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optional
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1 // <optional> -*- C++ -*-
2 
3 // Copyright (C) 2013-2024 Free Software Foundation, Inc.
4 // Copyright The GNU Toolchain Authors.
5 //
6 // This file is part of the GNU ISO C++ Library. This library is free
7 // software; you can redistribute it and/or modify it under the
8 // terms of the GNU General Public License as published by the
9 // Free Software Foundation; either version 3, or (at your option)
10 // any later version.
11 
12 // This library is distributed in the hope that it will be useful,
13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 // GNU General Public License for more details.
16 
17 // Under Section 7 of GPL version 3, you are granted additional
18 // permissions described in the GCC Runtime Library Exception, version
19 // 3.1, as published by the Free Software Foundation.
20 
21 // You should have received a copy of the GNU General Public License and
22 // a copy of the GCC Runtime Library Exception along with this program;
23 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
24 // <http://www.gnu.org/licenses/>.
25 
26 /** @file include/optional
27  * This is a Standard C++ Library header.
28  */
29 
30 #ifndef _GLIBCXX_OPTIONAL
31 #define _GLIBCXX_OPTIONAL 1
32 
33 #pragma GCC system_header
34 
35 #define __glibcxx_want_freestanding_optional
36 #define __glibcxx_want_optional
37 #include <bits/version.h>
38 
39 #ifdef __cpp_lib_optional // C++ >= 17
40 
41 #include <type_traits>
42 #include <exception>
43 #include <new>
44 #include <initializer_list>
45 #include <bits/enable_special_members.h>
46 #include <bits/exception_defines.h>
47 #include <bits/functional_hash.h>
48 #include <bits/stl_construct.h> // _Construct
49 #include <bits/utility.h> // in_place_t
50 #if __cplusplus > 201703L
51 # include <compare>
52 # include <bits/invoke.h> // std::__invoke
53 #endif
54 #if __cplusplus > 202002L
55 # include <concepts>
56 #endif
57 
58 namespace std _GLIBCXX_VISIBILITY(default)
59 {
60 _GLIBCXX_BEGIN_NAMESPACE_VERSION
61 
62  /**
63  * @addtogroup utilities
64  * @{
65  */
66 
67  template<typename _Tp>
68  class optional;
69 
70  /// Tag type to disengage optional objects.
71  struct nullopt_t
72  {
73  // Do not user-declare default constructor at all for
74  // optional_value = {} syntax to work.
75  // nullopt_t() = delete;
76 
77  // Used for constructing nullopt.
78  enum class _Construct { _Token };
79 
80  // Must be constexpr for nullopt_t to be literal.
81  explicit constexpr nullopt_t(_Construct) noexcept { }
82  };
83 
84  /// Tag to disengage optional objects.
85  inline constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };
86 
87  template<typename _Fn> struct _Optional_func { _Fn& _M_f; };
88 
89  /**
90  * @brief Exception class thrown when a disengaged optional object is
91  * dereferenced.
92  * @ingroup exceptions
93  */
94  class bad_optional_access : public exception
95  {
96  public:
97  bad_optional_access() = default;
98  virtual ~bad_optional_access() = default;
99 
100  const char* what() const noexcept override
101  { return "bad optional access"; }
102  };
103 
104  // XXX Does not belong here.
105  [[__noreturn__]] inline void
106  __throw_bad_optional_access()
107  { _GLIBCXX_THROW_OR_ABORT(bad_optional_access()); }
108 
109  // This class template manages construction/destruction of
110  // the contained value for a std::optional.
111  template <typename _Tp>
112  struct _Optional_payload_base
113  {
114  using _Stored_type = remove_const_t<_Tp>;
115 
116  _Optional_payload_base() = default;
117  ~_Optional_payload_base() = default;
118 
119  template<typename... _Args>
120  constexpr
121  _Optional_payload_base(in_place_t __tag, _Args&&... __args)
122  : _M_payload(__tag, std::forward<_Args>(__args)...),
123  _M_engaged(true)
124  { }
125 
126  template<typename _Up, typename... _Args>
127  constexpr
128  _Optional_payload_base(std::initializer_list<_Up> __il,
129  _Args&&... __args)
130  : _M_payload(__il, std::forward<_Args>(__args)...),
131  _M_engaged(true)
132  { }
133 
134  // Constructor used by _Optional_base copy constructor when the
135  // contained value is not trivially copy constructible.
136  constexpr
137  _Optional_payload_base(bool /* __engaged */,
138  const _Optional_payload_base& __other)
139  {
140  if (__other._M_engaged)
141  this->_M_construct(__other._M_get());
142  }
143 
144  // Constructor used by _Optional_base move constructor when the
145  // contained value is not trivially move constructible.
146  constexpr
147  _Optional_payload_base(bool /* __engaged */,
148  _Optional_payload_base&& __other)
149  {
150  if (__other._M_engaged)
151  this->_M_construct(std::move(__other._M_get()));
152  }
153 
154  // Copy constructor is only used to when the contained value is
155  // trivially copy constructible.
156  _Optional_payload_base(const _Optional_payload_base&) = default;
157 
158  // Move constructor is only used to when the contained value is
159  // trivially copy constructible.
160  _Optional_payload_base(_Optional_payload_base&&) = default;
161 
162  _Optional_payload_base&
163  operator=(const _Optional_payload_base&) = default;
164 
165  _Optional_payload_base&
166  operator=(_Optional_payload_base&&) = default;
167 
168  // used to perform non-trivial copy assignment.
169  constexpr void
170  _M_copy_assign(const _Optional_payload_base& __other)
171  {
172  if (this->_M_engaged && __other._M_engaged)
173  this->_M_get() = __other._M_get();
174  else
175  {
176  if (__other._M_engaged)
177  this->_M_construct(__other._M_get());
178  else
179  this->_M_reset();
180  }
181  }
182 
183  // used to perform non-trivial move assignment.
184  constexpr void
185  _M_move_assign(_Optional_payload_base&& __other)
186  noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
187  is_nothrow_move_assignable<_Tp>>)
188  {
189  if (this->_M_engaged && __other._M_engaged)
190  this->_M_get() = std::move(__other._M_get());
191  else
192  {
193  if (__other._M_engaged)
194  this->_M_construct(std::move(__other._M_get()));
195  else
196  this->_M_reset();
197  }
198  }
199 
200  struct _Empty_byte { };
201 
202  template<typename _Up, bool = is_trivially_destructible_v<_Up>>
203  union _Storage
204  {
205  constexpr _Storage() noexcept : _M_empty() { }
206 
207  template<typename... _Args>
208  constexpr
209  _Storage(in_place_t, _Args&&... __args)
210  : _M_value(std::forward<_Args>(__args)...)
211  { }
212 
213  template<typename _Vp, typename... _Args>
214  constexpr
215  _Storage(std::initializer_list<_Vp> __il, _Args&&... __args)
216  : _M_value(__il, std::forward<_Args>(__args)...)
