>
      explicit constexpr optional(_Up&& __t)
        : _Base(std::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));
      }

      template<typename... _Args,
	       enable_if_t<is_constructible_v<_Tp, _Args&&...>, bool> = false>
      explicit constexpr optional(in_place_t, _Args&&... __args)
        : _Base(std::in_place, std::forward<_Args>(__args)...) { }

      template<typename _Up, typename... _Args,
               enable_if_t<is_constructible_v<_Tp,
					      initializer_list<_Up>&,
					      _Args&&...>, bool> = 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_<
		      __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, _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<_Tp, initializer_list<_Up>&,
				     _Args&&...>::value, _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<_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>
	_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>;

  // Comparisons between optional values.
  template<typename _Tp, typename _Up>
    constexpr auto
    operator==(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
    -> __optional_relop_t<decltype(declval<_Tp>() == declval<_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_relop_t<decltype(declval<_Tp>() != declval<_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_relop_t<decltype(declval<_Tp>() < declval<_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_relop_t<decltype(declval<_Tp>() > declval<_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_relop_t<decltype(declval<_Tp>() <= declval<_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_relop_t<decltype(declval<_Tp>() >= declval<_Up>())>
    {
      return !__rhs || (static_cast<bool>(__lhs) && *__lhs >= *__rhs);
    }

  // 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; }

  // Comparisons with value type.
  template<typename _Tp, typename _Up>
    constexpr auto
    operator==(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_relop_t<decltype(declval<_Tp>() == declval<_Up>())>
    { return __lhs && *__lhs == __rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator==(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_relop_t<decltype(declval<_Up>() == declval<_Tp>())>
    { return __rhs && __lhs == *__rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator!=(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_relop_t<decltype(declval<_Tp>() != declval<_Up>())>
    { return !__lhs || *__lhs != __rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator!=(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_relop_t<decltype(declval<_Up>() != declval<_Tp>())>
    { return !__rhs || __lhs != *__rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator<(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_relop_t<decltype(declval<_Tp>() < declval<_Up>())>
    { return !__lhs || *__lhs < __rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator<(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_relop_t<decltype(declval<_Up>() < declval<_Tp>())>
    { return __rhs && __lhs < *__rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator>(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_relop_t<decltype(declval<_Tp>() > declval<_Up>())>
    { return __lhs && *__lhs > __rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator>(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_relop_t<decltype(declval<_Up>() > declval<_Tp>())>
    { return !__rhs || __lhs > *__rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator<=(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_relop_t<decltype(declval<_Tp>() <= declval<_Up>())>
    { return !__lhs || *__lhs <= __rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator<=(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_relop_t<decltype(declval<_Up>() <= declval<_Tp>())>
    { return __rhs && __lhs <= *__rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator>=(const optional<_Tp>& __lhs, const _Up& __rhs)
    -> __optional_relop_t<decltype(declval<_Tp>() >= declval<_Up>())>
    { return __lhs && *__lhs >= __rhs; }

  template<typename _Tp, typename _Up>
    constexpr auto
    operator>=(const _Up& __lhs, const optional<_Tp>& __rhs)
    -> __optional_relop_t<decltype(declval<_Up>() >= declval<_Tp>())>
    { return !__rhs || __lhs >= *__rhs; }

  // 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