The following example was submitted by Neal Gafter as a program that causes javac to get stuck in an infinite loop.

interface I<T> {}

interface A<T> extends I<A<A<T>>>{}

abstract class X {
   abstract <T> T foo(T x, T y);
   void bar(A<Integer> x, A<String> y){
       foo(x, y);
   }
}

I verified that the following variations still cause javac to get stuck:
- Replacing the invocation "foo(x,y)" with "Object o = true ? x : y"
- Redefining I and A as classes rather than interfaces
- Replacing "A<Integer>" and "A<String>" with "A<C>" and "A<D>", where C and D are unrelated classes (except by Object)
- Replacing "I<A<A<T>>>" with "I<A<A<A<T>>>>" or "I<A<I<A<T>>>>"

This change prevents the bug:
- Replacing "I<A<A<T>>>" with "I<A<T>>", or eliminating the extends clause entirely

This seems to be a problem with the lub implementation. It's not clear to me why the supertype of A has any impact on the implementation, since the lub of "A<Foo>" and "A<Bar>" is always just "A<? extends lub(Foo,Bar)>". But I know the specification gets there in a convoluted way...