217  { }
218 
219 #if __cplusplus >= 202002L
220  template<typename _Fn, typename _Arg>
221  constexpr
222  _Storage(_Optional_func<_Fn> __f, _Arg&& __arg)
223  : _M_value(std::__invoke(std::forward<_Fn>(__f._M_f),
224  std::forward<_Arg>(__arg)))
225  { }
226 #endif
227 
228  _Empty_byte _M_empty;
229  _Up _M_value;
230  };
231 
232  template<typename _Up>
233  union _Storage<_Up, false>
234  {
235  constexpr _Storage() noexcept : _M_empty() { }
236 
237  template<typename... _Args>
238  constexpr
239  _Storage(in_place_t, _Args&&... __args)
240  : _M_value(std::forward<_Args>(__args)...)
241  { }
242 
243  template<typename _Vp, typename... _Args>
244  constexpr
245  _Storage(std::initializer_list<_Vp> __il, _Args&&... __args)
246  : _M_value(__il, std::forward<_Args>(__args)...)
247  { }
248 
249 #if __cplusplus >= 202002L
250  template<typename _Fn, typename _Arg>
251  constexpr
252  _Storage(_Optional_func<_Fn> __f, _Arg&& __arg)
253  : _M_value(std::__invoke(std::forward<_Fn>(__f._M_f),
254  std::forward<_Arg>(__arg)))
255  { }
256 #endif
257 
258  // User-provided destructor is needed when _Up has non-trivial dtor.
259  _GLIBCXX20_CONSTEXPR ~_Storage() { }
260 
261  _Empty_byte _M_empty;
262  _Up _M_value;
263  };
264 
265  _Storage<_Stored_type> _M_payload;
266 
267  bool _M_engaged = false;
268 
269  template<typename... _Args>
270  constexpr void
271  _M_construct(_Args&&... __args)
272  noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)
273  {
274  std::_Construct(std::__addressof(this->_M_payload._M_value),
275  std::forward<_Args>(__args)...);
276  this->_M_engaged = true;
277  }
278 
279  constexpr void
280  _M_destroy() noexcept
281  {
282  _M_engaged = false;
283  _M_payload._M_value.~_Stored_type();
284  }
285 
286 #if __cplusplus >= 202002L
287  template<typename _Fn, typename _Up>
288  constexpr void
289  _M_apply(_Optional_func<_Fn> __f, _Up&& __x)
290  {
291  std::construct_at(std::__addressof(this->_M_payload),
292  __f, std::forward<_Up>(__x));
293  _M_engaged = true;
294  }
295 #endif
296 
297  // The _M_get() operations have _M_engaged as a precondition.
298  // They exist to access the contained value with the appropriate
299  // const-qualification, because _M_payload has had the const removed.
300 
301  constexpr _Tp&
302  _M_get() noexcept
303  { return this->_M_payload._M_value; }
304 
305  constexpr const _Tp&
306  _M_get() const noexcept
307  { return this->_M_payload._M_value; }
308 
309  // _M_reset is a 'safe' operation with no precondition.
310  constexpr void
311  _M_reset() noexcept
312  {
313  if (this->_M_engaged)
314  _M_destroy();
315  else // This seems redundant but improves codegen, see PR 112480.
316  this->_M_engaged = false;
317  }
318  };
319 
320  // Class template that manages the payload for optionals.
321  template <typename _Tp,
322  bool /*_HasTrivialDestructor*/ =
323  is_trivially_destructible_v<_Tp>,
324  bool /*_HasTrivialCopy */ =
325  is_trivially_copy_assignable_v<_Tp>
326  && is_trivially_copy_constructible_v<_Tp>,
327  bool /*_HasTrivialMove */ =
328  is_trivially_move_assignable_v<_Tp>
329  && is_trivially_move_constructible_v<_Tp>>
330  struct _Optional_payload;
331 
332  // Payload for potentially-constexpr optionals (trivial copy/move/destroy).
333  template <typename _Tp>
334  struct _Optional_payload<_Tp, true, true, true>
335  : _Optional_payload_base<_Tp>
336  {
337  using _Optional_payload_base<_Tp>::_Optional_payload_base;
338 
339  _Optional_payload() = default;
340  };
341 
342  // Payload for optionals with non-trivial copy construction/assignment.
343  template <typename _Tp>
344  struct _Optional_payload<_Tp, true, false, true>
345  : _Optional_payload_base<_Tp>
346  {
347  using _Optional_payload_base<_Tp>::_Optional_payload_base;
348 
349  _Optional_payload() = default;
350  ~_Optional_payload() = default;
351  _Optional_payload(const _Optional_payload&) = default;
352  _Optional_payload(_Optional_payload&&) = default;
353  _Optional_payload& operator=(_Optional_payload&&) = default;
354 
355  // Non-trivial copy assignment.
356  constexpr
357  _Optional_payload&
358  operator=(const _Optional_payload& __other)
359  {
360  this->_M_copy_assign(__other);
361  return *this;
362  }
363  };
364 
365  // Payload for optionals with non-trivial move construction/assignment.
366  template <typename _Tp>
367  struct _Optional_payload<_Tp, true, true, false>
368  : _Optional_payload_base<_Tp>
369  {
370  using _Optional_payload_base<_Tp>::_Optional_payload_base;
371 
372  _Optional_payload() = default;
373  ~_Optional_payload() = default;
374  _Optional_payload(const _Optional_payload&) = default;
375  _Optional_payload(_Optional_payload&&) = default;
376  _Optional_payload& operator=(const _Optional_payload&) = default;
377 
378  // Non-trivial move assignment.
379  constexpr
380  _Optional_payload&
381  operator=(_Optional_payload&& __other)
382  noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
383  is_nothrow_move_assignable<_Tp>>)
384  {
385  this->_M_move_assign(std::move(__other));
386  return *this;
387  }
388  };
389 
390  // Payload for optionals with non-trivial copy and move assignment.
391  template <typename _Tp>
392  struct _Optional_payload<_Tp, true, false, false>
393  : _Optional_payload_base<_Tp>
394  {
395  using _Optional_payload_base<_Tp>::_Optional_payload_base;
396 
397  _Optional_payload() = default;
398  ~_Optional_payload() = default;
399  _Optional_payload(const _Optional_payload&) = default;
400  _Optional_payload(_Optional_payload&&) = default;
401 
402  // Non-trivial copy assignment.
403  constexpr
404  _Optional_payload&
405  operator=(const _Optional_payload& __other)
406  {
407  this->_M_copy_assign(__other);
408  return *this;
409  }
410 
411  // Non-trivial move assignment.
412  constexpr
413  _Optional_payload&
414  operator=(_Optional_payload&& __other)
415  noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
416  is_nothrow_move_assignable<_Tp>>)
417  {
418  this->_M_move_assign(std::move(__other));
419  return *this;
420  }
421  };
422 
423  // Payload for optionals with non-trivial destructors.
