Adding upstream version 110.0.1.upstream/110.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 928 insertions, 0 deletions

diff --git a/mfbt/Variant.h b/mfbt/Variant.h
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+/* -*- 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/HashFunctions.h"
+#include "mozilla/OperatorNewExtensions.h"
+#include "mozilla/TemplateLib.h"
+#include <type_traits>
+#include <utility>
+
+#ifndef mozilla_Variant_h
+#  define mozilla_Variant_h
+
+namespace IPC {
+template <typename T>
+struct ParamTraits;
+}  // namespace IPC
+
+namespace mozilla {
+
+namespace ipc {
+template <typename T>
+struct IPDLParamTraits;
+}  // namespace ipc
+
+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<
+          std::remove_const_t<std::remove_reference_t<T>>, 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 = std::conditional_t<
+      (TypeCount <= 2), bool,
+      std::conditional_t<(TypeCount <= size_t(UINT_FAST8_MAX)), uint_fast8_t,
+                         size_t  // stop caring past a certain
+                                 // point :-)
+                         >>;
+};
+
+// 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(std::is_same_v<T, U>, "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 decltype(auto) match(Matcher&& aMatcher, ConcreteVariant&& aV) {
+    if constexpr (std::is_invocable_v<Matcher, Tag,
+                                      decltype(std::forward<ConcreteVariant>(aV)
+                                                   .template as<N>())>) {
+      return std::forward<Matcher>(aMatcher)(
+          Tag(N), std::forward<ConcreteVariant>(aV).template as<N>());
+    } else {
+      return std::forward<Matcher>(aMatcher)(
+          std::forward<ConcreteVariant>(aV).template as<N>());
+    }
+  }
+
+  template <typename ConcreteVariant, typename Matcher>
+  static decltype(auto) matchN(ConcreteVariant&& aV, Matcher&& aMatcher) {
+    if constexpr (std::is_invocable_v<Matcher, Tag,
+                                      decltype(std::forward<ConcreteVariant>(aV)
+                                                   .template as<N>())>) {
+      return std::forward<Matcher>(aMatcher)(
+          Tag(N), std::forward<ConcreteVariant>(aV).template as<N>());
+    } else {
+      return std::forward<Matcher>(aMatcher)(
+          std::forward<ConcreteVariant>(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, std::is_same_v<T, U>>::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, std::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 decltype(auto) match(Matcher&& aMatcher, ConcreteVariant&& aV) {
+    if (aV.template is<N>()) {
+      if constexpr (std::is_invocable_v<Matcher, Tag,
+                                        decltype(std::forward<ConcreteVariant>(
+                                                     aV)
+                                                     .template as<N>())>) {
+        return std::forward<Matcher>(aMatcher)(
+            Tag(N), std::forward<ConcreteVariant>(aV).template as<N>());
+      } else {
+        return std::forward<Matcher>(aMatcher)(
+            std::forward<ConcreteVariant>(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::operator()(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::operator()(T&) overloads are returning
+      // different types. They must all return the same type.
+      return Next::match(std::forward<Matcher>(aMatcher),
+                         std::forward<ConcreteVariant>(aV));
+    }
+  }
+
+  template <typename ConcreteVariant, typename Mi, typename... Ms>
+  static decltype(auto) matchN(ConcreteVariant&& aV, Mi&& aMi, Ms&&... aMs) {
+    if (aV.template is<N>()) {
+      if constexpr (std::is_invocable_v<Mi, Tag,
+                                        decltype(std::forward<ConcreteVariant>(
+                                                     aV)
+                                                     .template as<N>())>) {
+        static_assert(
+            std::is_same_v<
+                decltype(std::forward<Mi>(aMi)(
+                    Tag(N),
+                    std::forward<ConcreteVariant>(aV).template as<N>())),
+                decltype(Next::matchN(std::forward<ConcreteVariant>(aV),
+                                      std::forward<Ms>(aMs)...))>,
+            "all matchers must have the same return type");
+        return std::forward<Mi>(aMi)(
+            Tag(N), std::forward<ConcreteVariant>(aV).template as<N>());
+      } else {
+        static_assert(
+            std::is_same_v<
+                decltype(std::forward<Mi>(aMi)(
+                    std::forward<ConcreteVariant>(aV).template as<N>())),
+                decltype(Next::matchN(std::forward<ConcreteVariant>(aV),
+                                      std::forward<Ms>(aMs)...))>,
+            "all matchers must have the same return type");
+        return std::forward<Mi>(aMi)(
+            std::forward<ConcreteVariant>(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
+      // Matchers don't exhaust all variant types. There must exist a
+      // Matcher (with its operator()(T&)) for every variant type T, in the
+      // exact same order.
