Bug 1168008 - Make IsConvertible handle void. r=gerald

mfbt/TypeTraits.h

@@ -637,12 +637,31 @@ public:
  * For obscure reasons, you can't use IsConvertible when the types being tested
  * are related through private inheritance, and you'll get a compile error if
  * you try.  Just don't do it!
+ *
+ * Note - we need special handling for void, which ConvertibleTester doesn't
+ * handle. The void handling here doesn't handle const/volatile void correctly,
+ * which could be easily fixed if the need arises.
  */
 template<typename From, typename To>
 struct IsConvertible
   : IntegralConstant<bool, detail::ConvertibleTester<From, To>::value>
 {};
 
+template<typename B>
+struct IsConvertible<void, B>
+  : IntegralConstant<bool, IsVoid<B>::value>
+{};
+
+template<typename A>
+struct IsConvertible<A, void>
+  : IntegralConstant<bool, IsVoid<A>::value>
+{};
+
+template<>
+struct IsConvertible<void, void>
+  : TrueType
+{};
+
 /* 20.9.7 Transformations between types [meta.trans] */
 
 /* 20.9.7.1 Const-volatile modifications [meta.trans.cv] */

mfbt/tests/TestTypeTraits.cpp

@@ -354,6 +354,10 @@ TestIsConvertible()
   static_assert((!IsConvertible<A, D>::value),
                 "A and D are unrelated");
 
+  static_assert(IsConvertible<void, void>::value, "void is void");
+  static_assert(!IsConvertible<A, void>::value, "A shouldn't convert to void");
+  static_assert(!IsConvertible<void, B>::value, "void shouldn't convert to B");
+
   // These cases seem to require C++11 support to properly implement them, so
   // for now just disable them.
   //static_assert((!IsConvertible<C*, A*>::value),