> Actually, it's not in general decidable whether f.test(f) is safe because f gets read twice and the value might change in the meantime.

Might it? I can't see any way that actions in the current thread can change the value of f in between those two uses. Actions in another thread can be ignored until the receiving end of a happens-before relationship is encountered, and there isn't such a point inside that expression.

More generally, yes, two uses of the same wildcarded variable in one method might not yield objects with the same actual type argument. But in this case i think it's safe.