If someone comes along with a set of so specific requirements that there is currently no software fulfilling that needs, is it acceptable to answer
Currently no software fulfills that needs. If you can afford it you could hire someone developing that for you.
No, it's not an answer IMO. We want each answer to recommend a product. If there isn't a solution for a problem, the question should stay unanswered until one is found.
Sure. Quoting the ground rules:
You must also be prepared for answers that simply inform you that no single thing meets your needs, but goes on to recommend things that cover what you've identified to be the most important.
And if there is nothing that meets your needs, “write it” or “get someone to write it for you” may be the best answer.
Obviously, such answers need to be justified, just like positive answers.
Bad:
Use FooBarSoft, it's the bee's knees.
Good:
You can use FooBarSoft. Requirement #1 is met in that …. Requirement #2 is met in that …. To meet requirement #3, configure it in this way. Nice-to-have #4 is partially met: blah blah. Additionally, to do what you're trying to do, many people find this feature useful. A limitation of FooBarSoft is that it doesn't work in this rare circumstance.
Bad:
No. You'll have to write it yourself.
Good:
The de facto standard for requirements #1 and #2 is FooBarSoft, nothing else comes close. Requirement #3 has been requested (link to FooBarSoft's issue tracker); the author decided not to implement it because he had other priorities. He estimated the workload at 1 man-week. So you could adapt FooBarSoft yourself.
Very much related to this, where I suggest a very simple guideline - don't answer unless you can point to something that exists prior to your answer being written.
That said .. if you know something is open source, or can be modified with some kind of plug-in, and a combinations of plugins or known modifications could suit the criteria laid out by the question author, and the question author indicates that they're open to recommendations that entail a little develop-y stuff - fine. But we're getting more into granular case-by-case criteria there.
There's nothing wrong with saying:
I'm confident that what you want doesn't exist exactly how you laid it out, but [xyz] matches most of your criteria, and if used in conjunction with [abc], most of your needs would be met.
That's a far cry from you probably need to consult a developer to have something written to suit your need though.
Just make sure that answers point to something immediately actionable by the question author, and I think we're good.
No. It could be that it exists and you don't know it. It could be that it does not exist yet and the answer will be different in the future.
If a user asks a question and despite some views doesn't get an answer (maybe even after a bounty) he may just conclude that if nobody here knows a tool, there maybe isn't one.
Answers in the form of "I don't know a specific tool that does all of what you need but this tool here can do most of it and it can be improved to do all of it with a developer" should be good. Even more so if they are the only answer.
Is it acceptable for the answer to be summarized by "no, no such software"?
Yes.
Is it acceptable to answer literally with Currently no software fulfills that needs. If you can afford it you could hire someone developing that for you with nothing else in the answer?
NO!!!.
That answer is impossible to evaluate for correctness. You didn't provide any EVIDENCE that there's no such software. May be you don't know what you're talking about and randomly guessed. May be you just didn't hear about some useful software due to limited experience. May be you tried every single relevant software and found 100% wanting. By reading that threadbare answer, we can't tell which of those cases you represent.
As noted here, any good answer should be backed up.
A good "No such thing" answer should include:
List your qualifications for why you are familiar with this type of software. Maybe you have same needs as the OP and seeked similar software. Maybe you are a developer of similar software and competitor market research is your professional responsibility. Maybe you read trade publication dedicated to such software.
List software that you personally researched and evaluated that is in this area; and which features of each disqualify it from OP's needs.
