Hmm, is it error, value or value, error? In this situation, I'd prefer something like std::optional or std::variant, which you can't get wrong as easily as swap

Hmm, is it error, value or value, error? In this situation, I'd prefer something like std::optional or std::variant, which you can't get wrong as easily as swapping two variables in a structured bind. Those have their problems too, though, and I'd never claim std::variant is "nice" to use.