Import stripped Arm GNU Toolchain 13.2.Rel1HEADumineko

https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads

Change-Id: I7303388733328cd98ab9aa3c30236db67f2e9e9c

1 files changed, 1512 insertions, 0 deletions

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+// <optional> -*- C++ -*-
+
+// Copyright (C) 2013-2023 Free Software Foundation, Inc.
+// Copyright The GNU Toolchain Authors.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file include/optional
+ *  This is a Standard C++ Library header.
+ */
+
+#ifndef _GLIBCXX_OPTIONAL
+#define _GLIBCXX_OPTIONAL 1
+
+#pragma GCC system_header
+
+#if __cplusplus >= 201703L
+
+#include <type_traits>
+#include <exception>
+#include <new>
+#include <initializer_list>
+#include <bits/enable_special_members.h>
+#include <bits/exception_defines.h>
+#include <bits/functional_hash.h>
+#include <bits/stl_construct.h> // _Construct
+#include <bits/utility.h> // in_place_t
+#if __cplusplus > 201703L
+# include <compare>
+# include <bits/invoke.h> // std::__invoke
+#endif
+#if __cplusplus > 202002L
+# include <concepts>
+#endif
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  /**
+   *  @addtogroup utilities
+   *  @{
+   */
+
+#if __cplusplus > 202002L && __cpp_lib_concepts
+# define __cpp_lib_optional 202110L
+#elif __cplusplus >= 202002L
+# define __cpp_lib_optional 202106L
+#else
+# define __cpp_lib_optional 201606L
+#endif
+
+  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) noexcept { }
+  };
+
+  /// Tag to disengage optional objects.
+  inline constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };
+
+  template<typename _Fn> struct _Optional_func { _Fn& _M_f; };
+
+  /**
+   *  @brief Exception class thrown when a disengaged optional object is
+   *  dereferenced.
+   *  @ingroup exceptions
+   */
+  class bad_optional_access : public exception
+  {
+  public:
+    bad_optional_access() = default;
+    virtual ~bad_optional_access() = default;
+
+    const char* what() const noexcept override
+    { return "bad optional access"; }
+  };
+
+  // XXX Does not belong here.
+  [[__noreturn__]] inline void
+  __throw_bad_optional_access()
+  { _GLIBCXX_THROW_OR_ABORT(bad_optional_access()); }
+
+  // This class template manages construction/destruction of
+  // the contained value for a std::optional.
+  template <typename _Tp>
+    struct _Optional_payload_base
+    {
+      using _Stored_type = remove_const_t<_Tp>;
+
+      _Optional_payload_base() = default;
+      ~_Optional_payload_base() = default;
+
+      template<typename... _Args>
+	constexpr
+	_Optional_payload_base(in_place_t __tag, _Args&&... __args)
+	: _M_payload(__tag, std::forward<_Args>(__args)...),
+	  _M_engaged(true)
+	{ }
+
+      template<typename _Up, typename... _Args>
+	constexpr
+	_Optional_payload_base(std::initializer_list<_Up> __il,
+			       _Args&&... __args)
+	: _M_payload(__il, std::forward<_Args>(__args)...),
+	  _M_engaged(true)
+	{ }
+
+      // Constructor used by _Optional_base copy constructor when the
+      // contained value is not trivially copy constructible.
+      constexpr
+      _Optional_payload_base(bool __engaged,
+			     const _Optional_payload_base& __other)
+      {
+	if (__other._M_engaged)
+	  this->_M_construct(__other._M_get());
+      }
+
+      // Constructor used by _Optional_base move constructor when the
+      // contained value is not trivially move constructible.
+      constexpr
+      _Optional_payload_base(bool __engaged,
+			     _Optional_payload_base&& __other)
+      {
+	if (__other._M_engaged)
+	  this->_M_construct(std::move(__other._M_get()));
+      }
+
+      // Copy constructor is only used to when the contained value is
+      // trivially copy constructible.
+      _Optional_payload_base(const _Optional_payload_base&) = default;
+
+      // Move constructor is only used to when the contained value is
+      // trivially copy constructible.
+      _Optional_payload_base(_Optional_payload_base&&) = default;
+
+      _Optional_payload_base&
+      operator=(const _Optional_payload_base&) = default;
+
+      _Optional_payload_base&
+      operator=(_Optional_payload_base&&) = default;
+
+      // used to perform non-trivial copy assignment.
+      constexpr void
+      _M_copy_assign(const _Optional_payload_base& __other)
+      {
+	if (this->_M_engaged && __other._M_engaged)
+	  this->_M_get() = __other._M_get();
+	else
+	  {
+	    if (__other._M_engaged)
+	      this->_M_construct(__other._M_get());
+	    else
+	      this->_M_reset();
+	  }
+      }
+
+      // used to perform non-trivial move assignment.
+      constexpr void
+      _M_move_assign(_Optional_payload_base&& __other)
+      noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
+		       is_nothrow_move_assignable<_Tp>>)
+      {
+	if (this->_M_engaged && __other._M_engaged)
+	  this->_M_get() = std::move(__other._M_get());
+	else
+	  {
+	    if (__other._M_engaged)
+	      this->_M_construct(std::move(__other._M_get()));
+	    else
+	      this->_M_reset();
+	  }
+      }
+
+      struct _Empty_byte { };
+
+      template<typename _Up, bool = is_trivially_destructible_v<_Up>>
+	union _Storage
+	{
+	  constexpr _Storage() noexcept : _M_empty() { }
+
+	  template<typename... _Args>
+	    constexpr
+	    _Storage(in_place_t, _Args&&... __args)
+	    : _M_value(std::forward<_Args>(__args)...)
+	    { }
+
+	  template<typename _Vp, typename... _Args>
+	    constexpr
+	    _Storage(std::initializer_list<_Vp> __il, _Args&&... __args)
+	    : _M_value(__il, std::forward<_Args>(__args)...)