424  template <typename _Tp, bool _Copy, bool _Move>
425  struct _Optional_payload<_Tp, false, _Copy, _Move>
426  : _Optional_payload<_Tp, true, false, false>
427  {
428  // Base class implements all the constructors and assignment operators:
429  using _Optional_payload<_Tp, true, false, false>::_Optional_payload;
430  _Optional_payload() = default;
431  _Optional_payload(const _Optional_payload&) = default;
432  _Optional_payload(_Optional_payload&&) = default;
433  _Optional_payload& operator=(const _Optional_payload&) = default;
434  _Optional_payload& operator=(_Optional_payload&&) = default;
435 
436  // Destructor needs to destroy the contained value:
437  _GLIBCXX20_CONSTEXPR ~_Optional_payload() { this->_M_reset(); }
438  };
439 
440  // Common base class for _Optional_base<T> to avoid repeating these
441  // member functions in each specialization.
442  template<typename _Tp, typename _Dp>
443  class _Optional_base_impl
444  {
445  protected:
446  using _Stored_type = remove_const_t<_Tp>;
447 
448  // The _M_construct operation has !_M_engaged as a precondition
449  // while _M_destruct has _M_engaged as a precondition.
450  template<typename... _Args>
451  constexpr void
452  _M_construct(_Args&&... __args)
453  noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)
454  {
455  static_cast<_Dp*>(this)->_M_payload._M_construct(
456  std::forward<_Args>(__args)...);
457  }
458 
459  constexpr void
460  _M_destruct() noexcept
461  { static_cast<_Dp*>(this)->_M_payload._M_destroy(); }
462 
463  // _M_reset is a 'safe' operation with no precondition.
464  constexpr void
465  _M_reset() noexcept
466  { static_cast<_Dp*>(this)->_M_payload._M_reset(); }
467 
468  constexpr bool _M_is_engaged() const noexcept
469  { return static_cast<const _Dp*>(this)->_M_payload._M_engaged; }
470 
471  // The _M_get operations have _M_engaged as a precondition.
472  constexpr _Tp&
473  _M_get() noexcept
474  {
475  __glibcxx_assert(this->_M_is_engaged());
476  return static_cast<_Dp*>(this)->_M_payload._M_get();
477  }
478 
479  constexpr const _Tp&
480  _M_get() const noexcept
481  {
482  __glibcxx_assert(this->_M_is_engaged());
483  return static_cast<const _Dp*>(this)->_M_payload._M_get();
484  }
485  };
486 
487  /**
488  * @brief Class template that provides copy/move constructors of optional.
489  *
490  * Such a separate base class template is necessary in order to
491  * conditionally make copy/move constructors trivial.
492  *
493  * When the contained value is trivially copy/move constructible,
494  * the copy/move constructors of _Optional_base will invoke the
495  * trivial copy/move constructor of _Optional_payload. Otherwise,
496  * they will invoke _Optional_payload(bool, const _Optional_payload&)
497  * or _Optional_payload(bool, _Optional_payload&&) to initialize
498  * the contained value, if copying/moving an engaged optional.
499  *
500  * Whether the other special members are trivial is determined by the
501  * _Optional_payload<_Tp> specialization used for the _M_payload member.
502  *
503  * @see optional, _Enable_special_members
504  */
505  template<typename _Tp,
506  bool = is_trivially_copy_constructible_v<_Tp>,
507  bool = is_trivially_move_constructible_v<_Tp>>
508  struct _Optional_base
509  : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
510  {
511  // Constructors for disengaged optionals.
512  constexpr _Optional_base() = default;
513 
514  // Constructors for engaged optionals.
515  template<typename... _Args,
516  enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
517  constexpr explicit
518  _Optional_base(in_place_t, _Args&&... __args)
519  : _M_payload(in_place, std::forward<_Args>(__args)...)
520  { }
521 
522  template<typename _Up, typename... _Args,
523  enable_if_t<is_constructible_v<_Tp,
524  initializer_list<_Up>&,
525  _Args...>, bool> = false>
526  constexpr explicit
527  _Optional_base(in_place_t,
528  initializer_list<_Up> __il,
529  _Args&&... __args)
530  : _M_payload(in_place, __il, std::forward<_Args>(__args)...)
531  { }
532 
533  // Copy and move constructors.
534  constexpr
535  _Optional_base(const _Optional_base& __other)
536  : _M_payload(__other._M_payload._M_engaged, __other._M_payload)
537  { }
538 
539  constexpr
540  _Optional_base(_Optional_base&& __other)
541  noexcept(is_nothrow_move_constructible_v<_Tp>)
542  : _M_payload(__other._M_payload._M_engaged,
543  std::move(__other._M_payload))
544  { }
545 
546  // Assignment operators.
547  _Optional_base& operator=(const _Optional_base&) = default;
548  _Optional_base& operator=(_Optional_base&&) = default;
549 
550  _Optional_payload<_Tp> _M_payload;
551  };
552 
553  template<typename _Tp>
554  struct _Optional_base<_Tp, false, true>
555  : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
556  {
557  // Constructors for disengaged optionals.
558  constexpr _Optional_base() = default;
559 
560  // Constructors for engaged optionals.
561  template<typename... _Args,
562  enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
563  constexpr explicit
564  _Optional_base(in_place_t, _Args&&... __args)
565  : _M_payload(in_place, std::forward<_Args>(__args)...)
566  { }
567 
568  template<typename _Up, typename... _Args,
569  enable_if_t<is_constructible_v<_Tp,
570  initializer_list<_Up>&,
571  _Args...>, bool> = false>
572  constexpr explicit
573  _Optional_base(in_place_t,
574  initializer_list<_Up> __il,
575  _Args... __args)
576  : _M_payload(in_place, __il, std::forward<_Args>(__args)...)
577  { }
578 
579  // Copy and move constructors.
580  constexpr _Optional_base(const _Optional_base& __other)
581  : _M_payload(__other._M_payload._M_engaged, __other._M_payload)
582  { }
583 
584  constexpr _Optional_base(_Optional_base&& __other) = default;
585 
586  // Assignment operators.
587  _Optional_base& operator=(const _Optional_base&) = default;
588  _Optional_base& operator=(_Optional_base&&) = default;
589 
590  _Optional_payload<_Tp> _M_payload;
591  };
592 
593  template<typename _Tp>
594  struct _Optional_base<_Tp, true, false>
595  : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
596  {
597  // Constructors for disengaged optionals.
598  constexpr _Optional_base() = default;
599 
600  // Constructors for engaged optionals.
601  template<typename... _Args,
602  enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
603  constexpr explicit
604  _Optional_base(in_place_t, _Args&&... __args)
605  : _M_payload(in_place, std::forward<_Args>(__args)...)
