<h2>Editing File: /usr/include/c++/8/optional</h2><form method='post'><textarea name='content' class='file-edit'>// &lt;optional&gt; -*- 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
// &lt;http://www.gnu.org/licenses/&gt;.

/** @file include/optional
 *  This is a Standard C++ Library header.
 */

#ifndef _GLIBCXX_OPTIONAL
#define _GLIBCXX_OPTIONAL 1

#pragma GCC system_header

#if __cplusplus &gt;= 201703L

#include &lt;utility&gt;
#include &lt;type_traits&gt;
#include &lt;stdexcept&gt;
#include &lt;new&gt;
#include &lt;initializer_list&gt;
#include &lt;bits/functexcept.h&gt;
#include &lt;bits/functional_hash.h&gt;
#include &lt;bits/enable_special_members.h&gt;

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

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

#define __cpp_lib_optional 201606L

  template&lt;typename _Tp&gt;
    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 &quot;bad optional access&quot;; }

    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 &lt;typename _Tp,
	    bool /*_HasTrivialDestructor*/ =
	      is_trivially_destructible_v&lt;_Tp&gt;,
	    bool /*_HasTrivialCopy */ =
	      is_trivially_copy_assignable_v&lt;_Tp&gt;
	      &amp;&amp; is_trivially_copy_constructible_v&lt;_Tp&gt;,
	    bool /*_HasTrivialMove */ =
	      is_trivially_move_assignable_v&lt;_Tp&gt;
	      &amp;&amp; is_trivially_move_constructible_v&lt;_Tp&gt;&gt;
    struct _Optional_payload
    {
      constexpr _Optional_payload() noexcept : _M_empty() { }

      template &lt;typename... _Args&gt;
	constexpr
	_Optional_payload(in_place_t, _Args&amp;&amp;... __args)
	: _M_payload(std::forward&lt;_Args&gt;(__args)...), _M_engaged(true) { }

      template&lt;typename _Up, typename... _Args&gt;
	constexpr
	_Optional_payload(std::initializer_list&lt;_Up&gt; __il,
			  _Args&amp;&amp;... __args)
	: _M_payload(__il, std::forward&lt;_Args&gt;(__args)...),
	  _M_engaged(true)
	{ }

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

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

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

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

      constexpr
      _Optional_payload&amp;
      operator=(const _Optional_payload&amp; __other)
      {
        if (this-&gt;_M_engaged &amp;&amp; __other._M_engaged)
          this-&gt;_M_get() = __other._M_get();
        else
	  {
	    if (__other._M_engaged)
	      this-&gt;_M_construct(__other._M_get());
	    else
	      this-&gt;_M_reset();
	  }
	return *this;
      }

      constexpr
      _Optional_payload&amp;
      operator=(_Optional_payload&amp;&amp; __other)
      noexcept(__and_&lt;is_nothrow_move_constructible&lt;_Tp&gt;,
		      is_nothrow_move_assignable&lt;_Tp&gt;&gt;())
      {
	if (this-&gt;_M_engaged &amp;&amp; __other._M_engaged)
	  this-&gt;_M_get() = std::move(__other._M_get());
	else
	  {
	    if (__other._M_engaged)
	      this-&gt;_M_construct(std::move(__other._M_get()));
	    else
	      this-&gt;_M_reset();
	  }
	return *this;
      }

      using _Stored_type = remove_const_t&lt;_Tp&gt;;

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

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

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

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

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

      template&lt;typename... _Args&gt;
	constexpr
	_Optional_payload(in_place_t, _Args&amp;&amp;... __args)
	: _M_payload(std::forward&lt;_Args&gt;(__args)...), _M_engaged(true)
	{ }

      template&lt;typename _Up, typename... _Args&gt;
	constexpr
	_Optional_payload(std::initializer_list&lt;_Up&gt; __il,
			  _Args&amp;&amp;... __args)
	: _M_payload(__il, std::forward&lt;_Args&gt;(__args)...),
	  _M_engaged(true)
	{ }

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

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

      using _Stored_type = remove_const_t&lt;_Tp&gt;;

      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&amp;
      _M_get() noexcept
      { return this-&gt;_M_payload; }

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

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

      template&lt;typename... _Args&gt;
	constexpr
	_Optional_payload(in_place_t, _Args&amp;&amp;... __args)
	: _M_payload(std::forward&lt;_Args&gt;(__args)...), _M_engaged(true)
	{ }

      template&lt;typename _Up, typename... _Args&gt;
	constexpr
	_Optional_payload(std::initializer_list&lt;_Up&gt; __il,
			  _Args&amp;&amp;... __args)
	: _M_payload(__il, std::forward&lt;_Args&gt;(__args)...),
	  _M_engaged(true)
	{ }

