Constexpr math functions

[PaulXiCao]

Multiple functions in autodiff/forward/real/real.hpp are marked constexpr which does not align with the standard, e.g. exp, pow, ...

This is a non-standard gcc extension, see here.

Thus, the following example code compiles with gcc but not with clang nor msvc

#include <cmath>
int main(){ constexpr auto x = std::exp(1); }

You can try it live on godbolt here.

This problem will also occur when we try to apply those functions to real:

#include <autodiff/forward/real.hpp>
using namespace autodiff;

int main() { constexpr auto x = exp(real(1)); }

There are some drop-in replacements which support constexpr basic math functions. Maybe we should rely on one of those.

PS: I currently work on rewriting real.hpp abit and would remove the constexpr tag on those functions.