From https://www.facebook.com/ross.tate/posts/10102249566392775

Unsound.java

public class Unsound {
  static class Bound<A, B extends A> {}
  static class Bind<A> {
    <B extends A> A bad(Bound<A,B> bound, B b) {return b;}
  }
  public static <T,U> U coerce(T t) {
    Bound<U,? super T> bound = null;
    Bind<U> bind = new Bind<U>();
    return bind.bad(bound, t);
  }
  public static void main(String[] args) {
    String s = Unsound.coerce(new Object());
  }
}