Revision control

Line Code
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
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* A template class for tagged unions. */

#include <new>
#include <stdint.h>

#include "mozilla/Assertions.h"
#include "mozilla/Move.h"
#include "mozilla/OperatorNewExtensions.h"
#include "mozilla/TemplateLib.h"
#include "mozilla/TypeTraits.h"

#ifndef mozilla_Variant_h
#define mozilla_Variant_h

namespace mozilla {

template<typename... Ts>
class Variant;

namespace detail {

// Nth<N, types...>::Type is the Nth type (0-based) in the list of types Ts.
template<size_t N, typename... Ts>
struct Nth;

template<typename T, typename... Ts>
struct Nth<0, T, Ts...>
{
  using Type = T;
};

template<size_t N, typename T, typename... Ts>
struct Nth<N, T, Ts...>
{
  using Type = typename Nth<N - 1, Ts...>::Type;
};

/// SelectVariantTypeHelper is used in the implementation of SelectVariantType.
template<typename T, typename... Variants>
struct SelectVariantTypeHelper;

template<typename T>
struct SelectVariantTypeHelper<T>
{
  static constexpr size_t count = 0;
};

template<typename T, typename... Variants>
struct SelectVariantTypeHelper<T, T, Variants...>
{
  typedef T Type;
  static constexpr size_t count = 1 + SelectVariantTypeHelper<T, Variants...>::count;
};

template<typename T, typename... Variants>
struct SelectVariantTypeHelper<T, const T, Variants...>
{
  typedef const T Type;
  static constexpr size_t count = 1 + SelectVariantTypeHelper<T, Variants...>::count;
};

template<typename T, typename... Variants>
struct SelectVariantTypeHelper<T, const T&, Variants...>
{
  typedef const T& Type;
  static constexpr size_t count = 1 + SelectVariantTypeHelper<T, Variants...>::count;
};

template<typename T, typename... Variants>
struct SelectVariantTypeHelper<T, T&&, Variants...>
{
  typedef T&& Type;
  static constexpr size_t count = 1 + SelectVariantTypeHelper<T, Variants...>::count;
};

template<typename T, typename Head, typename... Variants>
struct SelectVariantTypeHelper<T, Head, Variants...>
  : public SelectVariantTypeHelper<T, Variants...>
{ };

/**
 * SelectVariantType takes a type T and a list of variant types Variants and
 * yields a type Type, selected from Variants, that can store a value of type T
 * or a reference to type T. If no such type was found, Type is not defined.
 * SelectVariantType also has a `count` member that contains the total number of
 * selectable types (which will be used to check that a requested type is not
 * ambiguously present twice.)
 */
template <typename T, typename... Variants>
struct SelectVariantType
  : public SelectVariantTypeHelper<typename RemoveConst<typename RemoveReference<T>::Type>::Type,
                                   Variants...>
{ };

// Compute a fast, compact type that can be used to hold integral values that
// distinctly map to every type in Ts.
template<typename... Ts>
struct VariantTag
{
private:
  static const size_t TypeCount = sizeof...(Ts);

public:
  using Type =
    typename Conditional<TypeCount < 3,
                         bool,
                         typename Conditional<TypeCount < (1 << 8),
                                              uint_fast8_t,
                                              size_t // stop caring past a certain point :-)
                                              >::Type
                         >::Type;
};

// TagHelper gets the given sentinel tag value for the given type T. This has to
// be split out from VariantImplementation because you can't nest a partial
// template specialization within a template class.

template<typename Tag, size_t N, typename T, typename U, typename Next, bool isMatch>
struct TagHelper;

// In the case where T != U, we continue recursion.
template<typename Tag, size_t N, typename T, typename U, typename Next>
struct TagHelper<Tag, N, T, U, Next, false>
{
  static Tag tag() { return Next::template tag<U>(); }
};

// In the case where T == U, return the tag number.
template<typename Tag, size_t N, typename T, typename U, typename Next>
struct TagHelper<Tag, N, T, U, Next, true>
{
  static Tag tag() { return Tag(N); }
};

// The VariantImplementation template provides the guts of mozilla::Variant.  We
// create a VariantImplementation for each T in Ts... which handles
// construction, destruction, etc for when the Variant's type is T.  If the
// Variant's type isn't T, it punts the request on to the next
// VariantImplementation.

