[boost] [type_traits] Is it possible to test for an explicit conversion?

OK, so we don't have such a trait at present, but one of the requests from the multiprecision review was "make conversions explicit in the front end if they are in the backend". Which to do it nicely (and semi-automagically) requires a trait that tests whether A is explicitly convertible to B. Which would obviously make a useful type trait in it's own right, but.... anyone have any ideas on how such a trait could be implemented? At present it looks "impossible" to me, but I've learned never to say never ;-) Thanks, John.