Sometimes you can see a question like How to do <something> in <some program>, asking for advice how to replicate a feature of competitive program or how to do something that OP expects to be possible, but couldn't find out how to do it.
If the feature just isn't available, like in Is it possible to crop and resize in one step in GIMP?, is it acceptable to post "It's impossible" as an answer, or should it be a comment?
Why it should be an answer:
- It actually answers the question if OP asks "Is it possible to do <something>".
- If it's wrong, it can be downvoted.
- If it's correct, it can be accepted. Otherwise the question looks like it's unanswered.
Why it should be a comment:
- It doesn't solve the real problem. If OP asks "Is <XYZ> possible", he really means "How do I do <XYZ>".
- In most cases, short answer is a poor answer. For example alternative solutions or workarounds could be provided.
- Such answers shouldn't receive points for upvotes or accepting, as they aren't helpful. Getting rep for not being helpful is bad.
- Adding a Unsolvable close reason would both indicate that the question is kinda answered and prevent getting rep for unhelpful answers. But it would block the question from being answered in the future if the feature is ever implemented in the mentioned application. Also, it looks like an overkill to me.