+      return Next::matchN(std::forward<ConcreteVariant>(aV),
+                          std::forward<Ms>(aMs)...);
+    }
+  }
+};
+
+/**
+ * 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(std::forward<U>(aValue)) {}
+
+  AsVariantTemporary(const AsVariantTemporary& aOther)
+      : mValue(aOther.mValue) {}
+
+  AsVariantTemporary(AsVariantTemporary&& aOther)
+      : mValue(std::move(aOther.mValue)) {}
+
+  AsVariantTemporary() = delete;
+  void operator=(const AsVariantTemporary&) = delete;
+  void operator=(AsVariantTemporary&&) = delete;
+
+  std::remove_const_t<std::remove_reference_t<T>> 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!
+ *       }
+ *     }
+ *
+ *     // Instead, a single function object (that can deal with all possible
+ *     // options) may be provided:
+ *     struct FooMatcher
+ *     {
+ *       // The return type of all matchers must be identical.
+ *       char* operator()(A& a) { ... }
+ *       char* operator()(B& b) { ... }
+ *       char* operator()(C& c) { ... }
+ *       char* operator()(D& d) { ... } // Compile-time error to forget D!
+ *     }
+ *     char* foo(Variant<A, B, C, D>& v) {
+ *       return v.match(FooMatcher());
+ *     }
+ *
+ *     // In some situations, a single generic lambda may also be appropriate:
+ *     char* foo(Variant<A, B, C, D>& v) {
+ *       return v.match([](auto&) {...});
+ *     }
+ *
+ *     // Alternatively, multiple function objects may be provided, each one
+ *     // corresponding to an option, in the same order:
+ *     char* foo(Variant<A, B, C, D>& v) {
+ *       return v.match([](A&) { ... },
+ *                      [](B&) { ... },
+ *                      [](C&) { ... },
+ *                      [](D&) { ... });
+ *     }
+ *
+ *     // In rare cases, the index of the currently-active alternative is
+ *     // needed, it may be obtained by adding a first parameter in the matcner
+ *     // callback, which will receive the index in its most compact type (just
+ *     // use `size_t` if the exact type is not important), e.g.:
+ *     char* foo(Variant<A, B, C, D>& v) {
+ *       return v.match([](auto aIndex, auto& aAlternative) {...});
+ *       // --OR--
+ *       return v.match([](size_t aIndex, auto& aAlternative) {...});
+ *     }
+ *
+ * ## 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 {
+  friend struct IPC::ParamTraits<mozilla::Variant<Ts...>>;
+  friend struct mozilla::ipc::IPDLParamTraits<mozilla::Variant<Ts...>>;
+
+  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(std::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(std::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(std::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(std::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(), std::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(std::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(std::move(aValue));
+    return *this;
+  }
+
+  ~Variant() { Impl::destroy(*this); }
+
+  template <typename T, typename... Args>
+  T& emplace(Args&&... aTs) {
+    Impl::destroy(*this);
+    tag = Impl::template tag<T>();
+    ::new (KnownNotNull, ptr()) T(std::forward<Args>(aTs)...);
+    return as<T>();
+  }
+
+  template <size_t N, typename... Args>
+  typename detail::Nth<N, Ts...>::Type& emplace(Args&&... aTs) {
+    using T = typename detail::Nth<N, Ts...>::Type;
+    Impl::destroy(*this);
+    tag = N;
+    ::new (KnownNotNull, ptr()) T(std::forward<Args>(aTs)...);
+    return as<N>();
+  }
+
+  /** 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 lvalue-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 lvalue-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());
+  }
+
+  /** Mutable rvalue-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 std::move(*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 std::move(
+        *static_cast<typename detail::Nth<N, Ts...>::Type*>(ptr()));
+  }
+
+  /** Immutable const rvalue-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 std::move(*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 std::move(
+        *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(std::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(std::move(as<N>()));
+  }
+
+  // Exhaustive matching of all variant types on the contained value.