template<typename Tag, size_t N, typename... Ts>
struct VariantImplementation;

// The singly typed Variant / recursion base case.
template<typename Tag, size_t N, typename T>
struct VariantImplementation<Tag, N, T>
{
  template<typename U>
  static Tag tag() {
    static_assert(mozilla::IsSame<T, U>::value,
                  "mozilla::Variant: tag: bad type!");
    return Tag(N);
  }

  template<typename Variant>
  static void copyConstruct(void* aLhs, const Variant& aRhs) {
    ::new (KnownNotNull, aLhs) T(aRhs.template as<N>());
  }

  template<typename Variant>
  static void moveConstruct(void* aLhs, Variant&& aRhs) {
    ::new (KnownNotNull, aLhs) T(aRhs.template extract<N>());
  }

  template<typename Variant>
  static void destroy(Variant& aV) {
    aV.template as<N>().~T();
  }

  template<typename Variant>
  static bool
  equal(const Variant& aLhs, const Variant& aRhs) {
      return aLhs.template as<N>() == aRhs.template as<N>();
  }

  template<typename Matcher, typename ConcreteVariant>
  static auto
  match(Matcher&& aMatcher, ConcreteVariant& aV)
    -> decltype(aMatcher.match(aV.template as<N>()))
  {
    return aMatcher.match(aV.template as<N>());
  }
};

// VariantImplementation for some variant type T.
template<typename Tag, size_t N, typename T, typename... Ts>
struct VariantImplementation<Tag, N, T, Ts...>
{
  // The next recursive VariantImplementation.
  using Next = VariantImplementation<Tag, N + 1, Ts...>;

  template<typename U>
  static Tag tag() {
    return TagHelper<Tag, N, T, U, Next, IsSame<T, U>::value>::tag();
  }

  template<typename Variant>
  static void copyConstruct(void* aLhs, const Variant& aRhs) {
    if (aRhs.template is<N>()) {
      ::new (KnownNotNull, aLhs) T(aRhs.template as<N>());
    } else {
      Next::copyConstruct(aLhs, aRhs);
    }
  }

  template<typename Variant>
  static void moveConstruct(void* aLhs, Variant&& aRhs) {
    if (aRhs.template is<N>()) {
      ::new (KnownNotNull, aLhs) T(aRhs.template extract<N>());
    } else {
      Next::moveConstruct(aLhs, Move(aRhs));
    }
  }

  template<typename Variant>
  static void destroy(Variant& aV) {
    if (aV.template is<N>()) {
      aV.template as<N>().~T();
    } else {
      Next::destroy(aV);
    }
  }

  template<typename Variant>
  static bool equal(const Variant& aLhs, const Variant& aRhs) {
    if (aLhs.template is<N>()) {
      MOZ_ASSERT(aRhs.template is<N>());
      return aLhs.template as<N>() == aRhs.template as<N>();
    } else {
      return Next::equal(aLhs, aRhs);
    }
  }

  template<typename Matcher, typename ConcreteVariant>
  static auto
  match(Matcher&& aMatcher, ConcreteVariant& aV)
    -> decltype(aMatcher.match(aV.template as<N>()))
  {
    if (aV.template is<N>()) {
      return aMatcher.match(aV.template as<N>());
    } else {
      // If you're seeing compilation errors here like "no matching
      // function for call to 'match'" then that means that the
      // Matcher doesn't exhaust all variant types. There must exist a
      // Matcher::match(T&) for every variant type T.
      //
      // If you're seeing compilation errors here like "cannot
      // initialize return object of type <...> with an rvalue of type
      // <...>" then that means that the Matcher::match(T&) overloads
      // are returning different types. They must all return the same
      // Matcher::ReturnType type.
      return Next::match(aMatcher, aV);
    }
  }
};

/**
 * AsVariantTemporary stores a value of type T to allow construction of a
 * Variant value via type inference. Because T is copied and there's no
 * guarantee that the copy can be elided, AsVariantTemporary is best used with
 * primitive or very small types.
 */
template <typename T>
struct AsVariantTemporary
{
  explicit AsVariantTemporary(const T& aValue)
    : mValue(aValue)
  {}

  template<typename U>
  explicit AsVariantTemporary(U&& aValue)
    : mValue(Forward<U>(aValue))
  {}