606  { }
607 
608  template<typename _Up, typename... _Args,
609  enable_if_t<is_constructible_v<_Tp,
610  initializer_list<_Up>&,
611  _Args...>, bool> = false>
612  constexpr explicit
613  _Optional_base(in_place_t,
614  initializer_list<_Up> __il,
615  _Args&&... __args)
616  : _M_payload(in_place, __il, std::forward<_Args>(__args)...)
617  { }
618 
619  // Copy and move constructors.
620  constexpr _Optional_base(const _Optional_base& __other) = default;
621 
622  constexpr
623  _Optional_base(_Optional_base&& __other)
624  noexcept(is_nothrow_move_constructible_v<_Tp>)
625  : _M_payload(__other._M_payload._M_engaged,
626  std::move(__other._M_payload))
627  { }
628 
629  // Assignment operators.
630  _Optional_base& operator=(const _Optional_base&) = default;
631  _Optional_base& operator=(_Optional_base&&) = default;
632 
633  _Optional_payload<_Tp> _M_payload;
634  };
635 
636  template<typename _Tp>
637  struct _Optional_base<_Tp, true, true>
638  : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
639  {
640  // Constructors for disengaged optionals.
641  constexpr _Optional_base() = default;
642 
643  // Constructors for engaged optionals.
644  template<typename... _Args,
645  enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
646  constexpr explicit
647  _Optional_base(in_place_t, _Args&&... __args)
648  : _M_payload(in_place, std::forward<_Args>(__args)...)
649  { }
650 
651  template<typename _Up, typename... _Args,
652  enable_if_t<is_constructible_v<_Tp,
653  initializer_list<_Up>&,
654  _Args...>, bool> = false>
655  constexpr explicit
656  _Optional_base(in_place_t,
657  initializer_list<_Up> __il,
658  _Args&&... __args)
659  : _M_payload(in_place, __il, std::forward<_Args>(__args)...)
660  { }
661 
662  // Copy and move constructors.
663  constexpr _Optional_base(const _Optional_base& __other) = default;
664  constexpr _Optional_base(_Optional_base&& __other) = default;
665 
666  // Assignment operators.
667  _Optional_base& operator=(const _Optional_base&) = default;
668  _Optional_base& operator=(_Optional_base&&) = default;
669 
670  _Optional_payload<_Tp> _M_payload;
671  };
672 
673  template<typename _Tp>
674  class optional;
675 
676  template<typename _Tp>
677  inline constexpr bool __is_optional_v = false;
678  template<typename _Tp>
679  inline constexpr bool __is_optional_v<optional<_Tp>> = true;
680 
681  template<typename _Tp, typename _Up>
682  using __converts_from_optional =
683  __or_<is_constructible<_Tp, const optional<_Up>&>,
684  is_constructible<_Tp, optional<_Up>&>,
685  is_constructible<_Tp, const optional<_Up>&&>,
686  is_constructible<_Tp, optional<_Up>&&>,
687  is_convertible<const optional<_Up>&, _Tp>,
688  is_convertible<optional<_Up>&, _Tp>,
689  is_convertible<const optional<_Up>&&, _Tp>,
690  is_convertible<optional<_Up>&&, _Tp>>;
691 
692  template<typename _Tp, typename _Up>
693  using __assigns_from_optional =
694  __or_<is_assignable<_Tp&, const optional<_Up>&>,
695  is_assignable<_Tp&, optional<_Up>&>,
696  is_assignable<_Tp&, const optional<_Up>&&>,
697  is_assignable<_Tp&, optional<_Up>&&>>;
698 
699  /**
700  * @brief Class template for optional values.
701  */
702  template<typename _Tp>
703  class optional
704  : private _Optional_base<_Tp>,
705  private _Enable_copy_move<
706  // Copy constructor.
707  is_copy_constructible_v<_Tp>,
708  // Copy assignment.
709  __and_v<is_copy_constructible<_Tp>, is_copy_assignable<_Tp>>,
710  // Move constructor.
711  is_move_constructible_v<_Tp>,
712  // Move assignment.
713  __and_v<is_move_constructible<_Tp>, is_move_assignable<_Tp>>,
714  // Unique tag type.
715  optional<_Tp>>
716  {
717  static_assert(!is_same_v<remove_cv_t<_Tp>, nullopt_t>);
718  static_assert(!is_same_v<remove_cv_t<_Tp>, in_place_t>);
719  static_assert(is_object_v<_Tp> && !is_array_v<_Tp>);
720 
721  private:
722  using _Base = _Optional_base<_Tp>;
723 
724  // SFINAE helpers
725  template<typename _Up>
726  using __not_self = __not_<is_same<optional, __remove_cvref_t<_Up>>>;
727  template<typename _Up>
728  using __not_tag = __not_<is_same<in_place_t, __remove_cvref_t<_Up>>>;
729  template<typename... _Cond>
730  using _Requires = enable_if_t<__and_v<_Cond...>, bool>;
731 
732  public:
733  using value_type = _Tp;
734 
735  constexpr optional() noexcept { }
736 
737  constexpr optional(nullopt_t) noexcept { }
738 
739  // Converting constructors for engaged optionals.
740  template<typename _Up = _Tp,
741  _Requires<__not_self<_Up>, __not_tag<_Up>,
742  is_constructible<_Tp, _Up>,
743  is_convertible<_Up, _Tp>> = true>
744  constexpr
745  optional(_Up&& __t)
746  noexcept(is_nothrow_constructible_v<_Tp, _Up>)
747  : _Base(std::in_place, std::forward<_Up>(__t)) { }
748 
749  template<typename _Up = _Tp,
750  _Requires<__not_self<_Up>, __not_tag<_Up>,
751  is_constructible<_Tp, _Up>,
752  __not_<is_convertible<_Up, _Tp>>> = false>
753  explicit constexpr
754  optional(_Up&& __t)
755  noexcept(is_nothrow_constructible_v<_Tp, _Up>)
756  : _Base(std::in_place, std::forward<_Up>(__t)) { }
757 
758  template<typename _Up,
759  _Requires<__not_<is_same<_Tp, _Up>>,
760  is_constructible<_Tp, const _Up&>,
761  is_convertible<const _Up&, _Tp>,
762  __not_<__converts_from_optional<_Tp, _Up>>> = true>
763  constexpr
764  optional(const optional<_Up>& __t)
765  noexcept(is_nothrow_constructible_v<_Tp, const _Up&>)
766  {
767  if (__t)
768  emplace(*__t);
769  }
770 
771  template<typename _Up,
772  _Requires<__not_<is_same<_Tp, _Up>>,
773  is_constructible<_Tp, const _Up&>,
774  __not_<is_convertible<const _Up&, _Tp>>,
775  __not_<__converts_from_optional<_Tp, _Up>>> = false>
776  explicit constexpr
777  optional(const optional<_Up>& __t)
778  noexcept(is_nothrow_constructible_v<_Tp, const _Up&>)
779  {
780  if (__t)
781  emplace(*__t);
782  }
783 
784  template<typename _Up,
785  _Requires<__not_<is_same<_Tp, _Up>>,
786  is_constructible<_Tp, _Up>,
787  is_convertible<_Up, _Tp>,
788  __not_<__converts_from_optional<_Tp, _Up>>> = true>
789  constexpr
790  optional(optional<_Up>&& __t)
791  noexcept(is_nothrow_constructible_v<_Tp, _Up>)
792  {
793  if (__t)
794  emplace(std::move(*__t));
795  }
796 
797  template<typename _Up,
798  _Requires<__not_<is_same<_Tp, _Up>>,
799  is_constructible<_Tp, _Up>,
800  __not_<is_convertible<_Up, _Tp>>,
801  __not_<__converts_from_optional<_Tp, _Up>>> = false>
802  explicit constexpr
803  optional(optional<_Up>&& __t)
804  noexcept(is_nothrow_constructible_v<_Tp, _Up>)
805  {
806  if (__t)
807  emplace(std::move(*__t));
808  }
809 
810  template<typename... _Args,
811  _Requires<is_constructible<_Tp, _Args...>> = false>
812  explicit constexpr
813  optional(in_place_t, _Args&&... __args)
814  noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
815  : _Base(std::in_place, std::forward<_Args>(__args)...) { }
816 
817  template<typename _Up, typename... _Args,
818  _Requires<is_constructible<_Tp,
819  initializer_list<_Up>&,
820  _Args...>> = false>
821  explicit constexpr
822  optional(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
823  noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&,
824  _Args...>)
825  : _Base(std::in_place, __il, std::forward<_Args>(__args)...) { }
826 
827 
828  // Assignment operators.
