[Bug libstdc++/78420] [5/6/7 Regression] std::less is not a total order with -O2 enabled

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78420

--- Comment #12 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
..which is http://cplusplus.github.io/LWG/lwg-active.html#2491