> If you wanted a c++26 preconditon for collatz tho, here it is.

  cpp bool collatz(unsigned long long n)
  {
    while (n != 1)
      n = n % 2 ? 3 * n + 1 : n / 2;
    return true;
  }

Ideally, you also need a way to detect runs towards infinity. That is harder.

Otherwise, an optimising compiler can use the forward progress guarantee and optimize that to:

  cpp bool collatz(unsigned long long n)
  {
    return true;
  }

I think I saw years ago that clang replaced a collatz impl with true. Why is it that the "forward progress garuntee" would let it do that?