diff --git a/lib/AST/ExprConstant.cpp b/lib/AST/ExprConstant.cpp
index 6fb2abe..a0ef746 100644
--- a/lib/AST/ExprConstant.cpp
+++ b/lib/AST/ExprConstant.cpp
@@ -696,6 +696,7 @@
 /// as GCC.
 static int EvaluateBuiltinClassifyType(const CallExpr *E) {
   // The following enum mimics the values returned by GCC.
+  // FIXME: Does GCC differ between lvalue and rvalue references here?
   enum gcc_type_class {
     no_type_class = -1,
     void_type_class, integer_type_class, char_type_class,