[Bug c++/66841] [concepts] bogus error "invalid reference to function concept" when function concept is overloaded

[eric.niebler at gmail dot com]

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.