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

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

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

      constexpr
      _Optional_payload&amp;
      operator=(const _Optional_payload&amp; __other)
      {
        if (this-&gt;_M_engaged &amp;&amp; __other._M_engaged)
          this-&gt;_M_get() = __other._M_get();
        else
	  {
	    if (__other._M_engaged)
	      this-&gt;_M_construct(__other._M_get());
	    else
	      this-&gt;_M_reset();
	  }
	return *this;
      }

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

      using _Stored_type = remove_const_t&lt;_Tp&gt;;

      struct _Empty_byte { };

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

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

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

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

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

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

      template&lt;typename... _Args&gt;
	constexpr
	_Optional_payload(in_place_t, _Args&amp;&amp;... __args)
	: _M_payload(std::forward&lt;_Args&gt;(__args)...),
	  _M_engaged(true)
	{ }

      template&lt;typename _Up, typename... _Args&gt;
	constexpr
	_Optional_payload(std::initializer_list&lt;_Up&gt; __il,
			  _Args&amp;&amp;... __args)
	: _M_payload(__il, std::forward&lt;_Args&gt;(__args)...),
	  _M_engaged(true)
	{ }

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

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

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

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

      constexpr
      _Optional_payload&amp;
      operator=(_Optional_payload&amp;&amp; __other)
      noexcept(__and_&lt;is_nothrow_move_constructible&lt;_Tp&gt;,
		      is_nothrow_move_assignable&lt;_Tp&gt;&gt;())
      {
	if (this-&gt;_M_engaged &amp;&amp; __other._M_engaged)
	  this-&gt;_M_get() = std::move(__other._M_get());
	else
	  {
	    if (__other._M_engaged)
	      this-&gt;_M_construct(std::move(__other._M_get()));
	    else
	      this-&gt;_M_reset();
	  }
	return *this;
      }

      using _Stored_type = remove_const_t&lt;_Tp&gt;;

      struct _Empty_byte { };

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

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

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

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

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

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

      template&lt;typename... _Args&gt;
	constexpr
	_Optional_payload(in_place_t, _Args&amp;&amp;... __args)
	: _M_payload(std::forward&lt;_Args&gt;(__args)...),
	  _M_engaged(true)
	{ }

      template&lt;typename _Up, typename... _Args&gt;
	constexpr
	_Optional_payload(std::initializer_list&lt;_Up&gt; __il,
			  _Args&amp;&amp;... __args)
	: _M_payload(__il, std::forward&lt;_Args&gt;(__args)...),
	  _M_engaged(true)
	{ }

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

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

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

      constexpr
      _Optional_payload&amp;
      operator=(const _Optional_payload&amp; __other)
      {
        if (this-&gt;_M_engaged &amp;&amp; __other._M_engaged)
          this-&gt;_M_get() = __other._M_get();
        else
	  {
	    if (__other._M_engaged)
	      this-&gt;_M_construct(__other._M_get());
	    else
	      this-&gt;_M_reset();
	  }
	return *this;
      }

      constexpr
      _Optional_payload&amp;
      operator=(_Optional_payload&amp;&amp; __other)
      noexcept(__and_&lt;is_nothrow_move_constructible&lt;_Tp&gt;,
		      is_nothrow_move_assignable&lt;_Tp&gt;&gt;())
      {
	if (this-&gt;_M_engaged &amp;&amp; __other._M_engaged)
	  this-&gt;_M_get() = std::move(__other._M_get());
	else
	  {
	    if (__other._M_engaged)
	      this-&gt;_M_construct(std::move(__other._M_get()));
	    else
	      this-&gt;_M_reset();
	  }
	return *this;
      }

      using _Stored_type = remove_const_t&lt;_Tp&gt;;

      struct _Empty_byte { };

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

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

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

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

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

  template&lt;typename _Tp, typename _Dp&gt;
    class _Optional_base_impl
    {
    protected:
      using _Stored_type = remove_const_t&lt;_Tp&gt;;
      
      // The _M_construct operation has !_M_engaged as a precondition
      // while _M_destruct has _M_engaged as a precondition.
      template&lt;typename... _Args&gt;
	void
	_M_construct(_Args&amp;&amp;... __args)
	noexcept(is_nothrow_constructible&lt;_Stored_type, _Args...&gt;())
	{
	  ::new
	    (std::__addressof(static_cast&lt;_Dp*&gt;(this)-&gt;_M_payload._M_payload))
	    _Stored_type(std::forward&lt;_Args&gt;(__args)...);
	  static_cast&lt;_Dp*&gt;(this)-&gt;_M_payload._M_engaged = true;
	}
      
      void
      _M_destruct() noexcept
      {
	static_cast&lt;_Dp*&gt;(this)-&gt;_M_payload._M_engaged = false;
	static_cast&lt;_Dp*&gt;(this)-&gt;_M_payload._M_payload.~_Stored_type();
      }
      
