Add assertion.

src/compiler/checker.ts

@@ -16130,6 +16130,7 @@ namespace ts {
 
                 if (isLiteralType(source) && !typeCouldHaveTopLevelSingletonTypes(target)) {
                     generalizedSource = getBaseTypeOfLiteralType(source);
+                    Debug.assert(!isTypeAssignableTo(generalizedSource, target), "generalized source shouldn't be assignable");
                     generalizedSourceType = getTypeNameForErrorDisplay(generalizedSource);
                 }