Is it possible to test if a constexpr function is evaluated at compile time?

Question

Since the extended versions of constexpr (I think from C++14) you can declare constexpr functions that could be used as "real" cons

Answer 1

C++20 introduces is_constant_evaluated, defined in header , which addresses this issue.

constexpr int foo(int s)
{
    if (std::is_constant_evaluated()) // note: not "if constexpr"
        /* evaluated at compile time */;
    else
        /* evaluated at run time */;
}

Note that here the ordinary if is used instead of if constexpr. If you use if constexpr, then the condition has to be evaluated at compile time, so is_constant_evaluated always returns true, rendering the test useless.