  AsVariantTemporary(const AsVariantTemporary& aOther)
    : mValue(aOther.mValue)
  {}

  AsVariantTemporary(AsVariantTemporary&& aOther)
    : mValue(Move(aOther.mValue))
  {}

  AsVariantTemporary() = delete;
  void operator=(const AsVariantTemporary&) = delete;
  void operator=(AsVariantTemporary&&) = delete;

  typename RemoveConst<typename RemoveReference<T>::Type>::Type mValue;
};

} // namespace detail

// Used to unambiguously specify one of the Variant's type.
template<typename T> struct VariantType { using Type = T; };

// Used to specify one of the Variant's type by index.
template<size_t N> struct VariantIndex { static constexpr size_t index = N; };

/**
 * # mozilla::Variant
 *
 * A variant / tagged union / heterogenous disjoint union / sum-type template
 * class. Similar in concept to (but not derived from) `boost::variant`.
 *
 * Sometimes, you may wish to use a C union with non-POD types. However, this is
 * forbidden in C++ because it is not clear which type in the union should have
 * its constructor and destructor run on creation and deletion
 * respectively. This is the problem that `mozilla::Variant` solves.
 *
 * ## Usage
 *
 * A `mozilla::Variant` instance is constructed (via move or copy) from one of
 * its variant types (ignoring const and references). It does *not* support
 * construction from subclasses of variant types or types that coerce to one of
 * the variant types.
 *
 *     Variant<char, uint32_t> v1('a');
 *     Variant<UniquePtr<A>, B, C> v2(MakeUnique<A>());
 *     Variant<bool, char> v3(VariantType<char>, 0); // disambiguation needed
 *     Variant<int, int> v4(VariantIndex<1>, 0); // 2nd int
 *
 * Because specifying the full type of a Variant value is often verbose,
 * there are two easier ways to construct values:
 *
 * A. AsVariant() can be used to construct a Variant value using type inference
 * in contexts such as expressions or when returning values from functions.
 * Because AsVariant() must copy or move the value into a temporary and this
 * cannot necessarily be elided by the compiler, it's mostly appropriate only
 * for use with primitive or very small types.
 *
 *     Variant<char, uint32_t> Foo() { return AsVariant('x'); }
 *     // ...
 *     Variant<char, uint32_t> v1 = Foo();  // v1 holds char('x').
 *
 * B. Brace-construction with VariantType or VariantIndex; this also allows
 * in-place construction with any number of arguments.
 *
 *     struct AB { AB(int, int){...} };
 *     static Variant<AB, bool> foo()
 *     {
 *       return {VariantIndex<0>{}, 1, 2};
 *     }
 *     // ...
 *     Variant<AB, bool> v0 = Foo();  // v0 holds AB(1,2).
 *
 * All access to the contained value goes through type-safe accessors.
 * Either the stored type, or the type index may be provided.
 *
 *     void
 *     Foo(Variant<A, B, C> v)
 *     {
 *       if (v.is<A>()) {
 *         A& ref = v.as<A>();
 *         ...
 *       } else (v.is<1>()) { // Instead of v.is<B>.
 *         ...
 *       } else {
 *         ...
 *       }
 *     }
 *
 * In some situation, a Variant may be constructed from templated types, in
 * which case it is possible that the same type could be given multiple times by
 * an external developer. Or seemingly-different types could be aliases.
 * In this case, repeated types can only be accessed through their index, to
 * prevent ambiguous access by type.
 *
 *    // Bad!
 *    template <typename T>
 *    struct ResultOrError
 *    {
 *      Variant<T, int> m;
 *      ResultOrError() : m(int(0)) {} // Error '0' by default
 *      ResultOrError(const T& r) : m(r) {}
 *      bool IsResult() const { return m.is<T>(); }
 *      bool IsError() const { return m.is<int>(); }
 *    };
 *    // Now instantiante with the result being an int too:
 *    ResultOrError<int> myResult(123); // Fail!