+	    { }
+
+#if __cplusplus >= 202002L
+	  template<typename _Fn, typename _Arg>
+	    constexpr
+	    _Storage(_Optional_func<_Fn> __f, _Arg&& __arg)
+	    : _M_value(std::__invoke(std::forward<_Fn>(__f._M_f),
+				     std::forward<_Arg>(__arg)))
+	    { }
+#endif
+
+	  _Empty_byte _M_empty;
+	  _Up _M_value;
+	};
+
+      template<typename _Up>
+	union _Storage<_Up, false>
+	{
+	  constexpr _Storage() noexcept : _M_empty() { }
+
+	  template<typename... _Args>
+	    constexpr
+	    _Storage(in_place_t, _Args&&... __args)
+	    : _M_value(std::forward<_Args>(__args)...)
+	    { }
+
+	  template<typename _Vp, typename... _Args>
+	    constexpr
+	    _Storage(std::initializer_list<_Vp> __il, _Args&&... __args)
+	    : _M_value(__il, std::forward<_Args>(__args)...)
+	    { }
+
+#if __cplusplus >= 202002L
+	  template<typename _Fn, typename _Arg>
+	    constexpr
+	    _Storage(_Optional_func<_Fn> __f, _Arg&& __arg)
+	    : _M_value(std::__invoke(std::forward<_Fn>(__f._M_f),
+				     std::forward<_Arg>(__arg)))
+	    { }
+#endif
+
+	  // User-provided destructor is needed when _Up has non-trivial dtor.
+	  _GLIBCXX20_CONSTEXPR ~_Storage() { }
+
+	  _Empty_byte _M_empty;
+	  _Up _M_value;
+	};
+
+      _Storage<_Stored_type> _M_payload;
+
+      bool _M_engaged = false;
+
+      template<typename... _Args>
+	constexpr void
+	_M_construct(_Args&&... __args)
+	noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)
+	{
+	  std::_Construct(std::__addressof(this->_M_payload._M_value),
+			  std::forward<_Args>(__args)...);
+	  this->_M_engaged = true;
+	}
+
+      constexpr void
+      _M_destroy() noexcept
+      {
+	_M_engaged = false;
+	_M_payload._M_value.~_Stored_type();
+      }
+
+#if __cplusplus >= 202002L
+      template<typename _Fn, typename _Up>
+	constexpr void
+	_M_apply(_Optional_func<_Fn> __f, _Up&& __x)
+	{
+	  std::construct_at(std::__addressof(this->_M_payload),
+			    __f, std::forward<_Up>(__x));
+	  _M_engaged = true;
+	}
+#endif
+
+      // The _M_get() operations have _M_engaged as a precondition.
+      // They exist to access the contained value with the appropriate
+      // const-qualification, because _M_payload has had the const removed.
+
+      constexpr _Tp&
+      _M_get() noexcept
+      { return this->_M_payload._M_value; }
+
+      constexpr const _Tp&
+      _M_get() const noexcept
+      { return this->_M_payload._M_value; }
+
+      // _M_reset is a 'safe' operation with no precondition.
+      constexpr void
+      _M_reset() noexcept
+      {
+	if (this->_M_engaged)
+	  _M_destroy();
+      }
+    };
+
+  // Class template that manages the payload for optionals.
+  template <typename _Tp,
+	    bool /*_HasTrivialDestructor*/ =
+	      is_trivially_destructible_v<_Tp>,
+	    bool /*_HasTrivialCopy */ =
+	      is_trivially_copy_assignable_v<_Tp>
+	      && is_trivially_copy_constructible_v<_Tp>,
+	    bool /*_HasTrivialMove */ =
+	      is_trivially_move_assignable_v<_Tp>
+	      && is_trivially_move_constructible_v<_Tp>>
+    struct _Optional_payload;
+
+  // Payload for potentially-constexpr optionals (trivial copy/move/destroy).
+  template <typename _Tp>
+    struct _Optional_payload<_Tp, true, true, true>
+    : _Optional_payload_base<_Tp>
+    {
+      using _Optional_payload_base<_Tp>::_Optional_payload_base;
+
+      _Optional_payload() = default;
+    };
+
+  // Payload for optionals with non-trivial copy construction/assignment.
+  template <typename _Tp>
+    struct _Optional_payload<_Tp, true, false, true>
+    : _Optional_payload_base<_Tp>
+    {
+      using _Optional_payload_base<_Tp>::_Optional_payload_base;
+
+      _Optional_payload() = default;
+      ~_Optional_payload() = default;
+      _Optional_payload(const _Optional_payload&) = default;
+      _Optional_payload(_Optional_payload&&) = default;
+      _Optional_payload& operator=(_Optional_payload&&) = default;
+
+      // Non-trivial copy assignment.
+      constexpr
+      _Optional_payload&
+      operator=(const _Optional_payload& __other)
+      {
+	this->_M_copy_assign(__other);
+	return *this;
+      }
+    };
+
+  // Payload for optionals with non-trivial move construction/assignment.
+  template <typename _Tp>
+    struct _Optional_payload<_Tp, true, true, false>
+    : _Optional_payload_base<_Tp>
+    {
+      using _Optional_payload_base<_Tp>::_Optional_payload_base;
+
+      _Optional_payload() = default;
+      ~_Optional_payload() = default;
+      _Optional_payload(const _Optional_payload&) = default;
+      _Optional_payload(_Optional_payload&&) = default;
+      _Optional_payload& operator=(const _Optional_payload&) = default;
+
+      // Non-trivial move assignment.
+      constexpr
+      _Optional_payload&
+      operator=(_Optional_payload&& __other)
+      noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
+		       is_nothrow_move_assignable<_Tp>>)
+      {
+	this->_M_move_assign(std::move(__other));
+	return *this;
+      }
+    };
+
+  // Payload for optionals with non-trivial copy and move assignment.
