aboutsummaryrefslogtreecommitdiffstats
path: root/include/optional
blob: c0fd0e7bc49ffe86e7e3a449e5f1316b62f8b1c0 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
// -*- C++ -*-
//===-------------------------- optional ----------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is dual licensed under the MIT and the University of Illinois Open
// Source Licenses. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP_OPTIONAL
#define _LIBCPP_OPTIONAL

/*
    optional synopsis

// C++1z

namespace std {
  // 23.6.3, optional for object types
  template <class T> class optional;

  // 23.6.4, no-value state indicator
  struct nullopt_t{see below };
  constexpr nullopt_t nullopt(unspecified );

  // 23.6.5, class bad_optional_access
  class bad_optional_access;

  // 23.6.6, relational operators
  template <class T, class U>
  constexpr bool operator==(const optional<T>&, const optional<U>&);
  template <class T, class U>
  constexpr bool operator!=(const optional<T>&, const optional<U>&);
  template <class T, class U>
  constexpr bool operator<(const optional<T>&, const optional<U>&);
  template <class T, class U>
  constexpr bool operator>(const optional<T>&, const optional<U>&);
  template <class T, class U>
  constexpr bool operator<=(const optional<T>&, const optional<U>&);
  template <class T, class U>
  constexpr bool operator>=(const optional<T>&, const optional<U>&);

  // 23.6.7 comparison with nullopt
  template <class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator==(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator!=(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator!=(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator<(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator<(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator<=(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator>(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator>(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator>=(nullopt_t, const optional<T>&) noexcept;

  // 23.6.8, comparison with T
  template <class T, class U> constexpr bool operator==(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator==(const U&, const optional<T>&);
  template <class T, class U> constexpr bool operator!=(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator!=(const U&, const optional<T>&);
  template <class T, class U> constexpr bool operator<(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator<(const U&, const optional<T>&);
  template <class T, class U> constexpr bool operator<=(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator<=(const U&, const optional<T>&);
  template <class T, class U> constexpr bool operator>(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator>(const U&, const optional<T>&);
  template <class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator>=(const U&, const optional<T>&);

  // 23.6.9, specialized algorithms
  template <class T> void swap(optional<T>&, optional<T>&) noexcept(see below );
  template <class T> constexpr optional<see below > make_optional(T&&);
  template <class T, class... Args>
    constexpr optional<T> make_optional(Args&&... args);
  template <class T, class U, class... Args>
    constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);

  // 23.6.10, hash support
  template <class T> struct hash;
  template <class T> struct hash<optional<T>>;

  template <class T> class optional {
  public:
    using value_type = T;

    // 23.6.3.1, constructors
    constexpr optional() noexcept;
    constexpr optional(nullopt_t) noexcept;
    optional(const optional &);
    optional(optional &&) noexcept(see below);
    template <class... Args> constexpr explicit optional(in_place_t, Args &&...);
    template <class U, class... Args>
      constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...);
    template <class U = T>
      constexpr EXPLICIT optional(U &&);
    template <class U>
      constexpr EXPLICIT optional(const optional<U> &);
    template <class U>
      constexpr EXPLICIT optional(optional<U> &&);

    // 23.6.3.2, destructor
    ~optional();

    // 23.6.3.3, assignment
    optional &operator=(nullopt_t) noexcept;
    optional &operator=(const optional &);
    optional &operator=(optional &&) noexcept(see below );
    template <class U = T> optional &operator=(U &&);
    template <class U> optional &operator=(const optional<U> &);
    template <class U> optional &operator=(optional<U> &&);
    template <class... Args> T& emplace(Args &&...);
    template <class U, class... Args>
      T& emplace(initializer_list<U>, Args &&...);

    // 23.6.3.4, swap
    void swap(optional &) noexcept(see below );

    // 23.6.3.5, observers
    constexpr T const *operator->() const;
    constexpr T *operator->();
    constexpr T const &operator*() const &;
    constexpr T &operator*() &;
    constexpr T &&operator*() &&;
    constexpr const T &&operator*() const &&;
    constexpr explicit operator bool() const noexcept;
    constexpr bool has_value() const noexcept;
    constexpr T const &value() const &;
    constexpr T &value() &;
    constexpr T &&value() &&;
    constexpr const T &&value() const &&;
    template <class U> constexpr T value_or(U &&) const &;
    template <class U> constexpr T value_or(U &&) &&;

