Compilation of the source below shows a problem with subtyping
generic classes.
- The class D<X,Y> is a subclass of C<X,Y>.
  This is confirmed by the correct an accepted cast in method f().
- The class D<Y,X> should a subclass of C<Y,X>,
  because permuting parameters X and Y in both classes C and D should not
  alter subclass relationship.
  So the cast in method g() should be correct, but the compiler rejects it.

class C<X, Y> {}

class D<X, Y> extends C<X, Y>
{
    void f(C<X, Y> c)
    {
        D<X, Y> d = (D<X, Y>) c;// OK because D extends C
    }

    void g(C<Y, X> c)// parameters X and Y are now permuted
    {
        D<Y, X> d = (D<Y, X>) c;// should also be OK but is not !?
        // ^
        // inconvertible types
        // found : C<Y,X>
        // required : D<Y,X>
    }
    void gWorkAround(C<Y, X> c)// but generates unchecked warning
    {
        D<Y, X> d = (D) ((C) c);
    }
}