+  template <typename _Tp>
+    struct _Optional_payload<_Tp, true, false, false>
+    : _Optional_payload_base<_Tp>
+    {
+      using _Optional_payload_base<_Tp>::_Optional_payload_base;
+
+      _Optional_payload() = default;
+      ~_Optional_payload() = default;
+      _Optional_payload(const _Optional_payload&) = default;
+      _Optional_payload(_Optional_payload&&) = default;
+
+      // Non-trivial copy assignment.
+      constexpr
+      _Optional_payload&
+      operator=(const _Optional_payload& __other)
+      {
+	this->_M_copy_assign(__other);
+	return *this;
+      }
+
+      // Non-trivial move assignment.
+      constexpr
+      _Optional_payload&
+      operator=(_Optional_payload&& __other)
+      noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
+		       is_nothrow_move_assignable<_Tp>>)
+      {
+	this->_M_move_assign(std::move(__other));
+	return *this;
+      }
+    };
+
+  // Payload for optionals with non-trivial destructors.
+  template <typename _Tp, bool _Copy, bool _Move>
+    struct _Optional_payload<_Tp, false, _Copy, _Move>
+    : _Optional_payload<_Tp, true, false, false>
+    {
+      // Base class implements all the constructors and assignment operators:
+      using _Optional_payload<_Tp, true, false, false>::_Optional_payload;
+      _Optional_payload() = default;
+      _Optional_payload(const _Optional_payload&) = default;
+      _Optional_payload(_Optional_payload&&) = default;
+      _Optional_payload& operator=(const _Optional_payload&) = default;
+      _Optional_payload& operator=(_Optional_payload&&) = default;
+
+      // Destructor needs to destroy the contained value:
+      _GLIBCXX20_CONSTEXPR ~_Optional_payload() { this->_M_reset(); }
+    };
+
+  // Common base class for _Optional_base<T> to avoid repeating these
+  // member functions in each specialization.
+  template<typename _Tp, typename _Dp>
+    class _Optional_base_impl
+    {
+    protected:
+      using _Stored_type = remove_const_t<_Tp>;
+
+      // The _M_construct operation has !_M_engaged as a precondition
+      // while _M_destruct has _M_engaged as a precondition.
+      template<typename... _Args>
+	constexpr void
+	_M_construct(_Args&&... __args)
+	noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)
+	{
+	  static_cast<_Dp*>(this)->_M_payload._M_construct(
+	    std::forward<_Args>(__args)...);
+	}
+
+      constexpr void
+      _M_destruct() noexcept
+      { static_cast<_Dp*>(this)->_M_payload._M_destroy(); }
+
+      // _M_reset is a 'safe' operation with no precondition.
+      constexpr void
+      _M_reset() noexcept
+      { static_cast<_Dp*>(this)->_M_payload._M_reset(); }
+
+      constexpr bool _M_is_engaged() const noexcept
+      { return static_cast<const _Dp*>(this)->_M_payload._M_engaged; }
+
+      // The _M_get operations have _M_engaged as a precondition.
+      constexpr _Tp&
+      _M_get() noexcept
+      {
+	__glibcxx_assert(this->_M_is_engaged());
+	return static_cast<_Dp*>(this)->_M_payload._M_get();
+      }
+
+      constexpr const _Tp&
+      _M_get() const noexcept
+      {
+	__glibcxx_assert(this->_M_is_engaged());
+	return static_cast<const _Dp*>(this)->_M_payload._M_get();
+      }
+    };
+
+  /**
+    * @brief Class template that provides copy/move constructors of optional.
+    *
+    * Such a separate base class template is necessary in order to
+    * conditionally make copy/move constructors trivial.
+    *
+    * When the contained value is trivially copy/move constructible,
+    * the copy/move constructors of _Optional_base will invoke the
+    * trivial copy/move constructor of _Optional_payload. Otherwise,
+    * they will invoke _Optional_payload(bool, const _Optional_payload&)
+    * or _Optional_payload(bool, _Optional_payload&&) to initialize
+    * the contained value, if copying/moving an engaged optional.
+    *
+    * Whether the other special members are trivial is determined by the
+    * _Optional_payload<_Tp> specialization used for the _M_payload member.
+    *
+    * @see optional, _Enable_special_members
+    */
+  template<typename _Tp,
+	   bool = is_trivially_copy_constructible_v<_Tp>,
+	   bool = is_trivially_move_constructible_v<_Tp>>
+    struct _Optional_base
+    : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
+    {
+      // Constructors for disengaged optionals.
+      constexpr _Optional_base() = default;
+
+      // Constructors for engaged optionals.
+      template<typename... _Args,
+	       enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
+	constexpr explicit
+	_Optional_base(in_place_t, _Args&&... __args)
+	: _M_payload(in_place, std::forward<_Args>(__args)...)
+	{ }
+
+      template<typename _Up, typename... _Args,
+	       enable_if_t<is_constructible_v<_Tp,
+					      initializer_list<_Up>&,
+					      _Args...>, bool> = false>
+	constexpr explicit
+	_Optional_base(in_place_t,
+		       initializer_list<_Up> __il,
+		       _Args&&... __args)
+	: _M_payload(in_place, __il, std::forward<_Args>(__args)...)
+	{ }
+
+      // Copy and move constructors.
+      constexpr
+      _Optional_base(const _Optional_base& __other)
+      : _M_payload(__other._M_payload._M_engaged, __other._M_payload)
+      { }
+
+      constexpr
+      _Optional_base(_Optional_base&& __other)
+      noexcept(is_nothrow_move_constructible_v<_Tp>)
+      : _M_payload(__other._M_payload._M_engaged,
+		   std::move(__other._M_payload))
+      { }
+
+      // Assignment operators.
+      _Optional_base& operator=(const _Optional_base&) = default;
+      _Optional_base& operator=(_Optional_base&&) = default;
+
+      _Optional_payload<_Tp> _M_payload;
+    };
+
+  template<typename _Tp>
+    struct _Optional_base<_Tp, false, true>
+    : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
+    {
+      // Constructors for disengaged optionals.
+      constexpr _Optional_base() = default;
+
+      // Constructors for engaged optionals.
