https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66841
--- Comment #2 from Eric Niebler <eric.niebler at gmail dot com> --- This answer is deeply unsatisfying. I want valid expressions, not traits. And if std::is_constructible doesn't do *exactly* what I want (and it doesn't) I have to author my own trait, when what I want to do is author my own concept. I'm not trying to do anything exotic here. There should be a way to do it without resorting to metaprogramming trickery and SFINAE.
