Use the correct cast kind as suggested by Doug.

llvm-svn: 78395

clang/lib/Sema/SemaOverload.cpp

@@ -2001,7 +2001,8 @@ Sema::PerformObjectArgumentInitialization(Expr *&From, CXXMethodDecl *Method) {
                                    From->getSourceRange()))
     return true;
 
-  ImpCastExprToType(From, DestType, CastExpr::CK_Unknown, /*isLvalue=*/true);
+  ImpCastExprToType(From, DestType, CastExpr::CK_DerivedToBase, 
+                    /*isLvalue=*/true);
   return false;
 }