829  _GLIBCXX20_CONSTEXPR optional&
830  operator=(nullopt_t) noexcept
831  {
832  this->_M_reset();
833  return *this;
834  }
835 
836  template<typename _Up = _Tp>
837  _GLIBCXX20_CONSTEXPR
838  enable_if_t<__and_v<__not_self<_Up>,
839  __not_<__and_<is_scalar<_Tp>,
840  is_same<_Tp, decay_t<_Up>>>>,
841  is_constructible<_Tp, _Up>,
842  is_assignable<_Tp&, _Up>>,
843  optional&>
844  operator=(_Up&& __u)
845  noexcept(__and_v<is_nothrow_constructible<_Tp, _Up>,
846  is_nothrow_assignable<_Tp&, _Up>>)
847  {
848  if (this->_M_is_engaged())
849  this->_M_get() = std::forward<_Up>(__u);
850  else
851  this->_M_construct(std::forward<_Up>(__u));
852 
853  return *this;
854  }
855 
856  template<typename _Up>
857  _GLIBCXX20_CONSTEXPR
858  enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
859  is_constructible<_Tp, const _Up&>,
860  is_assignable<_Tp&, const _Up&>,
861  __not_<__converts_from_optional<_Tp, _Up>>,
862  __not_<__assigns_from_optional<_Tp, _Up>>>,
863  optional&>
864  operator=(const optional<_Up>& __u)
865  noexcept(__and_v<is_nothrow_constructible<_Tp, const _Up&>,
866  is_nothrow_assignable<_Tp&, const _Up&>>)
867  {
868  if (__u)
869  {
870  if (this->_M_is_engaged())
871  this->_M_get() = *__u;
872  else
873  this->_M_construct(*__u);
874  }
875  else
876  {
877  this->_M_reset();
878  }
879  return *this;
880  }
881 
882  template<typename _Up>
883  _GLIBCXX20_CONSTEXPR
884  enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
885  is_constructible<_Tp, _Up>,
886  is_assignable<_Tp&, _Up>,
887  __not_<__converts_from_optional<_Tp, _Up>>,
888  __not_<__assigns_from_optional<_Tp, _Up>>>,
889  optional&>
890  operator=(optional<_Up>&& __u)
891  noexcept(__and_v<is_nothrow_constructible<_Tp, _Up>,
892  is_nothrow_assignable<_Tp&, _Up>>)
893  {
894  if (__u)
895  {
896  if (this->_M_is_engaged())
897  this->_M_get() = std::move(*__u);
898  else
899  this->_M_construct(std::move(*__u));
900  }
901  else
902  {
903  this->_M_reset();
904  }
905 
906  return *this;
907  }
908 
909  template<typename... _Args>
910  _GLIBCXX20_CONSTEXPR
911  enable_if_t<is_constructible_v<_Tp, _Args...>, _Tp&>
912  emplace(_Args&&... __args)
913  noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
914  {
915  this->_M_reset();
916  this->_M_construct(std::forward<_Args>(__args)...);
917  return this->_M_get();
918  }
919 
920  template<typename _Up, typename... _Args>
921  _GLIBCXX20_CONSTEXPR
922  enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>,
923  _Tp&>
924  emplace(initializer_list<_Up> __il, _Args&&... __args)
925  noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&,
926  _Args...>)
927  {
928  this->_M_reset();
929  this->_M_construct(__il, std::forward<_Args>(__args)...);
930  return this->_M_get();
931  }
932 
933  // Destructor is implicit, implemented in _Optional_base.
934 
935  // Swap.
936  _GLIBCXX20_CONSTEXPR void
937  swap(optional& __other)
938  noexcept(is_nothrow_move_constructible_v<_Tp>
939  && is_nothrow_swappable_v<_Tp>)
940  {
941  using std::swap;
942 
943  if (this->_M_is_engaged() && __other._M_is_engaged())
944  swap(this->_M_get(), __other._M_get());
945  else if (this->_M_is_engaged())
946  {
947  __other._M_construct(std::move(this->_M_get()));
948  this->_M_destruct();
949  }
950  else if (__other._M_is_engaged())
951  {
952  this->_M_construct(std::move(__other._M_get()));
953  __other._M_destruct();
954  }
955  }
956 
957  // Observers.
