diff --git a/lib/Sema/SemaObjCProperty.cpp b/lib/Sema/SemaObjCProperty.cpp
index 42a5c9d..9d849ff 100644
--- a/lib/Sema/SemaObjCProperty.cpp
+++ b/lib/Sema/SemaObjCProperty.cpp
@@ -281,7 +281,10 @@
     QualType ConvertedType;
     // Relax the strict type matching for property type in continuation class.
     // Allow property object type of continuation class to be different as long
-    // as it narrows the object type in its primary class property.
+    // as it narrows the object type in its primary class property. Note that
+    // this conversion is safe only because the wider type is for a 'readonly'
+    // property in primary class and 'narrowed' type for a 'readwrite' property
+    // in continuation class.
     if (!isa<ObjCObjectPointerType>(PIDecl->getType()) ||
         !isa<ObjCObjectPointerType>(PDecl->getType()) ||
         (!isObjCPointerConversion(PDecl->getType(), PIDecl->getType(),