[Bug c++/57588] [C++11][constexpr] static constexpr in class fails to link

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57588

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Daniel Krügler from comment #4)
> I don't think that this correct here (In C++11 the rules became relaxed).

It was post-C++11, but as a DR we should implement it, and I see you're right,
the initialization of int_ uses the lvalue-to-rvalue conversion immediately so
it isn't odr-used.

I've created PR 57594 and PR 57595 for the missing diagnostics.