    // 23.6.3.6, modifiers
    void reset() noexcept;

  private:
    T *val; // exposition only
  };
} // namespace std

*/

#include <__config>
#include <__debug>
#include <__functional_base>
#include <functional>
#include <initializer_list>
#include <new>
#include <stdexcept>
#include <type_traits>
#include <utility>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif

_LIBCPP_PUSH_MACROS
#include <__undef_macros>


namespace std  // purposefully not using versioning namespace
{

class _LIBCPP_EXCEPTION_ABI bad_optional_access
    : public exception
{
public:
    // Get the key function ~bad_optional_access() into the dylib
    virtual ~bad_optional_access() _NOEXCEPT;
    virtual const char* what() const _NOEXCEPT;
};

}  // std

#if _LIBCPP_STD_VER > 14

_LIBCPP_BEGIN_NAMESPACE_STD

_LIBCPP_NORETURN
inline _LIBCPP_INLINE_VISIBILITY
void __throw_bad_optional_access() {
#ifndef _LIBCPP_NO_EXCEPTIONS
        throw bad_optional_access();
#else
        _VSTD::abort();
#endif
}

struct nullopt_t
{
    struct __secret_tag { _LIBCPP_INLINE_VISIBILITY explicit __secret_tag() = default; };
    _LIBCPP_INLINE_VISIBILITY constexpr explicit nullopt_t(__secret_tag, __secret_tag) noexcept {}
};

/* inline */ constexpr nullopt_t nullopt{nullopt_t::__secret_tag{}, nullopt_t::__secret_tag{}};

template <class _Tp, bool = is_trivially_destructible<_Tp>::value>
struct __optional_destruct_base;

template <class _Tp>
struct __optional_destruct_base<_Tp, false>
{
    typedef _Tp value_type;
    static_assert(is_object_v<value_type>,
        "instantiation of optional with a non-object type is undefined behavior");
    union
    {
        char __null_state_;
        value_type __val_;
    };
    bool __engaged_;

    _LIBCPP_INLINE_VISIBILITY
    ~__optional_destruct_base()
    {
        if (__engaged_)
            __val_.~value_type();
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr __optional_destruct_base() noexcept
        :  __null_state_(),
           __engaged_(false) {}

    template <class... _Args>
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit __optional_destruct_base(in_place_t, _Args&&... __args)
        :  __val_(_VSTD::forward<_Args>(__args)...),
           __engaged_(true) {}

    _LIBCPP_INLINE_VISIBILITY
    void reset() noexcept
    {
        if (__engaged_)
        {
            __val_.~value_type();
            __engaged_ = false;
        }
    }
};

template <class _Tp>
struct __optional_destruct_base<_Tp, true>
{
    typedef _Tp value_type;
    static_assert(is_object_v<value_type>,
        "instantiation of optional with a non-object type is undefined behavior");
    union
    {
        char __null_state_;
        value_type __val_;
    };
    bool __engaged_;

    _LIBCPP_INLINE_VISIBILITY
    constexpr __optional_destruct_base() noexcept
        :  __null_state_(),
           __engaged_(false) {}

    template <class... _Args>
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit __optional_destruct_base(in_place_t, _Args&&... __args)
        :  __val_(_VSTD::forward<_Args>(__args)...),
           __engaged_(true) {}

    _LIBCPP_INLINE_VISIBILITY
    void reset() noexcept
    {
        if (__engaged_)
        {
            __engaged_ = false;
        }
    }
};

template <class _Tp, bool = is_reference<_Tp>::value>
struct __optional_storage_base : __optional_destruct_base<_Tp>
{
    using __base = __optional_destruct_base<_Tp>;
    using value_type = _Tp;
    using __base::__base;

