I've just browsed this question, that has no "answers", but actually the question is answered in one line as a comment, and the OP acknowledge it answered his question.
Hence, the right thing would be that the comment was actually an answer, and that this answer was accepted. What do you think is the appropriate behaviour if we see this case:
- Do nothing?
- Suggest to the answerer (as a follow-up comment) to post it as an actual answer?
- Answer the question ourself, possibly detailing a little more, giving credits to the actual answerer?
I first guessed that 2. was an obviously correct behaviour, but then I realized that if the answerer didn't do it in the first place, it is probably because he didn't want to spend time on writing a nice answer (and dislike the idea of a one-line answer). Hence I don't see why he would be inclined to so afterward, and I guess would be more annoyed than anything else.