In every interview I've been in during the last few decades, I've been asked (in addition to this question) what I'm currently making "in order for us to form an offer". I've tried explaining that I believe my compensation should reflect my skill levels and experience based on the current market, and not my current salary (since that was the sole reason for leaving some of my previous jobs [0]). Even when I tell them that I'm not allowed (by my current employer) to divulge that information (which was true on some cases), they were adamant that they cannot make me an offer or even "we cannot continue this interview" until I disclose my current salary. I've never walked out of an interview in a huff as I don't see that as being professional, but I've also never been able to get past this question - even though I question the ethics of asking it.

[0]: One particular company I worked for had an a cap on what percentage raise you could receive, even when changing to completely different positions in different departments. There was as running joke that "the only way to make money here is to leave and come back". Once I left and realized just how horrifically underpaid I was there, I saw no reason to return.