I agree ; however, the tricky part - I believe - is not computing the message string in case you're not going to print/record it.

Is IO allowed in constexpr functions?

I don't think so, but for zero-cost logging you just need the conditional for your branch to be constexpr. If the branch is known not to be taken at compile time, the compiler can just throw it away.

Something like:

  void MyDebugLog(text) {
    if (<constepxr check if logging enabled>) {
      // actually log...
    }
  }