Bug 891177 - Add an explanatory comment by the const_cast<> in the Move(const T&) overload. r=luke

--HG--
extra : rebase_source : 37a91bf2bdfe2b2f96ddebf276ad532d4419c42b

mfbt/Move.h

@@ -133,6 +133,20 @@ template<typename T>
 inline MoveRef<T>
 Move(const T& t)
 {
+  // With some versions of gcc, for a class C, there's an (incorrect) ambiguity
+  // between the C(const C&) constructor and the default C(C&&) C++11 move
+  // constructor, when the constructor is called with a const C& argument.
+  //
+  // This ambiguity manifests with the Move implementation above when Move is
+  // passed const U& for some class U.  Calling Move(const U&) returns a
+  // MoveRef<const U&>, which is then commonly passed to the U constructor,
+  // triggering an implicit conversion to const U&.  gcc doesn't know whether to
+  // call U(const U&) or U(U&&), so it wrongly reports a compile error.
+  //
+  // http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50442 has since been fixed, so
+  // this is no longer an issue for up-to-date compilers.  But there's no harm
+  // in keeping it around for older compilers, so we might as well.  See also
+  // bug 686280.
   return MoveRef<T>(const_cast<T&>(t));
 }