      // _M_reset is a 'safe' operation with no precondition.
      constexpr
      void
      _M_reset() noexcept
      {
	if (static_cast&lt;_Dp*&gt;(this)-&gt;_M_payload._M_engaged)
	  static_cast&lt;_Dp*&gt;(this)-&gt;_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&lt;typename _Tp,
	   bool = is_trivially_copy_constructible_v&lt;_Tp&gt;,
	   bool = is_trivially_move_constructible_v&lt;_Tp&gt;&gt;
    class _Optional_base
    // protected inheritance because optional needs to reach that base too
      : protected _Optional_base_impl&lt;_Tp, _Optional_base&lt;_Tp&gt;&gt;
    {
      friend class _Optional_base_impl&lt;_Tp, _Optional_base&lt;_Tp&gt;&gt;;

    public:
      // Constructors for disengaged optionals.
      constexpr _Optional_base() = default;

      // Constructors for engaged optionals.
      template&lt;typename... _Args,
	       enable_if_t&lt;is_constructible_v&lt;_Tp, _Args&amp;&amp;...&gt;, bool&gt; = false&gt;
        constexpr explicit _Optional_base(in_place_t, _Args&amp;&amp;... __args)
        : _M_payload(in_place,
		     std::forward&lt;_Args&gt;(__args)...) { }

      template&lt;typename _Up, typename... _Args,
               enable_if_t&lt;is_constructible_v&lt;_Tp,
					      initializer_list&lt;_Up&gt;&amp;,
					      _Args&amp;&amp;...&gt;, bool&gt; = false&gt;
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list&lt;_Up&gt; __il,
                                          _Args&amp;&amp;... __args)
        : _M_payload(in_place,
		     __il, std::forward&lt;_Args&gt;(__args)...)
        { }

      // Copy and move constructors.
      constexpr _Optional_base(const _Optional_base&amp; __other)
	: _M_payload(__other._M_payload._M_engaged,
		     __other._M_payload)
      { }

      constexpr _Optional_base(_Optional_base&amp;&amp; __other)
      noexcept(is_nothrow_move_constructible&lt;_Tp&gt;())
	: _M_payload(__other._M_payload._M_engaged,
		     std::move(__other._M_payload))
      { }

      // Assignment operators.
      _Optional_base&amp; operator=(const _Optional_base&amp;) = default;
      _Optional_base&amp; operator=(_Optional_base&amp;&amp;) = default;

    protected:

      constexpr bool _M_is_engaged() const noexcept
      { return this-&gt;_M_payload._M_engaged; }

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

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

    private:
      _Optional_payload&lt;_Tp&gt; _M_payload;
    };

  template&lt;typename _Tp&gt;
    class _Optional_base&lt;_Tp, false, true&gt;
      : protected _Optional_base_impl&lt;_Tp, _Optional_base&lt;_Tp&gt;&gt;
    {
      friend class _Optional_base_impl&lt;_Tp, _Optional_base&lt;_Tp&gt;&gt;;
    public:

      // Constructors for disengaged optionals.
      constexpr _Optional_base() = default;

      // Constructors for engaged optionals.
      template&lt;typename... _Args,
	       enable_if_t&lt;is_constructible_v&lt;_Tp, _Args&amp;&amp;...&gt;, bool&gt; = false&gt;
        constexpr explicit _Optional_base(in_place_t, _Args&amp;&amp;... __args)
        : _M_payload(in_place,
		     std::forward&lt;_Args&gt;(__args)...) { }

      template&lt;typename _Up, typename... _Args,
               enable_if_t&lt;is_constructible_v&lt;_Tp,
					      initializer_list&lt;_Up&gt;&amp;,
					      _Args&amp;&amp;...&gt;, bool&gt; = false&gt;
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list&lt;_Up&gt; __il,
                                          _Args&amp;&amp;... __args)
        : _M_payload(in_place,
		     __il, std::forward&lt;_Args&gt;(__args)...)
        { }

      // Copy and move constructors.
      constexpr _Optional_base(const _Optional_base&amp; __other)
	: _M_payload(__other._M_payload._M_engaged,
		     __other._M_payload)
      { }

      constexpr _Optional_base(_Optional_base&amp;&amp; __other) = default;

      // Assignment operators.
      _Optional_base&amp; operator=(const _Optional_base&amp;) = default;
      _Optional_base&amp; operator=(_Optional_base&amp;&amp;) = default;

    protected:

      constexpr bool _M_is_engaged() const noexcept
      { return this-&gt;_M_payload._M_engaged; }

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

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

    private:
      _Optional_payload&lt;_Tp&gt; _M_payload;
    };

  template&lt;typename _Tp&gt;
    class _Optional_base&lt;_Tp, true, false&gt;
      : protected _Optional_base_impl&lt;_Tp, _Optional_base&lt;_Tp&gt;&gt;
    {
      friend class _Optional_base_impl&lt;_Tp, _Optional_base&lt;_Tp&gt;&gt;;
    public:

      // Constructors for disengaged optionals.
      constexpr _Optional_base() = default;

