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

C++20 introduces is_constant_evaluated, defined in header <type_traits>, 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.