Per JLS 4.10.4, lub(List<? extends Exception>, List<? super Exception>) = List<Exception>.

lcta(? extends U, ? super V) = U if U = V, otherwise ?

This is not correct -- the result is not a supertype of either of the inputs. (Effectively this is computing a glb, not a lub.)

The right thing to do is to find a wildcard that includes the union of the ranges represented by two wildcards -- always an unbounded wildcard, '?'