On the other hand, an alternative design would be that the single-arm if wrapped the return type R to "Some(R)", and None for the "phantom else" arm. I haven't

On the other hand, an alternative design would be that the single-arm if wrapped the return type R to "Some(R)", and None for the "phantom else" arm. I haven't considered the ramifications of this, but I'd expect it to work well if the Option<R> type is defined flexibly and compose-ably enough.