# What to do when an existing answer to one question answers another question?

I would like to answer this question with this answer.

Trying to post a cross-reference as an answer results in the answer being converted into a comment.

But thou shalt not answer in the comments; comments are for suggesting more information or making improvements or similar. Answers belong in answers.

Duplicating the content of the answer seems like a bad idea.

Voting to close as duplicate is probably wrong as well — my impression is that the new question is not identical enough to make people happy with that action. (also, the old post doesn't have an accepted answer, if that matters)

What to do?

• What's the relevance of the other question not having an accepted answer? – quid Apr 8 '17 at 20:49
• @quid: I think it has come up in the past as a reason not to close something as a duplicate, or maybe as which of two posts should be closed as a duplicate of the other, or maybe both; I don't recall details. I did not really contemplate the point. – Hurkyl Apr 8 '17 at 20:51
• As it stands it seems like a distraction to mention this. What you may recall is that a post cannot be closed of a post as a dupe unless the target is considered as not unanswered, i.e., positively scored or accepted answer. – quid Apr 8 '17 at 21:06
• @quid: I have deemphasized that point. Thank you for the suggestion. – Hurkyl Apr 8 '17 at 21:07
• I was wondering whether to add specific-question tag, but it is probably better to leave the decision to the OP. (The tag should be applied if the question is mainly about this specific situation. If the linked question is merely and example and this meta post is about the general situation, then this tag should not be included.) – Martin Sleziak Apr 15 '17 at 9:30
• Maybe duplicate/adapt the content of the answer, properly referencing it, and make the new answer community wiki. – Olivier Apr 17 '17 at 4:00

## 3 Answers

I will usually put the link in a comment prefixed with

Possibly related:

or

Please see:

Then it's not "an answer in a comment" nor a request to close as a duplicate.

• I think such an action would motivate others to flag the question as a duplicate and if the question is not closed as such, it's probably going to end up on the unanswered list. – Ben Apr 21 '17 at 8:52

One thing that I've personally done in such situations is to post a (possibly Community Wiki) answer to the new question that includes:

1. a link to the previous answer,
2. a quote of the relevant parts of the answer, just in case the link breaks somehow (e.g. the older answer is deleted), and
3. a brief explanation of how the quoted answer solves the specific problem stated in the new question.

Here's an example of one such answer that I posted to our sister site Crypto.SE. To make this a self-contained (and self-descriptive!) example, let me quote the relevant parts of that answer below:

The function $f$ introduced by Maeher in this answer to a related question should also do the job here (as both $g$ and $h$). For convenience, let me quote that answer here:

[meta-quotation of Maeher's answer omitted for brevity]

As shown in Maeher's answer, $f$ is one-way if $h$ is. However, $f(f(x)) = 0^{n/2}\Vert h(0^{n/2})$ is constant, independently of $x$ (except for its length $n$), and thus finding pre-images is trivial.

Just give some context for the link, e.g., a short summary or a description of the situation in the thread you link to.