JLS 3 Chapter 8 "Classes", section 8.1.2 "Generic Classes and Type Parameters"
provides the following example of mutually recursive type variable bounds:

interface ConvertibleTo<T> {
   T convert();
}
class ReprChange<T implements ConvertibleTo<S>,
                 S implements ConvertibleTo<T>> {
   T t;
   void set(S s) { t = s.convert(); }
   S get() { return t.convert(); }
}

But keyword "implements" is not allowed within type parameter declaration. Accrding to the JLS3 syntax rules

TypeParameter:
        Identifier [extends Bound]

Due to that reason javac fails to compile the sample code.
Replacing "implements" with "extends" solves the problem.