@wataruu @Crime @bylin Huh, I don't get this judgement at all. The claim is that "which students left?" is a question that can be felicitously asked by a speaker who deems it possible that no students left???
Whatever the facts turn out to be, I have some work with Uli and Andreea on plural vs. singular simplex wh-phrases which might be relevant. We found that plural but not singular "who" carries an existence presupposition in languages with a morphological distinction.
@bylin @patrl @wataruu better to use a third person subject with negation, I believe those were the test cases: "John doesn't know which students came" vs "John doesnt know which student came". If you find the contrast there you may find it in the case of actual questions, so being agnostic whether any students came is still a possible case. At least that's how I think it went; the discussion started with presupposition of "care": "John doesnt care which students left" def has no presupposition