[Bug libstdc++/67617] Non-standard const requirements imposed on associative container comparison objects

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67617

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Both Clang/libc++ and MSVC/Dinkumware reject it for the same reason.