text: /* -*- 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 */
  