Bug 766347 - Implement a mozilla::EnableIf template struct for SFINAE…

… capabilities. r=luke

mfbt/TypeTraits.h

@@ -49,6 +49,34 @@ struct Conditional<false, A, B>
     typedef B Type;
 };
 
+/*
+ * EnableIf is a struct containing a typedef of T if and only if B is true.
+ *
+ * mozilla::EnableIf<true, int>::Type is int;
+ * mozilla::EnableIf<false, int>::Type is a compile-time error.
+ *
+ * Use this template to implement SFINAE-style (Substitution Failure Is not An
+ * Error) requirements.  For example, you might use it to impose a restriction
+ * on a template parameter:
+ *
+ *   template<typename T>
+ *   class PodVector // vector optimized to store POD (memcpy-able) types
+ *   {
+ *      EnableIf<IsPodType<T>, T>::Type* vector;
+ *      size_t length;
+ *      ...
+ *   };
+ */
+template<bool B, typename T = void>
+struct EnableIf
+{};
+
+template<typename T>
+struct EnableIf<true, T>
+{
+    typedef T Type;
+};
+
 } /* namespace mozilla */
 
 #endif  /* mozilla_TypeTraits_h_ */