“ … and here I forgot an ampersand so the parameter was passed by value instead of by reference and I spent so much time …. Oh. You meant the other kind of bugs, didn’t you?”

Weird, it should be standard C++20. Hope are you invoking gcc?

godbolt link: https://godbolt.org/z/6Tn4Kcjrs

Edit: be sure to call g++, not gcc.

#include <cmath>
#include <iostream>
#include <numbers>

int main()
{
  decltype ( NAN ) f { std::numbers::pi };
  std::cout << f << std::endl;
}