+
+  /** Match on an immutable const lvalue-reference. */
+  template <typename Matcher>
+  decltype(auto) match(Matcher&& aMatcher) const& {
+    return Impl::match(std::forward<Matcher>(aMatcher), *this);
+  }
+
+  template <typename M0, typename M1, typename... Ms>
+  decltype(auto) match(M0&& aM0, M1&& aM1, Ms&&... aMs) const& {
+    return matchN(*this, std::forward<M0>(aM0), std::forward<M1>(aM1),
+                  std::forward<Ms>(aMs)...);
+  }
+
+  /** Match on a mutable non-const lvalue-reference. */
+  template <typename Matcher>
+  decltype(auto) match(Matcher&& aMatcher) & {
+    return Impl::match(std::forward<Matcher>(aMatcher), *this);
+  }
+
+  template <typename M0, typename M1, typename... Ms>
+  decltype(auto) match(M0&& aM0, M1&& aM1, Ms&&... aMs) & {
+    return matchN(*this, std::forward<M0>(aM0), std::forward<M1>(aM1),
+                  std::forward<Ms>(aMs)...);
+  }
+
+  /** Match on an immutable const rvalue-reference. */
+  template <typename Matcher>
+  decltype(auto) match(Matcher&& aMatcher) const&& {
+    return Impl::match(std::forward<Matcher>(aMatcher), std::move(*this));
+  }
+
+  template <typename M0, typename M1, typename... Ms>
+  decltype(auto) match(M0&& aM0, M1&& aM1, Ms&&... aMs) const&& {
+    return matchN(std::move(*this), std::forward<M0>(aM0),
+                  std::forward<M1>(aM1), std::forward<Ms>(aMs)...);
+  }
+
+  /** Match on a mutable non-const rvalue-reference. */
+  template <typename Matcher>
+  decltype(auto) match(Matcher&& aMatcher) && {
+    return Impl::match(std::forward<Matcher>(aMatcher), std::move(*this));
+  }
+
+  template <typename M0, typename M1, typename... Ms>
+  decltype(auto) match(M0&& aM0, M1&& aM1, Ms&&... aMs) && {
+    return matchN(std::move(*this), std::forward<M0>(aM0),
+                  std::forward<M1>(aM1), std::forward<Ms>(aMs)...);
+  }
+
+  /**
+   * Incorporate the current variant's tag into hashValue.
+   * Note that this does not hash the actual contents; you must take
+   * care of that yourself, perhaps by using a match.
+   */
+  mozilla::HashNumber addTagToHash(mozilla::HashNumber hashValue) const {
+    return mozilla::AddToHash(hashValue, tag);
+  }
+
+ private:
+  template <typename ConcreteVariant, typename M0, typename M1, typename... Ms>
+  static decltype(auto) matchN(ConcreteVariant&& aVariant, M0&& aM0, M1&& aM1,
+                               Ms&&... aMs) {
+    static_assert(
+        2 + sizeof...(Ms) == sizeof...(Ts),
+        "Variant<T...>::match() takes either one callable argument that "
+        "accepts every type T; or one for each type T, in order");
+    return Impl::matchN(std::forward<ConcreteVariant>(aVariant),
+                        std::forward<M0>(aM0), std::forward<M1>(aM1),
+                        std::forward<Ms>(aMs)...);
+  }
+};
+
+/*
+ * 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>(std::forward<T>(aValue));
+}
+
+}  // namespace mozilla
+
+#endif /* mozilla_Variant_h */