I notice (just now) that unary & and [] are absent. Detection of operator[] needs an entirely different mechanism than the above operators, so I understand its absence (but maybe it could still be included?). I'm not sure detection of unary operator& would be all that useful most of the time, but I'm just wondering what the reasons were for its exclusion.
It needs also a different implementation because & always exists (it is always possible to get the address of something) , even if not defined by the user so if you define a less preferred operator (like in the current implementation), I think it becomes more preferred over the built-in one. I tried at a time and discovered it was not possible. I do not remember all details however. Frédéric