 *    // In Variant<int, int>, which 'int' are we refering to, from inside
 *    // ResultOrError functions?
 *
 *    // Good!
 *    template <typename T>
 *    struct ResultOrError
 *    {
 *      Variant<T, int> m;
 *      ResultOrError() : m(VariantIndex<1>{}, 0) {} // Error '0' by default
 *      ResultOrError(const T& r) : m(VariantIndex<0>{}, r) {}
 *      bool IsResult() const { return m.is<0>(); } // 0 -> T
 *      bool IsError() const { return m.is<1>(); } // 1 -> int
 *    };
 *    // Now instantiante with the result being an int too:
 *    ResultOrError<int> myResult(123); // It now works!
 *
 * Attempting to use the contained value as type `T1` when the `Variant`
 * instance contains a value of type `T2` causes an assertion failure.
 *
 *     A a;
 *     Variant<A, B, C> v(a);
 *     v.as<B>(); // <--- Assertion failure!
 *
 * Trying to use a `Variant<Ts...>` instance as some type `U` that is not a
 * member of the set of `Ts...` is a compiler error.
 *
 *     A a;
 *     Variant<A, B, C> v(a);
 *     v.as<SomeRandomType>(); // <--- Compiler error!
 *
 * Additionally, you can turn a `Variant` that `is<T>` into a `T` by moving it
 * out of the containing `Variant` instance with the `extract<T>` method:
 *
 *     Variant<UniquePtr<A>, B, C> v(MakeUnique<A>());
 *     auto ptr = v.extract<UniquePtr<A>>();
 *
 * Finally, you can exhaustively match on the contained variant and branch into
 * different code paths depending on which type is contained. This is preferred
 * to manually checking every variant type T with is<T>() because it provides
 * compile-time checking that you handled every type, rather than runtime
 * assertion failures.
 *
 *     // Bad!
 *     char* foo(Variant<A, B, C, D>& v) {
 *       if (v.is<A>()) {
 *         return ...;
 *       } else if (v.is<B>()) {
 *         return ...;
 *       } else {
 *         return doSomething(v.as<C>()); // Forgot about case D!
 *       }
 *     }
 *
 *     // Good!
 *     struct FooMatcher
 *     {
 *       // The return type of all matchers must be identical.
 *       char* match(A& a) { ... }
 *       char* match(B& b) { ... }
 *       char* match(C& c) { ... }
 *       char* match(D& d) { ... } // Compile-time error to forget D!
 *     }
 *     char* foo(Variant<A, B, C, D>& v) {
 *       return v.match(FooMatcher());
 *     }
 *
 * ## Examples
 *
 * A tree is either an empty leaf, or a node with a value and two children:
 *
 *     struct Leaf { };
 *
 *     template<typename T>
 *     struct Node
 *     {
 *       T value;
 *       Tree<T>* left;
 *       Tree<T>* right;
 *     };
 *
 *     template<typename T>
 *     using Tree = Variant<Leaf, Node<T>>;
 *
 * A copy-on-write string is either a non-owning reference to some existing
 * string, or an owning reference to our copy:
 *
 *     class CopyOnWriteString
 *     {
 *       Variant<const char*, UniquePtr<char[]>> string;
 *
 *       ...
 *     };
 *
 * Because Variant must be aligned suitable to hold any value stored within it,
 * and because |alignas| requirements don't affect platform ABI with respect to
 * how parameters are laid out in memory, Variant can't be used as the type of a
 * function parameter.  Pass Variant to functions by pointer or reference
 * instead.
 */
template<typename... Ts>
class MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS MOZ_NON_PARAM Variant
{
  using Tag = typename detail::VariantTag<Ts...>::Type;
  using Impl = detail::VariantImplementation<Tag, 0, Ts...>;

  static constexpr size_t RawDataAlignment = tl::Max<alignof(Ts)...>::value;
  static constexpr size_t RawDataSize = tl::Max<sizeof(Ts)...>::value;

  // Raw storage for the contained variant value.
  alignas(RawDataAlignment) unsigned char rawData[RawDataSize];

  // Each type is given a unique tag value that lets us keep track of the
  // contained variant value's type.
  Tag tag;