+      template<typename... _Args,
+	       enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
+	constexpr explicit
+	_Optional_base(in_place_t, _Args&&... __args)
+	: _M_payload(in_place, std::forward<_Args>(__args)...)
+	{ }
+
+      template<typename _Up, typename... _Args,
+	       enable_if_t<is_constructible_v<_Tp,
+					      initializer_list<_Up>&,
+					      _Args...>, bool> = false>
+	constexpr explicit
+	_Optional_base(in_place_t,
+		       initializer_list<_Up> __il,
+		       _Args... __args)
+	: _M_payload(in_place, __il, std::forward<_Args>(__args)...)
+	{ }
+
+      // Copy and move constructors.
+      constexpr _Optional_base(const _Optional_base& __other)
+      : _M_payload(__other._M_payload._M_engaged, __other._M_payload)
+      { }
+
+      constexpr _Optional_base(_Optional_base&& __other) = default;
+
+      // Assignment operators.
+      _Optional_base& operator=(const _Optional_base&) = default;
+      _Optional_base& operator=(_Optional_base&&) = default;
+
+      _Optional_payload<_Tp> _M_payload;
+    };
+
+  template<typename _Tp>
+    struct _Optional_base<_Tp, true, false>
+    : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
+    {
+      // Constructors for disengaged optionals.
+      constexpr _Optional_base() = default;
+
+      // Constructors for engaged optionals.
+      template<typename... _Args,
+	       enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
+	constexpr explicit
+	_Optional_base(in_place_t, _Args&&... __args)
+	: _M_payload(in_place, std::forward<_Args>(__args)...)
+	{ }
+
+      template<typename _Up, typename... _Args,
+	       enable_if_t<is_constructible_v<_Tp,
+					      initializer_list<_Up>&,
+					      _Args...>, bool> = false>
+	constexpr explicit
+	_Optional_base(in_place_t,
+		       initializer_list<_Up> __il,
+		       _Args&&... __args)
+	: _M_payload(in_place, __il, std::forward<_Args>(__args)...)
+	{ }
+
+      // Copy and move constructors.
+      constexpr _Optional_base(const _Optional_base& __other) = default;
+
+      constexpr
+      _Optional_base(_Optional_base&& __other)
+      noexcept(is_nothrow_move_constructible_v<_Tp>)
+      : _M_payload(__other._M_payload._M_engaged,
+		   std::move(__other._M_payload))
+      { }
+
+      // Assignment operators.
+      _Optional_base& operator=(const _Optional_base&) = default;
+      _Optional_base& operator=(_Optional_base&&) = default;
+
+      _Optional_payload<_Tp> _M_payload;
+    };
+
+  template<typename _Tp>
+    struct _Optional_base<_Tp, true, true>
+    : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
+    {
+      // Constructors for disengaged optionals.
+      constexpr _Optional_base() = default;
+
+      // Constructors for engaged optionals.
+      template<typename... _Args,
+	       enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
+	constexpr explicit
+	_Optional_base(in_place_t, _Args&&... __args)
+	: _M_payload(in_place, std::forward<_Args>(__args)...)
+	{ }
+
+      template<typename _Up, typename... _Args,
+	       enable_if_t<is_constructible_v<_Tp,
+					      initializer_list<_Up>&,
+					      _Args...>, bool> = false>
+	constexpr explicit
+	_Optional_base(in_place_t,
+		       initializer_list<_Up> __il,
+		       _Args&&... __args)
+	: _M_payload(in_place, __il, std::forward<_Args>(__args)...)
+	{ }
+
+      // Copy and move constructors.
+      constexpr _Optional_base(const _Optional_base& __other) = default;
+      constexpr _Optional_base(_Optional_base&& __other) = default;
+
+      // Assignment operators.
+      _Optional_base& operator=(const _Optional_base&) = default;
+      _Optional_base& operator=(_Optional_base&&) = default;
+
+      _Optional_payload<_Tp> _M_payload;
+    };
+
+  template<typename _Tp>
+  class optional;
+
+  template<typename _Tp>
+    inline constexpr bool __is_optional_v = false;
+  template<typename _Tp>
+    inline constexpr bool __is_optional_v<optional<_Tp>> = true;
+
+  template<typename _Tp, typename _Up>
+    using __converts_from_optional =
+      __or_<is_constructible<_Tp, const optional<_Up>&>,
+	    is_constructible<_Tp, optional<_Up>&>,
+	    is_constructible<_Tp, const optional<_Up>&&>,
+	    is_constructible<_Tp, optional<_Up>&&>,
+	    is_convertible<const optional<_Up>&, _Tp>,
+	    is_convertible<optional<_Up>&, _Tp>,
+	    is_convertible<const optional<_Up>&&, _Tp>,
+	    is_convertible<optional<_Up>&&, _Tp>>;
+
+  template<typename _Tp, typename _Up>
+    using __assigns_from_optional =
+      __or_<is_assignable<_Tp&, const optional<_Up>&>,
+	    is_assignable<_Tp&, optional<_Up>&>,
+	    is_assignable<_Tp&, const optional<_Up>&&>,
+	    is_assignable<_Tp&, optional<_Up>&&>>;
+
+  /**
+    * @brief Class template for optional values.
+    */
+  template<typename _Tp>
+    class optional
+    : private _Optional_base<_Tp>,
+      private _Enable_copy_move<
+	// Copy constructor.
+	is_copy_constructible_v<_Tp>,
+	// Copy assignment.
+	__and_v<is_copy_constructible<_Tp>, is_copy_assignable<_Tp>>,
+	// Move constructor.
+	is_move_constructible_v<_Tp>,
+	// Move assignment.
+	__and_v<is_move_constructible<_Tp>, is_move_assignable<_Tp>>,
+	// Unique tag type.