    _LIBCPP_INLINE_VISIBILITY
    constexpr bool has_value() const noexcept
    {
        return this->__engaged_;
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr value_type& __get() & noexcept
    {
        return this->__val_;
    }
    _LIBCPP_INLINE_VISIBILITY
    constexpr const value_type& __get() const& noexcept
    {
        return this->__val_;
    }
    _LIBCPP_INLINE_VISIBILITY
    constexpr value_type&& __get() && noexcept
    {
        return _VSTD::move(this->__val_);
    }
    _LIBCPP_INLINE_VISIBILITY
    constexpr const value_type&& __get() const&& noexcept
    {
        return _VSTD::move(this->__val_);
    }

    template <class... _Args>
    _LIBCPP_INLINE_VISIBILITY
    void __construct(_Args&&... __args)
    {
        _LIBCPP_ASSERT(!has_value(), "__construct called for engaged __optional_storage");
        ::new((void*)_VSTD::addressof(this->__val_)) value_type(_VSTD::forward<_Args>(__args)...);
        this->__engaged_ = true;
    }

    template <class _That>
    _LIBCPP_INLINE_VISIBILITY
    void __construct_from(_That&& __opt)
    {
        if (__opt.has_value())
            __construct(_VSTD::forward<_That>(__opt).__get());
    }

    template <class _That>
    _LIBCPP_INLINE_VISIBILITY
    void __assign_from(_That&& __opt)
    {
        if (this->__engaged_ == __opt.has_value())
        {
            if (this->__engaged_)
                this->__val_ = _VSTD::forward<_That>(__opt).__get();
        }
        else
        {
            if (this->__engaged_)
                this->reset();
            else
                __construct(_VSTD::forward<_That>(__opt).__get());
        }
    }
};

// optional<T&> is currently required ill-formed, however it may to be in the
// future. For this reason it has already been implemented to ensure we can
// make the change in an ABI compatible manner.
template <class _Tp>
struct __optional_storage_base<_Tp, true>
{
    using value_type = _Tp;
    using __raw_type = remove_reference_t<_Tp>;
    __raw_type* __value_;

    template <class _Up>
    static constexpr bool __can_bind_reference() {
        using _RawUp = typename remove_reference<_Up>::type;
        using _UpPtr = _RawUp*;
        using _RawTp = typename remove_reference<_Tp>::type;
        using _TpPtr = _RawTp*;
        using _CheckLValueArg = integral_constant<bool,
            (is_lvalue_reference<_Up>::value && is_convertible<_UpPtr, _TpPtr>::value)
        ||  is_same<_RawUp, reference_wrapper<_RawTp>>::value
        ||  is_same<_RawUp, reference_wrapper<typename remove_const<_RawTp>::type>>::value
        >;
        return (is_lvalue_reference<_Tp>::value && _CheckLValueArg::value)
            || (is_rvalue_reference<_Tp>::value && !is_lvalue_reference<_Up>::value &&
                is_convertible<_UpPtr, _TpPtr>::value);
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr __optional_storage_base() noexcept
        :  __value_(nullptr) {}

    template <class _UArg>
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit __optional_storage_base(in_place_t, _UArg&& __uarg)
        :  __value_(_VSTD::addressof(__uarg))
    {
      static_assert(__can_bind_reference<_UArg>(),
        "Attempted to construct a reference element in tuple from a "
        "possible temporary");
    }

    _LIBCPP_INLINE_VISIBILITY
    void reset() noexcept { __value_ = nullptr; }

    _LIBCPP_INLINE_VISIBILITY
    constexpr bool has_value() const noexcept
      { return __value_ != nullptr; }

    _LIBCPP_INLINE_VISIBILITY
    constexpr value_type& __get() const& noexcept
      { return *__value_; }

    _LIBCPP_INLINE_VISIBILITY
    constexpr value_type&& __get() const&& noexcept
      { return _VSTD::forward<value_type>(*__value_); }

    template <class _UArg>
    _LIBCPP_INLINE_VISIBILITY
    void __construct(_UArg&& __val)
    {
        _LIBCPP_ASSERT(!has_value(), "__construct called for engaged __optional_storage");
        static_assert(__can_bind_reference<_UArg>(),
            "Attempted to construct a reference element in tuple from a "
            "possible temporary");
        __value_ = _VSTD::addressof(__val);
    }

    template <class _That>
    _LIBCPP_INLINE_VISIBILITY
    void __construct_from(_That&& __opt)
    {
        if (__opt.has_value())
            __construct(_VSTD::forward<_That>(__opt).__get());
    }

    template <class _That>
    _LIBCPP_INLINE_VISIBILITY
    void __assign_from(_That&& __opt)
    {
        if (has_value() == __opt.has_value())
        {
            if (has_value())
                *__value_ = _VSTD::forward<_That>(__opt).__get();
        }
        else
        {
            if (has_value())
                reset();
            else
                __construct(_VSTD::forward<_That>(__opt).__get());
        }
    }
};

template <class _Tp, bool = is_trivially_copyable<_Tp>::value>
struct __optional_storage;

template <class _Tp>
struct __optional_storage<_Tp, true> : __optional_storage_base<_Tp>
{
    using __optional_storage_base<_Tp>::__optional_storage_base;
};

template <class _Tp>
struct __optional_storage<_Tp, false> : __optional_storage_base<_Tp>
{
    using value_type = _Tp;
    using __optional_storage_base<_Tp>::__optional_storage_base;

