[Bug libstdc++/113159] More robust std::sort for silly comparator functions

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113159

--- Comment #12 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
Also note even

bool cmp(Element a, Element b) { return false; }

is a *valid* comparator, per the standard.  Though it'll likely produce
completely meaningless result, it's just your own logical error as it violates
nothing in the standard.