[Bug libstdc++/60519] Debug mode should check comparators for irreflexivity

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60519

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #3)
> Your patch checks for antisymmetry instead, which is also required for a
> strict weak order, but is a different property.

Maybe we want both, because the irreflexivity check can be done very cheaply,
so is suitable for the Debug Mode Lite I've talked about, but the antisymmetry
check is more expensive.