Bug 1246841 - Allow construction of Variant values using type inferen…

…ce. r=waldo

mfbt/Variant.h

@@ -247,6 +247,39 @@ struct VariantImplementation<N, T, Ts...>
   }
 };
 
+/**
+ * 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
 
 /**
@@ -270,6 +303,18 @@ struct VariantImplementation<N, T, Ts...>
  *     Variant<char, uint32_t> v1('a');
  *     Variant<UniquePtr<A>, B, C> v2(MakeUnique<A>());
  *
+ * Because specifying the full type of a Variant value is often verbose,
+ * 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').
+ *
  * All access to the contained value goes through type-safe accessors.
  *
  *     void
@@ -391,6 +436,19 @@ class Variant
     new (ptr()) T(Forward<T>(aT));
   }
 
+  /**
+   * 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,
+           typename T = typename detail::SelectVariantType<RefT, Ts...>::Type>
+  MOZ_IMPLICIT Variant(detail::AsVariantTemporary<RefT>&& aValue)
+    : tag(Impl::template tag<T>())
+  {
+    new (ptr()) T(Move(aValue.mValue));
+  }
+
   /** Copy construction. */
   Variant(const Variant& aRhs)
     : tag(aRhs.tag)
@@ -421,6 +479,15 @@ class Variant
     return *this;
   }
 
+  /** Move assignment from AsVariant(). */
+  template <typename T>
+  Variant& operator=(detail::AsVariantTemporary<T>&& aValue)
+  {
+    this->~Variant();
+    new (this) Variant(Move(aValue));
+    return *this;
+  }
+
   ~Variant()
   {
     Impl::destroy(*this);
@@ -502,6 +569,26 @@ class Variant
   }
 };
 
+/*
+ * 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 */