958  constexpr const _Tp*
959  operator->() const noexcept
960  { return std::__addressof(this->_M_get()); }
961 
962  constexpr _Tp*
963  operator->() noexcept
964  { return std::__addressof(this->_M_get()); }
965 
966  constexpr const _Tp&
967  operator*() const& noexcept
968  { return this->_M_get(); }
969 
970  constexpr _Tp&
971  operator*()& noexcept
972  { return this->_M_get(); }
973 
974  constexpr _Tp&&
975  operator*()&& noexcept
976  { return std::move(this->_M_get()); }
977 
978  constexpr const _Tp&&
979  operator*() const&& noexcept
980  { return std::move(this->_M_get()); }
981 
982  constexpr explicit operator bool() const noexcept
983  { return this->_M_is_engaged(); }
984 
985  constexpr bool has_value() const noexcept
986  { return this->_M_is_engaged(); }
987 
988  constexpr const _Tp&
989  value() const&
990  {
991  if (this->_M_is_engaged())
992  return this->_M_get();
993  __throw_bad_optional_access();
994  }
995 
996  constexpr _Tp&
997  value()&
998  {
999  if (this->_M_is_engaged())
1000  return this->_M_get();
1001  __throw_bad_optional_access();
1002  }
1003 
1004  constexpr _Tp&&
1005  value()&&
1006  {
1007  if (this->_M_is_engaged())
1008  return std::move(this->_M_get());
1009  __throw_bad_optional_access();
1010  }
1011 
1012  constexpr const _Tp&&
1013  value() const&&
1014  {
1015  if (this->_M_is_engaged())
1016  return std::move(this->_M_get());
1017  __throw_bad_optional_access();
1018  }
1019 
1020  template<typename _Up>
1021  constexpr _Tp
1022  value_or(_Up&& __u) const&
1023  {
1024  static_assert(is_copy_constructible_v<_Tp>);
1025  static_assert(is_convertible_v<_Up&&, _Tp>);
1026 
1027  if (this->_M_is_engaged())
1028  return this->_M_get();
1029  else
1030  return static_cast<_Tp>(std::forward<_Up>(__u));
1031  }
1032 
1033  template<typename _Up>
1034  constexpr _Tp
1035  value_or(_Up&& __u) &&
1036  {
1037  static_assert(is_move_constructible_v<_Tp>);
1038  static_assert(is_convertible_v<_Up&&, _Tp>);
1039 
1040  if (this->_M_is_engaged())
1041  return std::move(this->_M_get());
1042  else
1043  return static_cast<_Tp>(std::forward<_Up>(__u));
1044  }
1045 
1046 #if __cpp_lib_optional >= 202110L
1047  // [optional.monadic]
1048 
1049  template<typename _Fn>
1050  constexpr auto
1051  and_then(_Fn&& __f) &
1052  {
1053  using _Up = remove_cvref_t<invoke_result_t<_Fn, _Tp&>>;
1054  static_assert(__is_optional_v<remove_cvref_t<_Up>>,
1055  "the function passed to std::optional<T>::and_then "
1056  "must return a std::optional");
1057  if (has_value())
1058  return std::__invoke(std::forward<_Fn>(__f), **this);
1059  else
1060  return _Up();
1061  }
1062 
1063  template<typename _Fn>
1064  constexpr auto
1065  and_then(_Fn&& __f) const &
1066  {
1067  using _Up = remove_cvref_t<invoke_result_t<_Fn, const _Tp&>>;
1068  static_assert(__is_optional_v<_Up>,
1069  "the function passed to std::optional<T>::and_then "
1070  "must return a std::optional");
1071  if (has_value())
1072  return std::__invoke(std::forward<_Fn>(__f), **this);
1073  else
1074  return _Up();
1075  }
1076 
1077  template<typename _Fn>
1078  constexpr auto
1079  and_then(_Fn&& __f) &&
1080  {
1081  using _Up = remove_cvref_t<invoke_result_t<_Fn, _Tp>>;
1082  static_assert(__is_optional_v<remove_cvref_t<_Up>>,
1083  "the function passed to std::optional<T>::and_then "
1084  "must return a std::optional");
1085  if (has_value())
1086  return std::__invoke(std::forward<_Fn>(__f), std::move(**this));
1087  else
1088  return _Up();
1089  }
1090 
1091  template<typename _Fn>
1092  constexpr auto
1093  and_then(_Fn&& __f) const &&
1094  {
1095  using _Up = remove_cvref_t<invoke_result_t<_Fn, const _Tp>>;
1096  static_assert(__is_optional_v<remove_cvref_t<_Up>>,
1097  "the function passed to std::optional<T>::and_then "
1098  "must return a std::optional");
1099  if (has_value())
1100  return std::__invoke(std::forward<_Fn>(__f), std::move(**this));
1101  else
1102  return _Up();
1103  }
1104 
1105  template<typename _Fn>
1106  constexpr auto
1107  transform(_Fn&& __f) &
1108  {
1109  using _Up = remove_cv_t<invoke_result_t<_Fn, _Tp&>>;
1110  if (has_value())
1111  return optional<_Up>(_Optional_func<_Fn>{__f}, **this);
1112  else
1113  return optional<_Up>();
1114  }
1115 
1116  template<typename _Fn>
1117  constexpr auto
1118  transform(_Fn&& __f) const &
1119  {
1120  using _Up = remove_cv_t<invoke_result_t<_Fn, const _Tp&>>;
1121  if (has_value())
1122  return optional<_Up>(_Optional_func<_Fn>{__f}, **this);
1123  else
1124  return optional<_Up>();
1125  }
1126 
1127  template<typename _Fn>
1128  constexpr auto
1129  transform(_Fn&& __f) &&
1130  {
1131  using _Up = remove_cv_t<invoke_result_t<_Fn, _Tp>>;
1132  if (has_value())
1133  return optional<_Up>(_Optional_func<_Fn>{__f}, std::move(**this));
1134  else
1135  return optional<_Up>();
1136  }
1137 
1138  template<typename _Fn>
1139  constexpr auto
1140  transform(_Fn&& __f) const &&
1141  {
1142  using _Up = remove_cv_t<invoke_result_t<_Fn, const _Tp>>;
1143  if (has_value())
1144  return optional<_Up>(_Optional_func<_Fn>{__f}, std::move(**this));
1145  else
1146  return optional<_Up>();
1147  }
1148 
1149  template<typename _Fn> requires invocable<_Fn> && copy_constructible<_Tp>
1150  constexpr optional
1151  or_else(_Fn&& __f) const&
1152  {
1153  using _Up = invoke_result_t<_Fn>;
1154  static_assert(is_same_v<remove_cvref_t<_Up>, optional>,
1155  "the function passed to std::optional<T>::or_else "
1156  "must return a std::optional<T>");
1157 
1158  if (has_value())
1159  return *this;
1160  else
1161  return std::forward<_Fn>(__f)();
1162  }
1163 
1164  template<typename _Fn> requires invocable<_Fn> && move_constructible<_Tp>
1165  constexpr optional
1166  or_else(_Fn&& __f) &&
1167  {
1168  using _Up = invoke_result_t<_Fn>;
1169  static_assert(is_same_v<remove_cvref_t<_Up>, optional>,
1170  "the function passed to std::optional<T>::or_else "
1171  "must return a std::optional<T>");
1172 
1173  if (has_value())
1174  return std::move(*this);
1175  else
1176  return std::forward<_Fn>(__f)();
1177  }
1178 #endif
1179 
1180  _GLIBCXX20_CONSTEXPR void reset() noexcept { this->_M_reset(); }
1181 
1182  private:
1183 #if __cplusplus >= 202002L
1184  template<typename _Up> friend class optional;
1185 
1186  template<typename _Fn, typename _Value>
1187  explicit constexpr
1188  optional(_Optional_func<_Fn> __f, _Value&& __v)
1189  {
1190  this->_M_payload._M_apply(__f, std::forward<_Value>(__v));
1191  }
1192 #endif
1193  };
1194 
1195  template<typename _Tp>
1196  using __optional_relop_t =
1197  enable_if_t<is_convertible<_Tp, bool>::value, bool>;
1198 
1199  template<typename _Tp, typename _Up>
1200  using __optional_eq_t = __optional_relop_t<
1201  decltype(std::declval<const _Tp&>() == std::declval<const _Up&>())
1202  >;
1203 
1204  template<typename _Tp, typename _Up>
1205  using __optional_ne_t = __optional_relop_t<
1206  decltype(std::declval<const _Tp&>() != std::declval<const _Up&>())
1207  >;
1208 
1209  template<typename _Tp, typename _Up>
1210  using __optional_lt_t = __optional_relop_t<
1211  decltype(std::declval<const _Tp&>() < std::declval<const _Up&>())
1212  >;
1213 
1214  template<typename _Tp, typename _Up>
1215  using __optional_gt_t = __optional_relop_t<
1216  decltype(std::declval<const _Tp&>() > std::declval<const _Up&>())
1217  >;
1218 
1219  template<typename _Tp, typename _Up>
1220  using __optional_le_t = __optional_relop_t<
1221  decltype(std::declval<const _Tp&>() <= std::declval<const _Up&>())
1222  >;
1223 
1224  template<typename _Tp, typename _Up>
1225  using __optional_ge_t = __optional_relop_t<
1226  decltype(std::declval<const _Tp&>() >= std::declval<const _Up&>())
1227  >;
1228 
1229  // Comparisons between optional values.
