StringLiteral is excluding typechecking against anything other than Object, String or Void. However, String implements several interfaces and StringLiteral can be type-checked aginst these interfaces when it appears as a call to method argument e.g. $1.contains("foo") will try to use CharSequence to type check the String "foo".

The typecheck method should use an isAssignableFrom test rather than explicitly testing against just these specific types.