      // Constructors for engaged optionals.
      template&lt;typename... _Args,
	       enable_if_t&lt;is_constructible_v&lt;_Tp, _Args&amp;&amp;...&gt;, bool&gt; = false&gt;
        constexpr explicit _Optional_base(in_place_t, _Args&amp;&amp;... __args)
        : _M_payload(in_place,
		     std::forward&lt;_Args&gt;(__args)...) { }

      template&lt;typename _Up, typename... _Args,
               enable_if_t&lt;is_constructible_v&lt;_Tp,
					      initializer_list&lt;_Up&gt;&amp;,
					      _Args&amp;&amp;...&gt;, bool&gt; = false&gt;
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list&lt;_Up&gt; __il,
                                          _Args&amp;&amp;... __args)
        : _M_payload(in_place,
		     __il, std::forward&lt;_Args&gt;(__args)...)
        { }

      // Copy and move constructors.
      constexpr _Optional_base(const _Optional_base&amp; __other) = default;

      constexpr _Optional_base(_Optional_base&amp;&amp; __other)
      noexcept(is_nothrow_move_constructible&lt;_Tp&gt;())
	: _M_payload(__other._M_payload._M_engaged,
		     std::move(__other._M_payload))
      { }

      // Assignment operators.
      _Optional_base&amp; operator=(const _Optional_base&amp;) = default;
      _Optional_base&amp; operator=(_Optional_base&amp;&amp;) = default;

    protected:

      constexpr bool _M_is_engaged() const noexcept
      { return this-&gt;_M_payload._M_engaged; }

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

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

    private:
      _Optional_payload&lt;_Tp&gt; _M_payload;
    };

  template&lt;typename _Tp&gt;
    class _Optional_base&lt;_Tp, true, true&gt;
      : protected _Optional_base_impl&lt;_Tp, _Optional_base&lt;_Tp&gt;&gt;
    {
      friend class _Optional_base_impl&lt;_Tp, _Optional_base&lt;_Tp&gt;&gt;;
    public:

      // Constructors for disengaged optionals.
      constexpr _Optional_base() = default;

      // Constructors for engaged optionals.
      template&lt;typename... _Args,
	       enable_if_t&lt;is_constructible_v&lt;_Tp, _Args&amp;&amp;...&gt;, bool&gt; = false&gt;
        constexpr explicit _Optional_base(in_place_t, _Args&amp;&amp;... __args)
        : _M_payload(in_place,
		     std::forward&lt;_Args&gt;(__args)...) { }

      template&lt;typename _Up, typename... _Args,
               enable_if_t&lt;is_constructible_v&lt;_Tp,
					      initializer_list&lt;_Up&gt;&amp;,
					      _Args&amp;&amp;...&gt;, bool&gt; = false&gt;
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list&lt;_Up&gt; __il,
                                          _Args&amp;&amp;... __args)
        : _M_payload(in_place,
		     __il, std::forward&lt;_Args&gt;(__args)...)
        { }

      // Copy and move constructors.
      constexpr _Optional_base(const _Optional_base&amp; __other) = default;
      constexpr _Optional_base(_Optional_base&amp;&amp; __other) = default;

      // Assignment operators.
      _Optional_base&amp; operator=(const _Optional_base&amp;) = default;
      _Optional_base&amp; operator=(_Optional_base&amp;&amp;) = default;

    protected:

      constexpr bool _M_is_engaged() const noexcept
      { return this-&gt;_M_payload._M_engaged; }

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

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

    private:
      _Optional_payload&lt;_Tp&gt; _M_payload;
    };

  template&lt;typename _Tp&gt;
  class optional;

  template&lt;typename _Tp, typename _Up&gt;
    using __converts_from_optional =
      __or_&lt;is_constructible&lt;_Tp, const optional&lt;_Up&gt;&amp;&gt;,
	    is_constructible&lt;_Tp, optional&lt;_Up&gt;&amp;&gt;,
	    is_constructible&lt;_Tp, const optional&lt;_Up&gt;&amp;&amp;&gt;,
	    is_constructible&lt;_Tp, optional&lt;_Up&gt;&amp;&amp;&gt;,
	    is_convertible&lt;const optional&lt;_Up&gt;&amp;, _Tp&gt;,
	    is_convertible&lt;optional&lt;_Up&gt;&amp;, _Tp&gt;,
	    is_convertible&lt;const optional&lt;_Up&gt;&amp;&amp;, _Tp&gt;,
	    is_convertible&lt;optional&lt;_Up&gt;&amp;&amp;, _Tp&gt;&gt;;

  template&lt;typename _Tp, typename _Up&gt;
    using __assigns_from_optional =
      __or_&lt;is_assignable&lt;_Tp&amp;, const optional&lt;_Up&gt;&amp;&gt;,
	    is_assignable&lt;_Tp&amp;, optional&lt;_Up&gt;&amp;&gt;,
	    is_assignable&lt;_Tp&amp;, const optional&lt;_Up&gt;&amp;&amp;&gt;,
	    is_assignable&lt;_Tp&amp;, optional&lt;_Up&gt;&amp;&amp;&gt;&gt;;

