There are several questions that are officially "unanswered," but which were effectively answered in the comments. For example, this one was answered by Russell Impagliazzo in the comments. It's been a long time (~5 years), and no better answer is forthcoming. It's a little annoying that this question still appears as "Unanswered." Yet I don't feel comfortable just posting Russell's answer, because then any upvotes it received would go to me instead of Russell. I could of course post a CW answer to avoid getting upvotes myself, but then it precludes Russell from ever posting an answer and getting upvotes. Is there an established norm here?
You can ping the author in the comments, if the person doesn't post an answer in some reasonable amount of time I think it is fine to post a CW answer, making question/answer useful outweighs votes and reputation.
$\begingroup$ I agree with this. But I would add: it is better for the original asker (rather than someone else) to post an answer from the comments, unless they don't understand how to flesh it out into a full answer. However, it is reasonable for someone else to bring up this suggestion like "I think this question has been answered in the comments: are @-tag-of-answerer (or OP after some time) interested in converting this comment into an answer?" $\endgroup$– Artem Kaznatcheev ModJan 23, 2017 at 10:14