https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87356
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0492r2.html#Late37 for the change to the spec that happened post-P0218R1.