+	optional<_Tp>>
+    {
+      static_assert(!is_same_v<remove_cv_t<_Tp>, nullopt_t>);
+      static_assert(!is_same_v<remove_cv_t<_Tp>, in_place_t>);
+      static_assert(is_object_v<_Tp> && !is_array_v<_Tp>);
+
+    private:
+      using _Base = _Optional_base<_Tp>;
+
+      // SFINAE helpers
+      template<typename _Up>
+	using __not_self = __not_<is_same<optional, __remove_cvref_t<_Up>>>;
+      template<typename _Up>
+	using __not_tag = __not_<is_same<in_place_t, __remove_cvref_t<_Up>>>;
+      template<typename... _Cond>
+	using _Requires = enable_if_t<__and_v<_Cond...>, bool>;
+
+    public:
+      using value_type = _Tp;
+
+      constexpr optional() noexcept { }
+
+      constexpr optional(nullopt_t) noexcept { }
+
+      // Converting constructors for engaged optionals.
+      template<typename _Up = _Tp,
+	       _Requires<__not_self<_Up>, __not_tag<_Up>,
+			 is_constructible<_Tp, _Up>,
+			 is_convertible<_Up, _Tp>> = true>
+	constexpr
+	optional(_Up&& __t)
+	noexcept(is_nothrow_constructible_v<_Tp, _Up>)
+	: _Base(std::in_place, std::forward<_Up>(__t)) { }
+
+      template<typename _Up = _Tp,
+	       _Requires<__not_self<_Up>, __not_tag<_Up>,
+			 is_constructible<_Tp, _Up>,
+			 __not_<is_convertible<_Up, _Tp>>> = false>
+	explicit constexpr
+	optional(_Up&& __t)
+	noexcept(is_nothrow_constructible_v<_Tp, _Up>)
+	: _Base(std::in_place, std::forward<_Up>(__t)) { }
+
+      template<typename _Up,
+	       _Requires<__not_<is_same<_Tp, _Up>>,
+			 is_constructible<_Tp, const _Up&>,
+			 is_convertible<const _Up&, _Tp>,
+			 __not_<__converts_from_optional<_Tp, _Up>>> = true>
+	constexpr
+	optional(const optional<_Up>& __t)
+	noexcept(is_nothrow_constructible_v<_Tp, const _Up&>)
+	{
+	  if (__t)
+	    emplace(*__t);
+	}
+
+      template<typename _Up,
+	       _Requires<__not_<is_same<_Tp, _Up>>,
+			 is_constructible<_Tp, const _Up&>,
+			 __not_<is_convertible<const _Up&, _Tp>>,
+			 __not_<__converts_from_optional<_Tp, _Up>>> = false>
+	explicit constexpr
+	optional(const optional<_Up>& __t)
+	noexcept(is_nothrow_constructible_v<_Tp, const _Up&>)
+	{
+	  if (__t)
+	    emplace(*__t);
+	}
+
+      template<typename _Up,
+	       _Requires<__not_<is_same<_Tp, _Up>>,
+			 is_constructible<_Tp, _Up>,
+			 is_convertible<_Up, _Tp>,
+			 __not_<__converts_from_optional<_Tp, _Up>>> = true>
+	constexpr
+	optional(optional<_Up>&& __t)
+	noexcept(is_nothrow_constructible_v<_Tp, _Up>)
+	{
+	  if (__t)
+	    emplace(std::move(*__t));
+	}
+
+      template<typename _Up,
+	       _Requires<__not_<is_same<_Tp, _Up>>,
+			 is_constructible<_Tp, _Up>,
+			 __not_<is_convertible<_Up, _Tp>>,
+			 __not_<__converts_from_optional<_Tp, _Up>>> = false>
+	explicit constexpr
+	optional(optional<_Up>&& __t)
+	noexcept(is_nothrow_constructible_v<_Tp, _Up>)
+	{
+	  if (__t)
+	    emplace(std::move(*__t));
+	}
+
+      template<typename... _Args,
+	       _Requires<is_constructible<_Tp, _Args...>> = false>
+	explicit constexpr
+	optional(in_place_t, _Args&&... __args)
+	noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
+	: _Base(std::in_place, std::forward<_Args>(__args)...) { }
+
+      template<typename _Up, typename... _Args,
+	       _Requires<is_constructible<_Tp,
+					  initializer_list<_Up>&,
+					  _Args...>> = false>
+	explicit constexpr
+	optional(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
+	noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&,
+					    _Args...>)
+	: _Base(std::in_place, __il, std::forward<_Args>(__args)...) { }
+
+
+      // Assignment operators.
+      _GLIBCXX20_CONSTEXPR optional&
+      operator=(nullopt_t) noexcept
+      {
+	this->_M_reset();
+	return *this;
+      }
+
+      template<typename _Up = _Tp>
+	_GLIBCXX20_CONSTEXPR
+	enable_if_t<__and_v<__not_self<_Up>,
+			    __not_<__and_<is_scalar<_Tp>,
+					  is_same<_Tp, decay_t<_Up>>>>,
+			    is_constructible<_Tp, _Up>,
+			    is_assignable<_Tp&, _Up>>,
+		    optional&>
+	operator=(_Up&& __u)
+	noexcept(__and_v<is_nothrow_constructible<_Tp, _Up>,
+			 is_nothrow_assignable<_Tp&, _Up>>)
+	{
+	  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>
+	_GLIBCXX20_CONSTEXPR
+	enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
+			    is_constructible<_Tp, const _Up&>,
+			    is_assignable<_Tp&, const _Up&>,
+			    __not_<__converts_from_optional<_Tp, _Up>>,
+			    __not_<__assigns_from_optional<_Tp, _Up>>>,
+		    optional&>
+	operator=(const optional<_Up>& __u)
+	noexcept(__and_v<is_nothrow_constructible<_Tp, const _Up&>,
+			 is_nothrow_assignable<_Tp&, const _Up&>>)
+	{
+	  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>
+	_GLIBCXX20_CONSTEXPR
+	enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
+			    is_constructible<_Tp, _Up>,
+			    is_assignable<_Tp&, _Up>,
+			    __not_<__converts_from_optional<_Tp, _Up>>,
+			    __not_<__assigns_from_optional<_Tp, _Up>>>,
+		    optional&>
+	operator=(optional<_Up>&& __u)
+	noexcept(__and_v<is_nothrow_constructible<_Tp, _Up>,
+			 is_nothrow_assignable<_Tp&, _Up>>)
+	{
+	  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>
+	_GLIBCXX20_CONSTEXPR
+	enable_if_t<is_constructible_v<_Tp, _Args...>, _Tp&>
+	emplace(_Args&&... __args)
+	noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
+	{
+	  this->_M_reset();
+	  this->_M_construct(std::forward<_Args>(__args)...);
+	  return this->_M_get();
+	}
+
+      template<typename _Up, typename... _Args>
+	_GLIBCXX20_CONSTEXPR
+	enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>,
+		    _Tp&>
+	emplace(initializer_list<_Up> __il, _Args&&... __args)
+	noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&,
+					    _Args...>)
+	{
+	  this->_M_reset();
+	  this->_M_construct(__il, std::forward<_Args>(__args)...);
+	  return this->_M_get();
+	}
+
+      // Destructor is implicit, implemented in _Optional_base.
