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

[redi at gcc dot gnu.org]

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

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't know what fixed it, or if it's worth backporting.