[Bug c++/67888] Compiling clang 3.7.0 results in is used but never defined

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67888

--- Comment #8 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Paolo Carlini from comment #7)
> I have no objections, the patch seems safe to backport. Jason, shall I go
> ahead if it regtests Ok on the branch?

Sure.