  // Some versions of GCC treat it as a -Wstrict-aliasing violation (ergo a
  // -Werror compile error) to reinterpret_cast<> |rawData| to |T*|, even
  // through |void*|.  Placing the latter cast in these separate functions
  // breaks the chain such that affected GCC versions no longer warn/error.
  void* ptr() {
    return rawData;
  }

  const void* ptr() const {
    return rawData;
  }

public:
  /** Perfect forwarding construction for some variant type T. */
  template<typename RefT,
           // RefT captures both const& as well as && (as intended, to support
           // perfect forwarding), so we have to remove those qualifiers here
           // when ensuring that T is a variant of this type, and getting T's
           // tag, etc.
           typename T = typename detail::SelectVariantType<RefT, Ts...>::Type>
  explicit Variant(RefT&& aT)
    : tag(Impl::template tag<T>())
  {
    static_assert(detail::SelectVariantType<RefT, Ts...>::count == 1,
                  "Variant can only be selected by type if that type is unique");
    ::new (KnownNotNull, ptr()) T(Forward<RefT>(aT));
  }

  /**
   * Perfect forwarding construction for some variant type T, by
   * explicitly giving the type.
   * This is necessary to construct from any number of arguments,
   * or to convert from a type that is not in the Variant's type list.
   */
  template<typename T, typename... Args>
  MOZ_IMPLICIT Variant(const VariantType<T>&, Args&&... aTs)
    : tag(Impl::template tag<T>())
  {
    ::new (KnownNotNull, ptr()) T(Forward<Args>(aTs)...);
  }

  /**
   * Perfect forwarding construction for some variant type T, by
   * explicitly giving the type index.
   * This is necessary to construct from any number of arguments,
   * or to convert from a type that is not in the Variant's type list,
   * or to construct a type that is present more than once in the Variant.
   */
  template<size_t N, typename... Args>
  MOZ_IMPLICIT Variant(const VariantIndex<N>&, Args&&... aTs)
    : tag(N)
  {
    using T = typename detail::Nth<N, Ts...>::Type;
    ::new (KnownNotNull, ptr()) T(Forward<Args>(aTs)...);
  }

  /**
   * Constructs this Variant from an AsVariantTemporary<T> such that T can be
   * stored in one of the types allowable in this Variant. This is used in the
   * implementation of AsVariant().
   */
  template<typename RefT>
  MOZ_IMPLICIT Variant(detail::AsVariantTemporary<RefT>&& aValue)
    : tag(Impl::template tag<typename detail::SelectVariantType<RefT, Ts...>::Type>())
  {
    using T = typename detail::SelectVariantType<RefT, Ts...>::Type;
    static_assert(detail::SelectVariantType<RefT, Ts...>::count == 1,
                  "Variant can only be selected by type if that type is unique");
    ::new (KnownNotNull, ptr()) T(Move(aValue.mValue));
  }

  /** Copy construction. */
  Variant(const Variant& aRhs)
    : tag(aRhs.tag)
  {
    Impl::copyConstruct(ptr(), aRhs);
  }

  /** Move construction. */
  Variant(Variant&& aRhs)
    : tag(aRhs.tag)
  {
    Impl::moveConstruct(ptr(), Move(aRhs));
  }

  /** Copy assignment. */
  Variant& operator=(const Variant& aRhs) {
    MOZ_ASSERT(&aRhs != this, "self-assign disallowed");
    this->~Variant();
    ::new (KnownNotNull, this) Variant(aRhs);
    return *this;
  }

  /** Move assignment. */
  Variant& operator=(Variant&& aRhs) {
    MOZ_ASSERT(&aRhs != this, "self-assign disallowed");
    this->~Variant();
    ::new (KnownNotNull, this) Variant(Move(aRhs));
    return *this;
  }

  /** Move assignment from AsVariant(). */
  template<typename T>
  Variant& operator=(detail::AsVariantTemporary<T>&& aValue)
  {
    static_assert(detail::SelectVariantType<T, Ts...>::count == 1,
                  "Variant can only be selected by type if that type is unique");
    this->~Variant();
    ::new (KnownNotNull, this) Variant(Move(aValue));
    return *this;
  }

  ~Variant()
  {
    Impl::destroy(*this);
  }

  /** Check which variant type is currently contained. */
  template<typename T>
  bool is() const {
    static_assert(detail::SelectVariantType<T, Ts...>::count == 1,
                  "provided a type not uniquely found in this Variant's type list");
    return Impl::template tag<T>() == tag;
  }

  template<size_t N>
  bool is() const
  {
    static_assert(N < sizeof...(Ts),
                  "provided an index outside of this Variant's type list");
    return N == size_t(tag);
  }

