Using C++20 for optimizing compile-time keys

[lemire]

We expect that most of the keys are known at compile-time, which you can leverage to validate them.
According to stack overflow, see https://stackoverflow.com/questions/46919582/is-it-possible-to-test-if-a-constexpr-function-is-evaluated-at-compile-time, C++20 solves the problem:

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

[jkeiser]

I was wondering if there was a way we could use initializer lists and have c++ 11 compatibility, too.