Use the correct cast kind as suggested by Doug.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@78395 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaOverload.cpp b/lib/Sema/SemaOverload.cpp
index 0fb515e..d7d2610 100644
--- a/lib/Sema/SemaOverload.cpp
+++ b/lib/Sema/SemaOverload.cpp
@@ -2001,7 +2001,8 @@
                                    From->getSourceRange()))
     return true;
 
-  ImpCastExprToType(From, DestType, CastExpr::CK_Unknown, /*isLvalue=*/true);
+  ImpCastExprToType(From, DestType, CastExpr::CK_DerivedToBase, 
+                    /*isLvalue=*/true);
   return false;
 }