    _LIBCPP_INLINE_VISIBILITY
    __optional_storage() = default;

    _LIBCPP_INLINE_VISIBILITY
    __optional_storage(const __optional_storage& __opt)
    {
        this->__construct_from(__opt);
    }

    _LIBCPP_INLINE_VISIBILITY
    __optional_storage(__optional_storage&& __opt)
        noexcept(is_nothrow_move_constructible_v<value_type>)
    {
        this->__construct_from(_VSTD::move(__opt));
    }

    _LIBCPP_INLINE_VISIBILITY
    __optional_storage& operator=(const __optional_storage& __opt)
    {
        this->__assign_from(__opt);
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    __optional_storage& operator=(__optional_storage&& __opt)
        noexcept(is_nothrow_move_assignable_v<value_type> &&
                 is_nothrow_move_constructible_v<value_type>)
    {
        this->__assign_from(_VSTD::move(__opt));
        return *this;
    }
};

template <class _Tp>
using __optional_sfinae_ctor_base_t = __sfinae_ctor_base<
    is_copy_constructible<_Tp>::value,
    is_move_constructible<_Tp>::value
>;

template <class _Tp>
using __optional_sfinae_assign_base_t = __sfinae_assign_base<
    (is_copy_constructible<_Tp>::value && is_copy_assignable<_Tp>::value),
    (is_move_constructible<_Tp>::value && is_move_assignable<_Tp>::value)
>;

template <class _Tp>
class optional
    : private __optional_storage<_Tp>
    , private __optional_sfinae_ctor_base_t<_Tp>
    , private __optional_sfinae_assign_base_t<_Tp>
{
    using __base = __optional_storage<_Tp>;
public:
    using value_type = _Tp;

private:
     // Disable the reference extension using this static assert.
    static_assert(!is_same_v<value_type, in_place_t>,
        "instantiation of optional with in_place_t is ill-formed");
    static_assert(!is_same_v<__uncvref_t<value_type>, nullopt_t>,
        "instantiation of optional with nullopt_t is ill-formed");
    static_assert(!is_reference_v<value_type>,
        "instantiation of optional with a reference type is ill-formed");
    static_assert(is_destructible_v<value_type>,
        "instantiation of optional with a non-destructible type is ill-formed");

    // LWG2756: conditionally explicit conversion from _Up
    struct _CheckOptionalArgsConstructor {
      template <class _Up>
      static constexpr bool __enable_implicit() {
          return is_constructible_v<_Tp, _Up&&> &&
                 is_convertible_v<_Up&&, _Tp>;
      }

      template <class _Up>
      static constexpr bool __enable_explicit() {
          return is_constructible_v<_Tp, _Up&&> &&
                 !is_convertible_v<_Up&&, _Tp>;
      }
    };
    template <class _Up>
    using _CheckOptionalArgsCtor = conditional_t<
        !is_same_v<decay_t<_Up>, in_place_t> &&
        !is_same_v<decay_t<_Up>, optional>,
        _CheckOptionalArgsConstructor,
        __check_tuple_constructor_fail
    >;
    template <class _QualUp>
    struct _CheckOptionalLikeConstructor {
      template <class _Up, class _Opt = optional<_Up>>
      using __check_constructible_from_opt = __lazy_or<
          is_constructible<_Tp, _Opt&>,
          is_constructible<_Tp, _Opt const&>,
          is_constructible<_Tp, _Opt&&>,
          is_constructible<_Tp, _Opt const&&>,
          is_convertible<_Opt&, _Tp>,
          is_convertible<_Opt const&, _Tp>,
          is_convertible<_Opt&&, _Tp>,
          is_convertible<_Opt const&&, _Tp>
      >;
      template <class _Up, class _Opt = optional<_Up>>
      using __check_assignable_from_opt = __lazy_or<
          is_assignable<_Tp&, _Opt&>,
          is_assignable<_Tp&, _Opt const&>,
          is_assignable<_Tp&, _Opt&&>,
          is_assignable<_Tp&, _Opt const&&>
      >;
      template <class _Up, class _QUp = _QualUp>
      static constexpr bool __enable_implicit() {
          return is_convertible<_QUp, _Tp>::value &&
              !__check_constructible_from_opt<_Up>::value;
      }
      template <class _Up, class _QUp = _QualUp>
      static constexpr bool __enable_explicit() {
          return !is_convertible<_QUp, _Tp>::value &&
              !__check_constructible_from_opt<_Up>::value;
      }
      template <class _Up, class _QUp = _QualUp>
      static constexpr bool __enable_assign() {
          // Construction and assignability of _Qup to _Tp has already been
          // checked.
          return !__check_constructible_from_opt<_Up>::value &&
              !__check_assignable_from_opt<_Up>::value;
      }
    };

