Unsound.java

class Unsound {
    static class Constrain<A, B extends A> {}
    static class Bind<A> {
        <B extends A> A upcast(Constrain<A,B> constrain, B b) {
            return b;
        }
    }
    static <T,U> U coerce(T t) {
        Constrain<U, ? super T> constrain = null;
        Bind<U> bind = new Bind<U>();
        return bind.upcast(constrain, t);
    }
    public static void main(String[] args) {
        String zero = Unsound.<Integer,String>coerce(0);
    }
}