https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80265
--- Comment #13 from Marc Glisse <glisse at gcc dot gnu.org> --- If we need the "if constexpr()" that is proposed for C++20, we might as well implement that (and enable it in system headers for C++17 if that's useful), it seems better than abusing __builtin_constant_p, which is getting contradictory requirements from its various uses: - constexpr (forces very early lowering) - warning/error (forbid splitting or anything that might create calls with constants that did not exist in the user's code, or lower to false before such transformation) - optimization (wants to delay lowering to false quite late (though not so late that the code without __bcp isn't properly optimized) and likes isolating a path that makes the argument constant) etc (though at first glance your latest version seems likely to work well enough)
