std::is_constant_evaluated behavior

Question

GCC9 already implements std::is_constant_evaluated. I played a little bit with it, and I realized it is somewhat tricky. Here’s my test:

constex         


        

Answer 1

if constexpr requires a constant expression for a condition. So is_constant_evaluated is of course always going to be true in such a context.

It's meant for a regular if. The purpose is to not go into a code path that is illegal in a constexpr function when evaluated in a constant expression. But to let it execute in runtime. It's not there to eliminate those code paths from the function altogether.