App.java

class App {
  public static class SuperClass {}
  public static class SubClass extends SuperClass {}

  public static <T, U extends T> U returnSub(T sup, U sub) {
    return sub;
  }

  public static void main(String[] args) {
    SuperClass parent = new SuperClass();
    SubClass child = new SubClass();

    returnSub(parent, child);
    returnSub(child, parent); // Java doesn't like that
  }
}