+
+      // Swap.
+      _GLIBCXX20_CONSTEXPR void
+      swap(optional& __other)
+      noexcept(is_nothrow_move_constructible_v<_Tp>
+	       && is_nothrow_swappable_v<_Tp>)
+      {
+	using std::swap;
+
+	if (this->_M_is_engaged() && __other._M_is_engaged())
+	  swap(this->_M_get(), __other._M_get());
+	else if (this->_M_is_engaged())
+	  {
+	    __other._M_construct(std::move(this->_M_get()));
+	    this->_M_destruct();
+	  }
+	else if (__other._M_is_engaged())
+	  {
+	    this->_M_construct(std::move(__other._M_get()));
+	    __other._M_destruct();
+	  }
+      }
+
+      // Observers.
+      constexpr const _Tp*
+      operator->() const noexcept
+      { return std::__addressof(this->_M_get()); }
+
+      constexpr _Tp*
+      operator->() noexcept
+      { return std::__addressof(this->_M_get()); }
+
+      constexpr const _Tp&
+      operator*() const& noexcept
+      { return this->_M_get(); }
+
+      constexpr _Tp&
+      operator*()& noexcept
+      { return this->_M_get(); }
+
+      constexpr _Tp&&
+      operator*()&& noexcept
+      { return std::move(this->_M_get()); }
+
+      constexpr const _Tp&&
+      operator*() const&& noexcept
+      { 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&
+      {
+	if (this->_M_is_engaged())
+	  return this->_M_get();
+	__throw_bad_optional_access();
+      }
+
+      constexpr _Tp&
+      value()&
+      {
+	if (this->_M_is_engaged())
+	  return this->_M_get();
+	__throw_bad_optional_access();
+      }
+
+      constexpr _Tp&&
+      value()&&
+      {
+	if (this->_M_is_engaged())
+	  return std::move(this->_M_get());
+	__throw_bad_optional_access();
+      }
+
+      constexpr const _Tp&&
+      value() const&&
+      {
+	if (this->_M_is_engaged())
+	  return std::move(this->_M_get());
+	__throw_bad_optional_access();
+      }
+
+      template<typename _Up>
+	constexpr _Tp
+	value_or(_Up&& __u) const&
+	{
+	  static_assert(is_copy_constructible_v<_Tp>);
+	  static_assert(is_convertible_v<_Up&&, _Tp>);
+
+	  if (this->_M_is_engaged())
+	    return this->_M_get();
+	  else
+	    return static_cast<_Tp>(std::forward<_Up>(__u));
+	}
+
+      template<typename _Up>
+	constexpr _Tp
+	value_or(_Up&& __u) &&
+	{
+	  static_assert(is_move_constructible_v<_Tp>);
+	  static_assert(is_convertible_v<_Up&&, _Tp>);
+
+	  if (this->_M_is_engaged())
+	    return std::move(this->_M_get());
+	  else
+	    return static_cast<_Tp>(std::forward<_Up>(__u));
+	}
+
+#if __cpp_lib_optional >= 202110L
+      // [optional.monadic]
+
+      template<typename _Fn>
+	constexpr auto
+	and_then(_Fn&& __f) &
+	{
+	  using _Up = remove_cvref_t<invoke_result_t<_Fn, _Tp&>>;
+	  static_assert(__is_optional_v<remove_cvref_t<_Up>>);
+	  if (has_value())
+	    return std::__invoke(std::forward<_Fn>(__f), **this);
+	  else
+	    return _Up();
+	}
+
+      template<typename _Fn>
+	constexpr auto
+	and_then(_Fn&& __f) const &
+	{
+	  using _Up = remove_cvref_t<invoke_result_t<_Fn, const _Tp&>>;
+	  static_assert(__is_optional_v<_Up>);
+	  if (has_value())
+	    return std::__invoke(std::forward<_Fn>(__f), **this);
+	  else
+	    return _Up();
+	}
+
+      template<typename _Fn>
+	constexpr auto
+	and_then(_Fn&& __f) &&
+	{
+	  using _Up = remove_cvref_t<invoke_result_t<_Fn, _Tp>>;
+	  static_assert(__is_optional_v<remove_cvref_t<_Up>>);
+	  if (has_value())
+	    return std::__invoke(std::forward<_Fn>(__f), std::move(**this));
+	  else
+	    return _Up();
+	}
+
+      template<typename _Fn>
+	constexpr auto
+	and_then(_Fn&& __f) const &&
+	{
+	  using _Up = remove_cvref_t<invoke_result_t<_Fn, const _Tp>>;
+	  static_assert(__is_optional_v<remove_cvref_t<_Up>>);
+	  if (has_value())
+	    return std::__invoke(std::forward<_Fn>(__f), std::move(**this));
+	  else
+	    return _Up();
+	}
+
+      template<typename _Fn>
+	constexpr auto
+	transform(_Fn&& __f) &
+	{
+	  using _Up = remove_cv_t<invoke_result_t<_Fn, _Tp&>>;
+	  if (has_value())
+	    return optional<_Up>(_Optional_func<_Fn>{__f}, **this);
+	  else
+	    return optional<_Up>();
+	}
+
+      template<typename _Fn>
+	constexpr auto
+	transform(_Fn&& __f) const &
+	{
+	  using _Up = remove_cv_t<invoke_result_t<_Fn, const _Tp&>>;
+	  if (has_value())
+	    return optional<_Up>(_Optional_func<_Fn>{__f}, **this);
+	  else
+	    return optional<_Up>();
+	}
+
+      template<typename _Fn>
+	constexpr auto
+	transform(_Fn&& __f) &&
+	{
+	  using _Up = remove_cv_t<invoke_result_t<_Fn, _Tp>>;
+	  if (has_value())