    template <class _Up, class _QualUp>
    using _CheckOptionalLikeCtor = conditional_t<
      __lazy_and<
          __lazy_not<is_same<_Up, _Tp>>,
          is_constructible<_Tp, _QualUp>
      >::value,
      _CheckOptionalLikeConstructor<_QualUp>,
      __check_tuple_constructor_fail
    >;
    template <class _Up, class _QualUp>
    using _CheckOptionalLikeAssign = conditional_t<
      __lazy_and<
          __lazy_not<is_same<_Up, _Tp>>,
          is_constructible<_Tp, _QualUp>,
          is_assignable<_Tp&, _QualUp>
      >::value,
      _CheckOptionalLikeConstructor<_QualUp>,
      __check_tuple_constructor_fail
    >;
public:

    _LIBCPP_INLINE_VISIBILITY constexpr optional() noexcept {}
    _LIBCPP_INLINE_VISIBILITY constexpr optional(const optional&) = default;
    _LIBCPP_INLINE_VISIBILITY constexpr optional(optional&&) = default;
    _LIBCPP_INLINE_VISIBILITY constexpr optional(nullopt_t) noexcept {}

    template <class... _Args, class = enable_if_t<
        is_constructible_v<value_type, _Args...>>
    >
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit optional(in_place_t, _Args&&... __args)
        : __base(in_place, _VSTD::forward<_Args>(__args)...) {}

    template <class _Up, class... _Args, class = enable_if_t<
        is_constructible_v<value_type, initializer_list<_Up>&, _Args...>>
    >
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit optional(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
        : __base(in_place, __il, _VSTD::forward<_Args>(__args)...) {}

    template <class _Up = value_type, enable_if_t<
        _CheckOptionalArgsCtor<_Up>::template __enable_implicit<_Up>()
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY
    constexpr optional(_Up&& __v)
        : __base(in_place, _VSTD::forward<_Up>(__v)) {}

    template <class _Up, enable_if_t<
        _CheckOptionalArgsCtor<_Up>::template __enable_explicit<_Up>()
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit optional(_Up&& __v)
        : __base(in_place, _VSTD::forward<_Up>(__v)) {}

    // LWG2756: conditionally explicit conversion from const optional<_Up>&
    template <class _Up, enable_if_t<
        _CheckOptionalLikeCtor<_Up, _Up const&>::template __enable_implicit<_Up>()
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY
    optional(const optional<_Up>& __v)
    {
        this->__construct_from(__v);
    }
    template <class _Up, enable_if_t<
        _CheckOptionalLikeCtor<_Up, _Up const&>::template __enable_explicit<_Up>()
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY
    explicit optional(const optional<_Up>& __v)
    {
        this->__construct_from(__v);
    }

    // LWG2756: conditionally explicit conversion from optional<_Up>&&
    template <class _Up, enable_if_t<
        _CheckOptionalLikeCtor<_Up, _Up &&>::template __enable_implicit<_Up>()
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY
    optional(optional<_Up>&& __v)
    {
        this->__construct_from(_VSTD::move(__v));
    }
    template <class _Up, enable_if_t<
        _CheckOptionalLikeCtor<_Up, _Up &&>::template __enable_explicit<_Up>()
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY
    explicit optional(optional<_Up>&& __v)
    {
        this->__construct_from(_VSTD::move(__v));
    }

    _LIBCPP_INLINE_VISIBILITY
    optional& operator=(nullopt_t) noexcept
    {
        reset();
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY optional& operator=(const optional&) = default;
    _LIBCPP_INLINE_VISIBILITY optional& operator=(optional&&) = default;

