Last year, you were partially employed, maybe for cash, and in November you took a retail job just to muster up a couple of holiday gifts for the kiddos. "Last year's W-2" has as little to do with your current salary expectations as... well... as that partial employment had to do with your career plans. So, how does one answer such an immaterial question? Do you:
a) lie, and hope no one checks?
b) answer honestly, and hope for the best?
c) plead the fifth?
d) answer a different question?
Let's think through this. First, A has to be wrong because, hello, never lie in an interview. You'll get caught and be embarrassed and that's never good. B is no good, either, because it requires you to put irrelevant information on the table, which you must then figure out how to remove during a negotiation - never easy. C is a nonstarter because you're not in a court room and while it sometimes seems like prosecutor and interviewer might be synonymous, they're not. Don't go there.
That leaves D. However common, the W-2 question is a bad one. And when the question is bad, you must either provide a bad answer or answer a different question. In this case, having done your research, you can respond confidently to the question the interviewer really intended to ask: what do we have to pay you to bring you on board? You know what a fair range is for the position under discussion and you know how well you fit into that range, so say so. If you lack one or two key components, you'll say, honestly, that you know the common range for the position, but realize you'll come in at the lower end for this reason. If you're an old pro, frame your answer that way, asking for the higher end.
Asking for too much or too little rightly disqualifies many candidates. Know and be up front about a fair wage. You're not looking to gouge anyone, and they are not planning to rip you off. A good company wants to pay you fairly, and that's all you can ask or expect.