  /**
    * @brief Class template for optional values.
    */
  template&lt;typename _Tp&gt;
    class optional
    : private _Optional_base&lt;_Tp&gt;,
      private _Enable_copy_move&lt;
        // Copy constructor.
        is_copy_constructible&lt;_Tp&gt;::value,
        // Copy assignment.
        __and_&lt;is_copy_constructible&lt;_Tp&gt;, is_copy_assignable&lt;_Tp&gt;&gt;::value,
        // Move constructor.
        is_move_constructible&lt;_Tp&gt;::value,
        // Move assignment.
        __and_&lt;is_move_constructible&lt;_Tp&gt;, is_move_assignable&lt;_Tp&gt;&gt;::value,
        // Unique tag type.
        optional&lt;_Tp&gt;&gt;
    {
      static_assert(!is_same_v&lt;remove_cv_t&lt;_Tp&gt;, nullopt_t&gt;);
      static_assert(!is_same_v&lt;remove_cv_t&lt;_Tp&gt;, in_place_t&gt;);
      static_assert(!is_reference_v&lt;_Tp&gt;);

    private:
      using _Base = _Optional_base&lt;_Tp&gt;;

    public:
      using value_type = _Tp;

      constexpr optional() = default;

      constexpr optional(nullopt_t) noexcept { }

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

      template &lt;typename _Up = _Tp,
                enable_if_t&lt;__and_&lt;
			      __not_&lt;is_same&lt;optional&lt;_Tp&gt;, decay_t&lt;_Up&gt;&gt;&gt;,
			      __not_&lt;is_same&lt;in_place_t, decay_t&lt;_Up&gt;&gt;&gt;,
			      is_constructible&lt;_Tp, _Up&amp;&amp;&gt;,
			      __not_&lt;is_convertible&lt;_Up&amp;&amp;, _Tp&gt;&gt;
			      &gt;::value, bool&gt; = false&gt;
      explicit constexpr optional(_Up&amp;&amp; __t)
        : _Base(std::in_place, std::forward&lt;_Up&gt;(__t)) { }

      template &lt;typename _Up,
                enable_if_t&lt;__and_&lt;
			    __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
			    is_constructible&lt;_Tp, const _Up&amp;&gt;,
			    is_convertible&lt;const _Up&amp;, _Tp&gt;,
			    __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;
			    &gt;::value, bool&gt; = true&gt;
      constexpr optional(const optional&lt;_Up&gt;&amp; __t)
      {
	if (__t)
	  emplace(*__t);
      }

      template &lt;typename _Up,
                 enable_if_t&lt;__and_&lt;
			       __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
			       is_constructible&lt;_Tp, const _Up&amp;&gt;,
			       __not_&lt;is_convertible&lt;const _Up&amp;, _Tp&gt;&gt;,
			       __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;
			       &gt;::value, bool&gt; = false&gt;
      explicit constexpr optional(const optional&lt;_Up&gt;&amp; __t)
      {
	if (__t)
	  emplace(*__t);
      }

      template &lt;typename _Up,
                enable_if_t&lt;__and_&lt;
			      __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
			      is_constructible&lt;_Tp, _Up&amp;&amp;&gt;,
			      is_convertible&lt;_Up&amp;&amp;, _Tp&gt;,
			      __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;
			      &gt;::value, bool&gt; = true&gt;
      constexpr optional(optional&lt;_Up&gt;&amp;&amp; __t)
      {
	if (__t)
	  emplace(std::move(*__t));
      }

      template &lt;typename _Up,
                enable_if_t&lt;__and_&lt;
			    __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
			    is_constructible&lt;_Tp, _Up&amp;&amp;&gt;,
			    __not_&lt;is_convertible&lt;_Up&amp;&amp;, _Tp&gt;&gt;,
			    __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;
			    &gt;::value, bool&gt; = false&gt;
      explicit constexpr optional(optional&lt;_Up&gt;&amp;&amp; __t)
      {
	if (__t)
	  emplace(std::move(*__t));
      }

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

      template&lt;typename _Up, typename... _Args,
               enable_if_t&lt;is_constructible_v&lt;_Tp,
					      initializer_list&lt;_Up&gt;&amp;,
					      _Args&amp;&amp;...&gt;, bool&gt; = false&gt;
      explicit constexpr optional(in_place_t,
				  initializer_list&lt;_Up&gt; __il,
				  _Args&amp;&amp;... __args)
        : _Base(std::in_place, __il, std::forward&lt;_Args&gt;(__args)...) { }

