[Bug c++/52321] poor diagnostic of invalid cast

["igodard at pacbell dot net" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52321

--- Comment #4 from Ivan Godard <igodard at pacbell dot net> 2012-02-21 17:38:30 UTC ---
Define an enum of reasons with "success" first, flop the sense of the test so
that false means coercion was OK (grep to find all calls and put a "!" in front
of each), and return the reason enum instead of bool. The code that is
reason-aware saves the enum and builds a good message; the legacy code that is
not reason-aware treats the enum as a bool and works as before except for the
inverted sense of the test. Maybe half an hour of work.

Plausible?