    // LWG2756
    template <class _Up = value_type,
              class = enable_if_t
                      <__lazy_and<
                          integral_constant<bool,
                              !is_same_v<decay_t<_Up>, optional> &&
                              !(is_same_v<_Up, value_type> && is_scalar_v<value_type>)
                          >,
                          is_constructible<value_type, _Up>,
                          is_assignable<value_type&, _Up>
                      >::value>
             >
    _LIBCPP_INLINE_VISIBILITY
    optional&
    operator=(_Up&& __v)
    {
        if (this->has_value())
            this->__get() = _VSTD::forward<_Up>(__v);
        else
            this->__construct(_VSTD::forward<_Up>(__v));
        return *this;
    }

    // LWG2756
    template <class _Up, enable_if_t<
        _CheckOptionalLikeAssign<_Up, _Up const&>::template __enable_assign<_Up>()
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY
    optional&
    operator=(const optional<_Up>& __v)
    {
        this->__assign_from(__v);
        return *this;
    }

    // LWG2756
    template <class _Up, enable_if_t<
        _CheckOptionalLikeCtor<_Up, _Up &&>::template __enable_assign<_Up>()
    , int> = 0>
    _LIBCPP_INLINE_VISIBILITY
    optional&
    operator=(optional<_Up>&& __v)
    {
        this->__assign_from(_VSTD::move(__v));
        return *this;
    }

    template <class... _Args,
              class = enable_if_t
                      <
                          is_constructible_v<value_type, _Args...>
                      >
             >
    _LIBCPP_INLINE_VISIBILITY
    _Tp &
    emplace(_Args&&... __args)
    {
        reset();
        this->__construct(_VSTD::forward<_Args>(__args)...);
        return this->__get();
    }

    template <class _Up, class... _Args,
              class = enable_if_t
                      <
                          is_constructible_v<value_type, initializer_list<_Up>&, _Args...>
                      >
             >
    _LIBCPP_INLINE_VISIBILITY
    _Tp &
    emplace(initializer_list<_Up> __il, _Args&&... __args)
    {
        reset();
        this->__construct(__il, _VSTD::forward<_Args>(__args)...);
        return this->__get();
    }

    _LIBCPP_INLINE_VISIBILITY
    void swap(optional& __opt)
        noexcept(is_nothrow_move_constructible_v<value_type> &&
                 is_nothrow_swappable_v<value_type>)
    {
        if (this->has_value() == __opt.has_value())
        {
            using _VSTD::swap;
            if (this->has_value())
                swap(this->__get(), __opt.__get());
        }
        else
        {
            if (this->has_value())
            {
                __opt.__construct(_VSTD::move(this->__get()));
                reset();
            }
            else
            {
                this->__construct(_VSTD::move(__opt.__get()));
                __opt.reset();
            }
        }
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr
    add_pointer_t<value_type const>
    operator->() const
    {
        _LIBCPP_ASSERT(this->has_value(), "optional operator-> called for disengaged value");
#ifndef _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF
        return _VSTD::addressof(this->__get());
#else
        return __operator_arrow(__has_operator_addressof<value_type>{}, this->__get());
#endif
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr
    add_pointer_t<value_type>
    operator->()
    {
        _LIBCPP_ASSERT(this->has_value(), "optional operator-> called for disengaged value");
#ifndef _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF
        return _VSTD::addressof(this->__get());
#else
        return __operator_arrow(__has_operator_addressof<value_type>{}, this->__get());
#endif
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr
    const value_type&
    operator*() const&
    {
        _LIBCPP_ASSERT(this->has_value(), "optional operator* called for disengaged value");
        return this->__get();
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr
    value_type&
    operator*() &
    {
        _LIBCPP_ASSERT(this->has_value(), "optional operator* called for disengaged value");
        return this->__get();
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr
    value_type&&
    operator*() &&
    {
        _LIBCPP_ASSERT(this->has_value(), "optional operator* called for disengaged value");
        return _VSTD::move(this->__get());
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr
    const value_type&&
    operator*() const&&
    {
        _LIBCPP_ASSERT(this->has_value(), "optional operator* called for disengaged value");
        return _VSTD::move(this->__get());
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit operator bool() const noexcept { return has_value(); }

    using __base::has_value;
    using __base::__get;