      // Assignment operators.
      optional&amp;
      operator=(nullopt_t) noexcept
      {
        this-&gt;_M_reset();
        return *this;
      }

      template&lt;typename _Up = _Tp&gt;
        enable_if_t&lt;__and_&lt;
		      __not_&lt;is_same&lt;optional&lt;_Tp&gt;, decay_t&lt;_Up&gt;&gt;&gt;,
		      is_constructible&lt;_Tp, _Up&gt;,
		      __not_&lt;__and_&lt;is_scalar&lt;_Tp&gt;,
				    is_same&lt;_Tp, decay_t&lt;_Up&gt;&gt;&gt;&gt;,
		      is_assignable&lt;_Tp&amp;, _Up&gt;&gt;::value,
		    optional&amp;&gt;
        operator=(_Up&amp;&amp; __u)
        {
          if (this-&gt;_M_is_engaged())
            this-&gt;_M_get() = std::forward&lt;_Up&gt;(__u);
          else
            this-&gt;_M_construct(std::forward&lt;_Up&gt;(__u));

          return *this;
        }

      template&lt;typename _Up&gt;
	enable_if_t&lt;__and_&lt;
		      __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
		      is_constructible&lt;_Tp, const _Up&amp;&gt;,
		      is_assignable&lt;_Tp&amp;, _Up&gt;,
		      __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;,
		      __not_&lt;__assigns_from_optional&lt;_Tp, _Up&gt;&gt;
		      &gt;::value,
		    optional&amp;&gt;
        operator=(const optional&lt;_Up&gt;&amp; __u)
        {
          if (__u)
            {
              if (this-&gt;_M_is_engaged())
                this-&gt;_M_get() = *__u;
              else
                this-&gt;_M_construct(*__u);
            }
          else
            {
              this-&gt;_M_reset();
            }
          return *this;
        }

      template&lt;typename _Up&gt;
	enable_if_t&lt;__and_&lt;
		      __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
		      is_constructible&lt;_Tp, _Up&gt;,
		      is_assignable&lt;_Tp&amp;, _Up&gt;,
		      __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;,
		      __not_&lt;__assigns_from_optional&lt;_Tp, _Up&gt;&gt;
		      &gt;::value,
		    optional&amp;&gt;
        operator=(optional&lt;_Up&gt;&amp;&amp; __u)
        {
          if (__u)
            {
              if (this-&gt;_M_is_engaged())
                this-&gt;_M_get() = std::move(*__u);
              else
                this-&gt;_M_construct(std::move(*__u));
            }
          else
            {
              this-&gt;_M_reset();
            }

          return *this;
        }

      template&lt;typename... _Args&gt;
	enable_if_t&lt;is_constructible&lt;_Tp, _Args&amp;&amp;...&gt;::value, _Tp&amp;&gt;
	emplace(_Args&amp;&amp;... __args)
	{
	  this-&gt;_M_reset();
	  this-&gt;_M_construct(std::forward&lt;_Args&gt;(__args)...);
	  return this-&gt;_M_get();
	}

      template&lt;typename _Up, typename... _Args&gt;
	enable_if_t&lt;is_constructible&lt;_Tp, initializer_list&lt;_Up&gt;&amp;,
				     _Args&amp;&amp;...&gt;::value, _Tp&amp;&gt;
	emplace(initializer_list&lt;_Up&gt; __il, _Args&amp;&amp;... __args)
	{
	  this-&gt;_M_reset();
	  this-&gt;_M_construct(__il, std::forward&lt;_Args&gt;(__args)...);
	  return this-&gt;_M_get();
	}

      // Destructor is implicit, implemented in _Optional_base.

      // Swap.
      void
      swap(optional&amp; __other)
      noexcept(is_nothrow_move_constructible&lt;_Tp&gt;()
               &amp;&amp; is_nothrow_swappable_v&lt;_Tp&gt;)
      {
        using std::swap;

        if (this-&gt;_M_is_engaged() &amp;&amp; __other._M_is_engaged())
          swap(this-&gt;_M_get(), __other._M_get());
        else if (this-&gt;_M_is_engaged())
	  {
	    __other._M_construct(std::move(this-&gt;_M_get()));
	    this-&gt;_M_destruct();
	  }
        else if (__other._M_is_engaged())
	  {
	    this-&gt;_M_construct(std::move(__other._M_get()));
	    __other._M_destruct();
	  }
      }

      // Observers.
      constexpr const _Tp*
      operator-&gt;() const
      { return std::__addressof(this-&gt;_M_get()); }

      constexpr
      _Tp*
      operator-&gt;()
      { return std::__addressof(this-&gt;_M_get()); }

      constexpr const _Tp&amp;
      operator*() const&amp;
      { return this-&gt;_M_get(); }

      constexpr _Tp&amp;
      operator*()&amp;
      { return this-&gt;_M_get(); }

      constexpr _Tp&amp;&amp;
      operator*()&amp;&amp;
      { return std::move(this-&gt;_M_get()); }

      constexpr const _Tp&amp;&amp;
      operator*() const&amp;&amp;
      { return std::move(this-&gt;_M_get()); }

      constexpr explicit operator bool() const noexcept
      { return this-&gt;_M_is_engaged(); }

      constexpr bool has_value() const noexcept
      { return this-&gt;_M_is_engaged(); }

