https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113159
--- Comment #15 from Xi Ruoyao <xry111 at gcc dot gnu.org> --- (In reply to Yury Gribov from comment #14) > (In reply to Xi Ruoyao from comment #12) > > Also note even > > > > bool cmp(Element a, Element b) { return false; } > > > > is a *valid* comparator, per the standard. > > Hm, doesn't it violate the asymmetry axiom (cmp(x, y) => !cmp(y, x)) ? No there's no such axiom. cmp(x, y) && !cmp(y, x) just means x and y will be treat as equal elements.