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. In the example a simple flow analysis will tell you it doesn't but the general case is equivalent to the halting problem. The generic method solves the problem because now there is only one read in boom() and the concrete type in testFoo ensures that f can't change in an incompatible way between reads.

A more sophisticated type system could allow access if it can be proven safe with simple flow analysis but wildcards are already complicated, that would make them much more so for not much benefit.