[Java Spec Report] Java 1.5 subtyping rules undecidable?

Martin Odersky found that the Java 1.5 subtyping rules are undecidable
as written.  These examples are unlikely to occur "in the wild", and
it is likely some unburdensome restrictions on F-bounds can be added
to the spec to avoid these situations.  Here's a simplified
example of the problem:

class F<T> {}
class C<X extends F<F<? super X>>> {
   C(X x) {
       F<? super X> f = x;
   }
}

To verify that

 X <: F<? super X>

we need to unfold X to its upper bound, yielding:

 F<F<? super X>>> <: F<? super X>

This leads back to the original problem:

 X <: F<? super X>

The correct answer, incidentally, is that the subtype relation does
not hold. JDK5 javac goes into an infinite recursion on this example.





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