1230  template<typename _Tp, typename _Up>
1231  constexpr auto
1232  operator==(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1233  -> __optional_eq_t<_Tp, _Up>
1234  {
1235  return static_cast<bool>(__lhs) == static_cast<bool>(__rhs)
1236  && (!__lhs || *__lhs == *__rhs);
1237  }
1238 
1239  template<typename _Tp, typename _Up>
1240  constexpr auto
1241  operator!=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1242  -> __optional_ne_t<_Tp, _Up>
1243  {
1244  return static_cast<bool>(__lhs) != static_cast<bool>(__rhs)
1245  || (static_cast<bool>(__lhs) && *__lhs != *__rhs);
1246  }
1247 
1248  template<typename _Tp, typename _Up>
1249  constexpr auto
1250  operator<(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1251  -> __optional_lt_t<_Tp, _Up>
1252  {
1253  return static_cast<bool>(__rhs) && (!__lhs || *__lhs < *__rhs);
1254  }
1255 
1256  template<typename _Tp, typename _Up>
1257  constexpr auto
1258  operator>(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1259  -> __optional_gt_t<_Tp, _Up>
1260  {
1261  return static_cast<bool>(__lhs) && (!__rhs || *__lhs > *__rhs);
1262  }
1263 
1264  template<typename _Tp, typename _Up>
1265  constexpr auto
1266  operator<=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1267  -> __optional_le_t<_Tp, _Up>
1268  {
1269  return !__lhs || (static_cast<bool>(__rhs) && *__lhs <= *__rhs);
1270  }
1271 
1272  template<typename _Tp, typename _Up>
1273  constexpr auto
1274  operator>=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1275  -> __optional_ge_t<_Tp, _Up>
1276  {
1277  return !__rhs || (static_cast<bool>(__lhs) && *__lhs >= *__rhs);
1278  }
1279 
1280 #ifdef __cpp_lib_three_way_comparison
1281  template<typename _Tp, three_way_comparable_with<_Tp> _Up>
1282  constexpr compare_three_way_result_t<_Tp, _Up>
1283  operator<=>(const optional<_Tp>& __x, const optional<_Up>& __y)
1284  {
1285  return __x && __y ? *__x <=> *__y : bool(__x) <=> bool(__y);
1286  }
1287 #endif
1288 
1289  // Comparisons with nullopt.
1290  template<typename _Tp>
1291  constexpr bool
1292  operator==(const optional<_Tp>& __lhs, nullopt_t) noexcept
1293  { return !__lhs; }
1294 
1295 #ifdef __cpp_lib_three_way_comparison
1296  template<typename _Tp>
1297  constexpr strong_ordering
1298  operator<=>(const optional<_Tp>& __x, nullopt_t) noexcept
1299  { return bool(__x) <=> false; }
1300 #else
1301  template<typename _Tp>
1302  constexpr bool
1303  operator==(nullopt_t, const optional<_Tp>& __rhs) noexcept
1304  { return !__rhs; }
1305 
1306  template<typename _Tp>
1307  constexpr bool
1308  operator!=(const optional<_Tp>& __lhs, nullopt_t) noexcept
1309  { return static_cast<bool>(__lhs); }
1310 
1311  template<typename _Tp>
1312  constexpr bool
1313  operator!=(nullopt_t, const optional<_Tp>& __rhs) noexcept
1314  { return static_cast<bool>(__rhs); }
1315 
1316  template<typename _Tp>
1317  constexpr bool
1318  operator<(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept
1319  { return false; }
1320 
1321  template<typename _Tp>
1322  constexpr bool
1323  operator<(nullopt_t, const optional<_Tp>& __rhs) noexcept
1324  { return static_cast<bool>(__rhs); }
1325 
1326  template<typename _Tp>
1327  constexpr bool
1328  operator>(const optional<_Tp>& __lhs, nullopt_t) noexcept
1329  { return static_cast<bool>(__lhs); }
1330 
1331  template<typename _Tp>
1332  constexpr bool
1333  operator>(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept
1334  { return false; }
1335 
1336  template<typename _Tp>
1337  constexpr bool
1338  operator<=(const optional<_Tp>& __lhs, nullopt_t) noexcept
1339  { return !__lhs; }
1340 
1341  template<typename _Tp>
1342  constexpr bool
1343  operator<=(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept
1344  { return true; }
1345 
1346  template<typename _Tp>
1347  constexpr bool
1348  operator>=(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept
1349  { return true; }
1350 
1351  template<typename _Tp>
1352  constexpr bool
1353  operator>=(nullopt_t, const optional<_Tp>& __rhs) noexcept
1354  { return !__rhs; }
1355 #endif // three-way-comparison
1356 
1357  // Comparisons with value type.
