Include property in union type only if all underlying properties are public

src/compiler/checker.ts

@@ -2945,7 +2945,7 @@ module ts {
                 let type = getApparentType(current);
                 if (type !== unknownType) {
                     let prop = getPropertyOfType(type, name);
-                    if (!prop) {
+                    if (!prop || getDeclarationFlagsFromSymbol(prop) & (NodeFlags.Private | NodeFlags.Protected)) {
                         return undefined;
                     }
                     if (!props) {