+	    return optional<_Up>(_Optional_func<_Fn>{__f}, std::move(**this));
+	  else
+	    return optional<_Up>();
+	}
+
+      template<typename _Fn>
+	constexpr auto
+	transform(_Fn&& __f) const &&
+	{
+	  using _Up = remove_cv_t<invoke_result_t<_Fn, const _Tp>>;
+	  if (has_value())
+	    return optional<_Up>(_Optional_func<_Fn>{__f}, std::move(**this));
+	  else
+	    return optional<_Up>();
+	}
+
+      template<typename _Fn> requires invocable<_Fn> && copy_constructible<_Tp>
+	constexpr optional
+	or_else(_Fn&& __f) const&
+	{
+	  using _Up = invoke_result_t<_Fn>;
+	  static_assert( is_same_v<remove_cvref_t<_Up>, optional> );
+
+	  if (has_value())
+	    return *this;
+	  else
+	    return std::forward<_Fn>(__f)();
+	}
+
+      template<typename _Fn> requires invocable<_Fn> && move_constructible<_Tp>
+	constexpr optional
+	or_else(_Fn&& __f) &&
+	{
+	  using _Up = invoke_result_t<_Fn>;
+	  static_assert( is_same_v<remove_cvref_t<_Up>, optional> );
+
+	  if (has_value())
+	    return std::move(*this);
+	  else
+	    return std::forward<_Fn>(__f)();
+	}
+#endif
+
+      _GLIBCXX20_CONSTEXPR void reset() noexcept { this->_M_reset(); }
+
+    private:
+#if __cplusplus >= 202002L
+      template<typename _Up> friend class optional;
+
+      template<typename _Fn, typename _Value>
+	explicit constexpr
+	optional(_Optional_func<_Fn> __f, _Value&& __v)
+	{
+	  this->_M_payload._M_apply(__f, std::forward<_Value>(__v));
+	}
+#endif
+    };
+
+  template<typename _Tp>
+    using __optional_relop_t =
+      enable_if_t<is_convertible<_Tp, bool>::value, bool>;
+
+  template<typename _Tp, typename _Up>
+    using __optional_eq_t = __optional_relop_t<
+      decltype(std::declval<const _Tp&>() == std::declval<const _Up&>())
+      >;
+
+  template<typename _Tp, typename _Up>
+    using __optional_ne_t = __optional_relop_t<
+      decltype(std::declval<const _Tp&>() != std::declval<const _Up&>())
+      >;
+
+  template<typename _Tp, typename _Up>
+    using __optional_lt_t = __optional_relop_t<
+      decltype(std::declval<const _Tp&>() < std::declval<const _Up&>())
+      >;
+
+  template<typename _Tp, typename _Up>
+    using __optional_gt_t = __optional_relop_t<
+      decltype(std::declval<const _Tp&>() > std::declval<const _Up&>())
+      >;
+
+  template<typename _Tp, typename _Up>
+    using __optional_le_t = __optional_relop_t<
+      decltype(std::declval<const _Tp&>() <= std::declval<const _Up&>())
+      >;
+
+  template<typename _Tp, typename _Up>
+    using __optional_ge_t = __optional_relop_t<
+      decltype(std::declval<const _Tp&>() >= std::declval<const _Up&>())
+      >;
+
+  // Comparisons between optional values.
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator==(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
+    -> __optional_eq_t<_Tp, _Up>
+    {
+      return static_cast<bool>(__lhs) == static_cast<bool>(__rhs)
+	     && (!__lhs || *__lhs == *__rhs);
+    }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator!=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
+    -> __optional_ne_t<_Tp, _Up>
+    {
+      return static_cast<bool>(__lhs) != static_cast<bool>(__rhs)
+	|| (static_cast<bool>(__lhs) && *__lhs != *__rhs);
+    }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator<(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
+    -> __optional_lt_t<_Tp, _Up>
+    {
+      return static_cast<bool>(__rhs) && (!__lhs || *__lhs < *__rhs);
+    }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator>(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
+    -> __optional_gt_t<_Tp, _Up>
+    {
+      return static_cast<bool>(__lhs) && (!__rhs || *__lhs > *__rhs);
+    }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator<=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
+    -> __optional_le_t<_Tp, _Up>
+    {
+      return !__lhs || (static_cast<bool>(__rhs) && *__lhs <= *__rhs);
+    }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator>=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
+    -> __optional_ge_t<_Tp, _Up>
+    {
+      return !__rhs || (static_cast<bool>(__lhs) && *__lhs >= *__rhs);
+    }
+
+#ifdef __cpp_lib_three_way_comparison
+  template<typename _Tp, three_way_comparable_with<_Tp> _Up>
+    constexpr compare_three_way_result_t<_Tp, _Up>
+    operator<=>(const optional<_Tp>& __x, const optional<_Up>& __y)
+    {
+      return __x && __y ? *__x <=> *__y : bool(__x) <=> bool(__y);
+    }
+#endif
+
+  // Comparisons with nullopt.