    _LIBCPP_INLINE_VISIBILITY
    constexpr value_type const& value() const&
    {
        if (!this->has_value())
            __throw_bad_optional_access();
        return this->__get();
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr value_type& value() &
    {
        if (!this->has_value())
            __throw_bad_optional_access();
        return this->__get();
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr value_type&& value() &&
    {
        if (!this->has_value())
            __throw_bad_optional_access();
        return _VSTD::move(this->__get());
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr value_type const&& value() const&&
    {
        if (!this->has_value())
            __throw_bad_optional_access();
        return _VSTD::move(this->__get());
    }

    template <class _Up>
    _LIBCPP_INLINE_VISIBILITY
    constexpr value_type value_or(_Up&& __v) const&
    {
        static_assert(is_copy_constructible_v<value_type>,
                      "optional<T>::value_or: T must be copy constructible");
        static_assert(is_convertible_v<_Up, value_type>,
                      "optional<T>::value_or: U must be convertible to T");
        return this->has_value() ? this->__get() :
                                  static_cast<value_type>(_VSTD::forward<_Up>(__v));
    }

    template <class _Up>
    _LIBCPP_INLINE_VISIBILITY
    constexpr value_type value_or(_Up&& __v) &&
    {
        static_assert(is_move_constructible_v<value_type>,
                      "optional<T>::value_or: T must be move constructible");
        static_assert(is_convertible_v<_Up, value_type>,
                      "optional<T>::value_or: U must be convertible to T");
        return this->has_value() ? _VSTD::move(this->__get()) :
                                  static_cast<value_type>(_VSTD::forward<_Up>(__v));
    }

    using __base::reset;

private:
    template <class _Up>
    _LIBCPP_INLINE_VISIBILITY
    static _Up*
    __operator_arrow(true_type, _Up& __x)
    {
        return _VSTD::addressof(__x);
    }

    template <class _Up>
    _LIBCPP_INLINE_VISIBILITY
    static constexpr _Up*
    __operator_arrow(false_type, _Up& __x)
    {
        return &__x;
    }
};

// Comparisons between optionals
template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() ==
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator==(const optional<_Tp>& __x, const optional<_Up>& __y)
{
    if (static_cast<bool>(__x) != static_cast<bool>(__y))
        return false;
    if (!static_cast<bool>(__x))
        return true;
    return *__x == *__y;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() !=
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator!=(const optional<_Tp>& __x, const optional<_Up>& __y)
{
    if (static_cast<bool>(__x) != static_cast<bool>(__y))
        return true;
    if (!static_cast<bool>(__x))
        return false;
    return *__x != *__y;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator<(const optional<_Tp>& __x, const optional<_Up>& __y)
{
    if (!static_cast<bool>(__y))
        return false;
    if (!static_cast<bool>(__x))
        return true;
    return *__x < *__y;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator>(const optional<_Tp>& __x, const optional<_Up>& __y)
{
    if (!static_cast<bool>(__x))
        return false;
    if (!static_cast<bool>(__y))
        return true;
    return *__x > *__y;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <=
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator<=(const optional<_Tp>& __x, const optional<_Up>& __y)
{
    if (!static_cast<bool>(__x))
        return true;
    if (!static_cast<bool>(__y))
        return false;
    return *__x <= *__y;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >=
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator>=(const optional<_Tp>& __x, const optional<_Up>& __y)
{
    if (!static_cast<bool>(__y))
        return true;
    if (!static_cast<bool>(__x))
        return false;
    return *__x >= *__y;
}

// Comparisons with nullopt
template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator==(const optional<_Tp>& __x, nullopt_t) noexcept
{
    return !static_cast<bool>(__x);
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator==(nullopt_t, const optional<_Tp>& __x) noexcept
{
    return !static_cast<bool>(__x);
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator!=(const optional<_Tp>& __x, nullopt_t) noexcept
{
    return static_cast<bool>(__x);
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator!=(nullopt_t, const optional<_Tp>& __x) noexcept
{
    return static_cast<bool>(__x);
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator<(const optional<_Tp>&, nullopt_t) noexcept
{
    return false;
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator<(nullopt_t, const optional<_Tp>& __x) noexcept
{
    return static_cast<bool>(__x);
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator<=(const optional<_Tp>& __x, nullopt_t) noexcept
{
    return !static_cast<bool>(__x);
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator<=(nullopt_t, const optional<_Tp>&) noexcept
{
    return true;
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator>(const optional<_Tp>& __x, nullopt_t) noexcept
{
    return static_cast<bool>(__x);
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator>(nullopt_t, const optional<_Tp>&) noexcept
{
    return false;
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator>=(const optional<_Tp>&, nullopt_t) noexcept
{
    return true;
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
bool
operator>=(nullopt_t, const optional<_Tp>& __x) noexcept
{
    return !static_cast<bool>(__x);
}