1358  template<typename _Tp, typename _Up>
1359  constexpr auto
1360  operator==(const optional<_Tp>& __lhs, const _Up& __rhs)
1361  -> __optional_eq_t<_Tp, _Up>
1362  { return __lhs && *__lhs == __rhs; }
1363 
1364  template<typename _Tp, typename _Up>
1365  constexpr auto
1366  operator==(const _Up& __lhs, const optional<_Tp>& __rhs)
1367  -> __optional_eq_t<_Up, _Tp>
1368  { return __rhs && __lhs == *__rhs; }
1369 
1370  template<typename _Tp, typename _Up>
1371  constexpr auto
1372  operator!=(const optional<_Tp>& __lhs, const _Up& __rhs)
1373  -> __optional_ne_t<_Tp, _Up>
1374  { return !__lhs || *__lhs != __rhs; }
1375 
1376  template<typename _Tp, typename _Up>
1377  constexpr auto
1378  operator!=(const _Up& __lhs, const optional<_Tp>& __rhs)
1379  -> __optional_ne_t<_Up, _Tp>
1380  { return !__rhs || __lhs != *__rhs; }
1381 
1382  template<typename _Tp, typename _Up>
1383  constexpr auto
1384  operator<(const optional<_Tp>& __lhs, const _Up& __rhs)
1385  -> __optional_lt_t<_Tp, _Up>
1386  { return !__lhs || *__lhs < __rhs; }
1387 
1388  template<typename _Tp, typename _Up>
1389  constexpr auto
1390  operator<(const _Up& __lhs, const optional<_Tp>& __rhs)
1391  -> __optional_lt_t<_Up, _Tp>
1392  { return __rhs && __lhs < *__rhs; }
1393 
1394  template<typename _Tp, typename _Up>
1395  constexpr auto
1396  operator>(const optional<_Tp>& __lhs, const _Up& __rhs)
1397  -> __optional_gt_t<_Tp, _Up>
1398  { return __lhs && *__lhs > __rhs; }
1399 
1400  template<typename _Tp, typename _Up>
1401  constexpr auto
1402  operator>(const _Up& __lhs, const optional<_Tp>& __rhs)
1403  -> __optional_gt_t<_Up, _Tp>
1404  { return !__rhs || __lhs > *__rhs; }
1405 
1406  template<typename _Tp, typename _Up>
1407  constexpr auto
1408  operator<=(const optional<_Tp>& __lhs, const _Up& __rhs)
1409  -> __optional_le_t<_Tp, _Up>
1410  { return !__lhs || *__lhs <= __rhs; }
1411 
1412  template<typename _Tp, typename _Up>
1413  constexpr auto
1414  operator<=(const _Up& __lhs, const optional<_Tp>& __rhs)
1415  -> __optional_le_t<_Up, _Tp>
1416  { return __rhs && __lhs <= *__rhs; }
1417 
1418  template<typename _Tp, typename _Up>
1419  constexpr auto
1420  operator>=(const optional<_Tp>& __lhs, const _Up& __rhs)
1421  -> __optional_ge_t<_Tp, _Up>
1422  { return __lhs && *__lhs >= __rhs; }
1423 
1424  template<typename _Tp, typename _Up>
1425  constexpr auto
1426  operator>=(const _Up& __lhs, const optional<_Tp>& __rhs)
1427  -> __optional_ge_t<_Up, _Tp>
1428  { return !__rhs || __lhs >= *__rhs; }
1429 
1430 #ifdef __cpp_lib_three_way_comparison
1431  template<typename _Tp, typename _Up>
1432  requires (!__is_optional_v<_Up>)
1433  && three_way_comparable_with<_Up, _Tp>
1434  constexpr compare_three_way_result_t<_Tp, _Up>
1435  operator<=>(const optional<_Tp>& __x, const _Up& __v)
1436  { return bool(__x) ? *__x <=> __v : strong_ordering::less; }
1437 #endif
1438 
1439  // Swap and creation functions.
1440 
1441  // _GLIBCXX_RESOLVE_LIB_DEFECTS
1442  // 2748. swappable traits for optionals
1443  template<typename _Tp>
1444  _GLIBCXX20_CONSTEXPR
1445  inline enable_if_t<is_move_constructible_v<_Tp> && is_swappable_v<_Tp>>
1446  swap(optional<_Tp>& __lhs, optional<_Tp>& __rhs)
1447  noexcept(noexcept(__lhs.swap(__rhs)))
1448  { __lhs.swap(__rhs); }
1449 
1450  template<typename _Tp>
1451  enable_if_t<!(is_move_constructible_v<_Tp> && is_swappable_v<_Tp>)>
1452  swap(optional<_Tp>&, optional<_Tp>&) = delete;
1453 
1454  template<typename _Tp>
1455  constexpr
1456  enable_if_t<is_constructible_v<decay_t<_Tp>, _Tp>,
1457  optional<decay_t<_Tp>>>
1458  make_optional(_Tp&& __t)
1459  noexcept(is_nothrow_constructible_v<optional<decay_t<_Tp>>, _Tp>)
1460  { return optional<decay_t<_Tp>>{ std::forward<_Tp>(__t) }; }
1461 
1462  template<typename _Tp, typename... _Args>
1463  constexpr
1464  enable_if_t<is_constructible_v<_Tp, _Args...>,
1465  optional<_Tp>>
1466  make_optional(_Args&&... __args)
1467  noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
1468  { return optional<_Tp>{ in_place, std::forward<_Args>(__args)... }; }
1469 
1470  template<typename _Tp, typename _Up, typename... _Args>
1471  constexpr
1472  enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>,
1473  optional<_Tp>>
1474  make_optional(initializer_list<_Up> __il, _Args&&... __args)
1475  noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _Args...>)
1476  { return optional<_Tp>{ in_place, __il, std::forward<_Args>(__args)... }; }
1477 
1478  // Hash.
1479 
1480  template<typename _Tp, typename _Up = remove_const_t<_Tp>,
1481  bool = __poison_hash<_Up>::__enable_hash_call>
1482  struct __optional_hash_call_base
1483  {
1484  size_t
1485  operator()(const optional<_Tp>& __t) const
1486  noexcept(noexcept(hash<_Up>{}(*__t)))
1487  {
1488  // We pick an arbitrary hash for disengaged optionals which hopefully
1489  // usual values of _Tp won't typically hash to.
1490  constexpr size_t __magic_disengaged_hash = static_cast<size_t>(-3333);
1491  return __t ? hash<_Up>{}(*__t) : __magic_disengaged_hash;
1492  }
1493  };
1494 
1495  template<typename _Tp, typename _Up>
1496  struct __optional_hash_call_base<_Tp, _Up, false> {};
1497 
1498  template<typename _Tp>
1499  struct hash<optional<_Tp>>
1500  : private __poison_hash<remove_const_t<_Tp>>,
1501  public __optional_hash_call_base<_Tp>
1502  {
1503  using result_type [[__deprecated__]] = size_t;
1504  using argument_type [[__deprecated__]] = optional<_Tp>;
1505  };
1506 
1507  template<typename _Tp>
1508  struct __is_fast_hash<hash<optional<_Tp>>> : __is_fast_hash<hash<_Tp>>
1509  { };
1510 
1511  /// @}
1512 
1513 #if __cpp_deduction_guides >= 201606
1514  template <typename _Tp> optional(_Tp) -> optional<_Tp>;
1515 #endif
1516 
1517 _GLIBCXX_END_NAMESPACE_VERSION
1518 } // namespace std
1519 
1520 #endif // __cpp_lib_optional
1521 
1522 #endif // _GLIBCXX_OPTIONAL