  /**
   * Operator == overload that defers to the variant type's operator==
   * implementation if the rhs is tagged as the same type as this one.
   */
  bool operator==(const Variant& aRhs) const {
    return tag == aRhs.tag && Impl::equal(*this, aRhs);
  }

  /**
   * Operator != overload that defers to the negation of the variant type's
   * operator== implementation if the rhs is tagged as the same type as this
   * one.
   */
  bool operator!=(const Variant& aRhs) const {
    return !(*this == aRhs);
  }

  // Accessors for working with the contained variant value.

  /** Mutable reference. */
  template<typename T>
  T& as() {
    static_assert(detail::SelectVariantType<T, Ts...>::count == 1,
                  "provided a type not uniquely found in this Variant's type list");
    MOZ_RELEASE_ASSERT(is<T>());
    return *static_cast<T*>(ptr());
  }

  template<size_t N>
  typename detail::Nth<N, Ts...>::Type& as()
  {
    static_assert(N < sizeof...(Ts),
                  "provided an index outside of this Variant's type list");
    MOZ_RELEASE_ASSERT(is<N>());
    return *static_cast<typename detail::Nth<N, Ts...>::Type*>(ptr());
  }

  /** Immutable const reference. */
  template<typename T>
  const T& as() const {
    static_assert(detail::SelectVariantType<T, Ts...>::count == 1,
                  "provided a type not found in this Variant's type list");
    MOZ_RELEASE_ASSERT(is<T>());
    return *static_cast<const T*>(ptr());
  }

  template<size_t N>
  const typename detail::Nth<N, Ts...>::Type& as() const
  {
    static_assert(N < sizeof...(Ts),
                  "provided an index outside of this Variant's type list");
    MOZ_RELEASE_ASSERT(is<N>());
    return *static_cast<const typename detail::Nth<N, Ts...>::Type*>(ptr());
  }

  /**
   * Extract the contained variant value from this container into a temporary
   * value.  On completion, the value in the variant will be in a
   * safely-destructible state, as determined by the behavior of T's move
   * constructor when provided the variant's internal value.
   */
  template<typename T>
  T extract() {
    static_assert(detail::SelectVariantType<T, Ts...>::count == 1,
                  "provided a type not uniquely found in this Variant's type list");
    MOZ_ASSERT(is<T>());
    return T(Move(as<T>()));
  }

  template<size_t N>
  typename detail::Nth<N, Ts...>::Type extract()
  {
    static_assert(N < sizeof...(Ts),
                  "provided an index outside of this Variant's type list");
    MOZ_RELEASE_ASSERT(is<N>());
    return typename detail::Nth<N, Ts...>::Type(Move(as<N>()));
  }

  // Exhaustive matching of all variant types on the contained value.

  /** Match on an immutable const reference. */
  template<typename Matcher>
  auto
  match(Matcher&& aMatcher) const
    -> decltype(Impl::match(aMatcher, *this))
  {
    return Impl::match(aMatcher, *this);
  }

  /** Match on a mutable non-const reference. */
  template<typename Matcher>
  auto
  match(Matcher&& aMatcher)
    -> decltype(Impl::match(aMatcher, *this))
  {
    return Impl::match(aMatcher, *this);
  }
};

/*
 * AsVariant() is used to construct a Variant<T,...> value containing the
 * provided T value using type inference. It can be used to construct Variant
 * values in expressions or return them from functions without specifying the
 * entire Variant type.
 *
 * Because AsVariant() must copy or move the value into a temporary and this
 * cannot necessarily be elided by the compiler, it's mostly appropriate only
 * for use with primitive or very small types.
 *
 * AsVariant() returns a AsVariantTemporary value which is implicitly
 * convertible to any Variant that can hold a value of type T.
 */
template<typename T>
detail::AsVariantTemporary<T>
AsVariant(T&& aValue)
{
  return detail::AsVariantTemporary<T>(Forward<T>(aValue));
}

} // namespace mozilla

#endif /* mozilla_Variant_h */