// Comparisons with T
template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() ==
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator==(const optional<_Tp>& __x, const _Up& __v)
{
    return static_cast<bool>(__x) ? *__x == __v : false;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() ==
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator==(const _Tp& __v, const optional<_Up>& __x)
{
    return static_cast<bool>(__x) ? __v == *__x : false;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() !=
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator!=(const optional<_Tp>& __x, const _Up& __v)
{
    return static_cast<bool>(__x) ? *__x != __v : true;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() !=
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator!=(const _Tp& __v, const optional<_Up>& __x)
{
    return static_cast<bool>(__x) ? __v != *__x : true;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator<(const optional<_Tp>& __x, const _Up& __v)
{
    return static_cast<bool>(__x) ? *__x < __v : true;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator<(const _Tp& __v, const optional<_Up>& __x)
{
    return static_cast<bool>(__x) ? __v < *__x : false;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <=
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator<=(const optional<_Tp>& __x, const _Up& __v)
{
    return static_cast<bool>(__x) ? *__x <= __v : true;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <=
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator<=(const _Tp& __v, const optional<_Up>& __x)
{
    return static_cast<bool>(__x) ? __v <= *__x : false;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator>(const optional<_Tp>& __x, const _Up& __v)
{
    return static_cast<bool>(__x) ? *__x > __v : false;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator>(const _Tp& __v, const optional<_Up>& __x)
{
    return static_cast<bool>(__x) ? __v > *__x : true;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >=
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator>=(const optional<_Tp>& __x, const _Up& __v)
{
    return static_cast<bool>(__x) ? *__x >= __v : false;
}

template <class _Tp, class _Up>
_LIBCPP_INLINE_VISIBILITY constexpr
enable_if_t<
    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >=
        _VSTD::declval<const _Up&>()), bool>,
    bool
>
operator>=(const _Tp& __v, const optional<_Up>& __x)
{
    return static_cast<bool>(__x) ? __v >= *__x : true;
}


template <class _Tp>
inline _LIBCPP_INLINE_VISIBILITY
enable_if_t<
    is_move_constructible_v<_Tp> && is_swappable_v<_Tp>,
    void
>
swap(optional<_Tp>& __x, optional<_Tp>& __y) noexcept(noexcept(__x.swap(__y)))
{
    __x.swap(__y);
}

template <class _Tp>
_LIBCPP_INLINE_VISIBILITY constexpr
optional<decay_t<_Tp>> make_optional(_Tp&& __v)
{
    return optional<decay_t<_Tp>>(_VSTD::forward<_Tp>(__v));
}

template <class _Tp, class... _Args>
_LIBCPP_INLINE_VISIBILITY constexpr
optional<_Tp> make_optional(_Args&&... __args)
{
    return optional<_Tp>(in_place, _VSTD::forward<_Args>(__args)...);
}

template <class _Tp, class _Up, class... _Args>
_LIBCPP_INLINE_VISIBILITY constexpr
optional<_Tp> make_optional(initializer_list<_Up> __il,  _Args&&... __args)
{
    return optional<_Tp>(in_place, __il, _VSTD::forward<_Args>(__args)...);
}

template <class _Tp>
struct _LIBCPP_TEMPLATE_VIS hash<
    __enable_hash_helper<optional<_Tp>, remove_const_t<_Tp>>
>
{
    typedef optional<_Tp> argument_type;
    typedef size_t        result_type;

    _LIBCPP_INLINE_VISIBILITY
    result_type operator()(const argument_type& __opt) const
    {
        return static_cast<bool>(__opt) ? hash<remove_const_t<_Tp>>()(*__opt) : 0;
    }
};

_LIBCPP_END_NAMESPACE_STD

#endif  // _LIBCPP_STD_VER > 14

_LIBCPP_POP_MACROS

#endif  // _LIBCPP_OPTIONAL