[Bug libstdc++/65942] [5/6 Regression] [C++14] cannot use std::function as comparator in algorithms

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65942

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's due to the addition of C++14 constexpr throughout the library, the program
works if preceded by 

#include <bits/c++config.h>
#undef _GLIBCXX14_CONSTEXPR
#define _GLIBCXX14_CONSTEXPR

so that the newly-constexpr functions are not constexpr.