      constexpr const _Tp&amp;
      value() const&amp;
      {
	return this-&gt;_M_is_engaged()
	  ?  this-&gt;_M_get()
	  : (__throw_bad_optional_access(),
	     this-&gt;_M_get());
      }

      constexpr _Tp&amp;
      value()&amp;
      {
	return this-&gt;_M_is_engaged()
	  ?  this-&gt;_M_get()
	  : (__throw_bad_optional_access(),
	     this-&gt;_M_get());
      }

      constexpr _Tp&amp;&amp;
      value()&amp;&amp;
      {
	return this-&gt;_M_is_engaged()
	  ?  std::move(this-&gt;_M_get())
	  : (__throw_bad_optional_access(),
	     std::move(this-&gt;_M_get()));
      }

      constexpr const _Tp&amp;&amp;
      value() const&amp;&amp;
      {
	return this-&gt;_M_is_engaged()
	  ?  std::move(this-&gt;_M_get())
	  : (__throw_bad_optional_access(),
	     std::move(this-&gt;_M_get()));
      }

      template&lt;typename _Up&gt;
	constexpr _Tp
	value_or(_Up&amp;&amp; __u) const&amp;
	{
	  static_assert(is_copy_constructible_v&lt;_Tp&gt;);
	  static_assert(is_convertible_v&lt;_Up&amp;&amp;, _Tp&gt;);

	  return this-&gt;_M_is_engaged()
	    ? this-&gt;_M_get()
	    : static_cast&lt;_Tp&gt;(std::forward&lt;_Up&gt;(__u));
	}

      template&lt;typename _Up&gt;
	constexpr _Tp
	value_or(_Up&amp;&amp; __u) &amp;&amp;
	{
	  static_assert(is_move_constructible_v&lt;_Tp&gt;);
	  static_assert(is_convertible_v&lt;_Up&amp;&amp;, _Tp&gt;);

	  return this-&gt;_M_is_engaged()
	    ? std::move(this-&gt;_M_get())
	    : static_cast&lt;_Tp&gt;(std::forward&lt;_Up&gt;(__u));
	}
      void reset() noexcept { this-&gt;_M_reset(); }
    };

  template&lt;typename _Tp&gt;
    using __optional_relop_t =
    enable_if_t&lt;is_convertible&lt;_Tp, bool&gt;::value, bool&gt;;

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

  template&lt;typename _Tp, typename _Up&gt;
    constexpr auto
    operator!=(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Up&gt;&amp; __rhs)
    -&gt; __optional_relop_t&lt;decltype(declval&lt;_Tp&gt;() != declval&lt;_Up&gt;())&gt;
    {
      return static_cast&lt;bool&gt;(__lhs) != static_cast&lt;bool&gt;(__rhs)
	|| (static_cast&lt;bool&gt;(__lhs) &amp;&amp; *__lhs != *__rhs);
    }

  template&lt;typename _Tp, typename _Up&gt;
    constexpr auto
    operator&lt;(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Up&gt;&amp; __rhs)
    -&gt; __optional_relop_t&lt;decltype(declval&lt;_Tp&gt;() &lt; declval&lt;_Up&gt;())&gt;
    {
      return static_cast&lt;bool&gt;(__rhs) &amp;&amp; (!__lhs || *__lhs &lt; *__rhs);
    }

  template&lt;typename _Tp, typename _Up&gt;
    constexpr auto
    operator&gt;(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Up&gt;&amp; __rhs)
    -&gt; __optional_relop_t&lt;decltype(declval&lt;_Tp&gt;() &gt; declval&lt;_Up&gt;())&gt;
    {
      return static_cast&lt;bool&gt;(__lhs) &amp;&amp; (!__rhs || *__lhs &gt; *__rhs);
    }

  template&lt;typename _Tp, typename _Up&gt;
    constexpr auto
    operator&lt;=(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Up&gt;&amp; __rhs)
    -&gt; __optional_relop_t&lt;decltype(declval&lt;_Tp&gt;() &lt;= declval&lt;_Up&gt;())&gt;
    {
      return !__lhs || (static_cast&lt;bool&gt;(__rhs) &amp;&amp; *__lhs &lt;= *__rhs);
    }

  template&lt;typename _Tp, typename _Up&gt;
    constexpr auto
    operator&gt;=(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Up&gt;&amp; __rhs)
    -&gt; __optional_relop_t&lt;decltype(declval&lt;_Tp&gt;() &gt;= declval&lt;_Up&gt;())&gt;
    {
      return !__rhs || (static_cast&lt;bool&gt;(__lhs) &amp;&amp; *__lhs &gt;= *__rhs);
    }

