Constexpr is a keyword in C++ that allows the compiler
to generate some code at compiler time and inject it into the static
part of your executable.
It transparently allows code that is not constexpr to
still compile – it would generate that code at runtime, however.
It can also be used in branches with if constexpr, where
it only compiles a branch for the code that can be verified at compile
time to take one branch.