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

Subject: [boost] [type_traits] Is it possible to test for an explicit conversion?
From: John Maddock (boost.regex_at_[hidden])
Date: 2012-07-20 06:23:30

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.