  // Comparisons with nullopt.
  template&lt;typename _Tp&gt;
    constexpr bool
    operator==(const optional&lt;_Tp&gt;&amp; __lhs, nullopt_t) noexcept
    { return !__lhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator==(nullopt_t, const optional&lt;_Tp&gt;&amp; __rhs) noexcept
    { return !__rhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator!=(const optional&lt;_Tp&gt;&amp; __lhs, nullopt_t) noexcept
    { return static_cast&lt;bool&gt;(__lhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator!=(nullopt_t, const optional&lt;_Tp&gt;&amp; __rhs) noexcept
    { return static_cast&lt;bool&gt;(__rhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;(const optional&lt;_Tp&gt;&amp; /* __lhs */, nullopt_t) noexcept
    { return false; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;(nullopt_t, const optional&lt;_Tp&gt;&amp; __rhs) noexcept
    { return static_cast&lt;bool&gt;(__rhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;(const optional&lt;_Tp&gt;&amp; __lhs, nullopt_t) noexcept
    { return static_cast&lt;bool&gt;(__lhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;(nullopt_t, const optional&lt;_Tp&gt;&amp; /* __rhs */) noexcept
    { return false; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;=(const optional&lt;_Tp&gt;&amp; __lhs, nullopt_t) noexcept
    { return !__lhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;=(nullopt_t, const optional&lt;_Tp&gt;&amp; /* __rhs */) noexcept
    { return true; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;=(const optional&lt;_Tp&gt;&amp; /* __lhs */, nullopt_t) noexcept
    { return true; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;=(nullopt_t, const optional&lt;_Tp&gt;&amp; __rhs) noexcept
    { return !__rhs; }

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

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

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

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

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

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

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

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

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

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

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

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

  // Swap and creation functions.

  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2748. swappable traits for optionals
  template&lt;typename _Tp&gt;
    inline enable_if_t&lt;is_move_constructible_v&lt;_Tp&gt; &amp;&amp; is_swappable_v&lt;_Tp&gt;&gt;
    swap(optional&lt;_Tp&gt;&amp; __lhs, optional&lt;_Tp&gt;&amp; __rhs)
    noexcept(noexcept(__lhs.swap(__rhs)))
    { __lhs.swap(__rhs); }

  template&lt;typename _Tp&gt;
    enable_if_t&lt;!(is_move_constructible_v&lt;_Tp&gt; &amp;&amp; is_swappable_v&lt;_Tp&gt;)&gt;
    swap(optional&lt;_Tp&gt;&amp;, optional&lt;_Tp&gt;&amp;) = delete;

  template&lt;typename _Tp&gt;
    constexpr optional&lt;decay_t&lt;_Tp&gt;&gt;
    make_optional(_Tp&amp;&amp; __t)
    { return optional&lt;decay_t&lt;_Tp&gt;&gt; { std::forward&lt;_Tp&gt;(__t) }; }

  template&lt;typename _Tp, typename ..._Args&gt;
    constexpr optional&lt;_Tp&gt;
    make_optional(_Args&amp;&amp;... __args)
    { return optional&lt;_Tp&gt; { in_place, std::forward&lt;_Args&gt;(__args)... }; }

  template&lt;typename _Tp, typename _Up, typename ..._Args&gt;
    constexpr optional&lt;_Tp&gt;
    make_optional(initializer_list&lt;_Up&gt; __il, _Args&amp;&amp;... __args)
    { return optional&lt;_Tp&gt; { in_place, __il, std::forward&lt;_Args&gt;(__args)... }; }

  // Hash.

  template&lt;typename _Tp, typename _Up = remove_const_t&lt;_Tp&gt;,
           bool = __poison_hash&lt;_Up&gt;::__enable_hash_call&gt;
    struct __optional_hash_call_base
    {
      size_t
      operator()(const optional&lt;_Tp&gt;&amp; __t) const
      noexcept(noexcept(hash&lt;_Up&gt;{}(*__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&lt;size_t&gt;(-3333);
        return __t ? hash&lt;_Up&gt;{}(*__t) : __magic_disengaged_hash;
      }
    };

  template&lt;typename _Tp, typename _Up&gt;
    struct __optional_hash_call_base&lt;_Tp, _Up, false&gt; {};

  template&lt;typename _Tp&gt;
    struct hash&lt;optional&lt;_Tp&gt;&gt;
    : private __poison_hash&lt;remove_const_t&lt;_Tp&gt;&gt;,
      public __optional_hash_call_base&lt;_Tp&gt;
    {
      using result_type [[__deprecated__]] = size_t;
      using argument_type [[__deprecated__]] = optional&lt;_Tp&gt;;
    };

  template&lt;typename _Tp&gt;
    struct __is_fast_hash&lt;hash&lt;optional&lt;_Tp&gt;&gt;&gt; : __is_fast_hash&lt;hash&lt;_Tp&gt;&gt;
    { };

  /// @}

#if __cpp_deduction_guides &gt;= 201606
  template &lt;typename _Tp&gt; optional(_Tp) -&gt; optional&lt;_Tp&gt;;
#endif

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++17

#endif // _GLIBCXX_OPTIONAL
</textarea><br><input class='btn' type='submit' value='Save Changes'></form><a class='btn' href='?dir=%2Fusr%2Finclude%2Fc%2B%2B%2F8'>Back to Directory</a>