I have found a question without an answer, but there is a comment that answers it.
What should be done in such cases?
If you are able to write an exhaustive answer basing on what reported on that comment, you can write an answer. You should first comment suggesting to the user who wrote the comment to write it as answer; If that user doesn't add his own answer, then you can write the answer yourself, but:
Instead of an answer like it is not possible to achieve what you want, a more exhaustive answer should be given, for example explaining in detail why what the OP wants to achieve cannot be achieved.