+  template<typename _Tp>
+    constexpr bool
+    operator==(const optional<_Tp>& __lhs, nullopt_t) noexcept
+    { return !__lhs; }
+
+#ifdef __cpp_lib_three_way_comparison
+  template<typename _Tp>
+    constexpr strong_ordering
+    operator<=>(const optional<_Tp>& __x, nullopt_t) noexcept
+    { return bool(__x) <=> false; }
+#else
+  template<typename _Tp>
+    constexpr bool
+    operator==(nullopt_t, const optional<_Tp>& __rhs) noexcept
+    { return !__rhs; }
+
+  template<typename _Tp>
+    constexpr bool
+    operator!=(const optional<_Tp>& __lhs, nullopt_t) noexcept
+    { return static_cast<bool>(__lhs); }
+
+  template<typename _Tp>
+    constexpr bool
+    operator!=(nullopt_t, const optional<_Tp>& __rhs) noexcept
+    { return static_cast<bool>(__rhs); }
+
+  template<typename _Tp>
+    constexpr bool
+    operator<(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept
+    { return false; }
+
+  template<typename _Tp>
+    constexpr bool
+    operator<(nullopt_t, const optional<_Tp>& __rhs) noexcept
+    { return static_cast<bool>(__rhs); }
+
+  template<typename _Tp>
+    constexpr bool
+    operator>(const optional<_Tp>& __lhs, nullopt_t) noexcept
+    { return static_cast<bool>(__lhs); }
+
+  template<typename _Tp>
+    constexpr bool
+    operator>(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept
+    { return false; }
+
+  template<typename _Tp>
+    constexpr bool
+    operator<=(const optional<_Tp>& __lhs, nullopt_t) noexcept
+    { return !__lhs; }
+
+  template<typename _Tp>
+    constexpr bool
+    operator<=(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept
+    { return true; }
+
+  template<typename _Tp>
+    constexpr bool
+    operator>=(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept
+    { return true; }
+
+  template<typename _Tp>
+    constexpr bool
+    operator>=(nullopt_t, const optional<_Tp>& __rhs) noexcept
+    { return !__rhs; }
+#endif // three-way-comparison
+
+  // Comparisons with value type.
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator==(const optional<_Tp>& __lhs, const _Up& __rhs)
+    -> __optional_eq_t<_Tp, _Up>
+    { return __lhs && *__lhs == __rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator==(const _Up& __lhs, const optional<_Tp>& __rhs)
+    -> __optional_eq_t<_Up, _Tp>
+    { return __rhs && __lhs == *__rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator!=(const optional<_Tp>& __lhs, const _Up& __rhs)
+    -> __optional_ne_t<_Tp, _Up>
+    { return !__lhs || *__lhs != __rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator!=(const _Up& __lhs, const optional<_Tp>& __rhs)
+    -> __optional_ne_t<_Up, _Tp>
+    { return !__rhs || __lhs != *__rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator<(const optional<_Tp>& __lhs, const _Up& __rhs)
+    -> __optional_lt_t<_Tp, _Up>
+    { return !__lhs || *__lhs < __rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator<(const _Up& __lhs, const optional<_Tp>& __rhs)
+    -> __optional_lt_t<_Up, _Tp>
+    { return __rhs && __lhs < *__rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator>(const optional<_Tp>& __lhs, const _Up& __rhs)
+    -> __optional_gt_t<_Tp, _Up>
+    { return __lhs && *__lhs > __rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator>(const _Up& __lhs, const optional<_Tp>& __rhs)
+    -> __optional_gt_t<_Up, _Tp>
+    { return !__rhs || __lhs > *__rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator<=(const optional<_Tp>& __lhs, const _Up& __rhs)
+    -> __optional_le_t<_Tp, _Up>
+    { return !__lhs || *__lhs <= __rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator<=(const _Up& __lhs, const optional<_Tp>& __rhs)
+    -> __optional_le_t<_Up, _Tp>
+    { return __rhs && __lhs <= *__rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator>=(const optional<_Tp>& __lhs, const _Up& __rhs)
+    -> __optional_ge_t<_Tp, _Up>
+    { return __lhs && *__lhs >= __rhs; }
+
+  template<typename _Tp, typename _Up>
+    constexpr auto
+    operator>=(const _Up& __lhs, const optional<_Tp>& __rhs)
+    -> __optional_ge_t<_Up, _Tp>
+    { return !__rhs || __lhs >= *__rhs; }
+
+#ifdef __cpp_lib_three_way_comparison
+  template<typename _Tp, typename _Up>
+    requires (!__is_optional_v<_Up>)
+      && three_way_comparable_with<_Tp, _Up>
+    constexpr compare_three_way_result_t<_Tp, _Up>
+    operator<=>(const optional<_Tp>& __x, const _Up& __v)
+    { return bool(__x) ? *__x <=> __v : strong_ordering::less; }
+#endif
+
+  // Swap and creation functions.
+
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2748. swappable traits for optionals
+  template<typename _Tp>
+    _GLIBCXX20_CONSTEXPR
+    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
+    enable_if_t<is_constructible_v<decay_t<_Tp>, _Tp>,
+		optional<decay_t<_Tp>>>
+    make_optional(_Tp&& __t)
+    noexcept(is_nothrow_constructible_v<optional<decay_t<_Tp>>, _Tp>)
+    { return optional<decay_t<_Tp>>{ std::forward<_Tp>(__t) }; }
+
+  template<typename _Tp, typename... _Args>
+    constexpr
+    enable_if_t<is_constructible_v<_Tp, _Args...>,
+		optional<_Tp>>
+    make_optional(_Args&&... __args)
+    noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
+    { return optional<_Tp>{ in_place, std::forward<_Args>(__args)... }; }
+
+  template<typename _Tp, typename _Up, typename... _Args>
+    constexpr
+    enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>,
+		optional<_Tp>>
+    make_optional(initializer_list<_Up> __il, _Args&&... __args)
+    noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _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