PR feedback and another test

src/compiler/checker.ts

@@ -3574,7 +3574,8 @@ module ts {
 
         // Since removeSubtypes checks the subtype relation, and the subtype relation on a union
         // may attempt to reduce a union, it is possible that removeSubtypes could be called
-        // recursively on the same set of types.
+        // recursively on the same set of types. The removeSubtypesStack is used to track which
+        // sets of types are currently undergoing subtype reduction.
         let removeSubtypesStack: string[] = [];
         function removeSubtypes(types: Type[]) {
             let typeListId = getTypeListId(types);