REF: dont _try_cast for user-defined functions
Also add a whatsnew note for #29641 and a comment in core.apply that ive been meaning to get in
Can you add test(s) for this? > Can you add test(s) for this?

This is turning out to be surprisingly difficult.  Which suggests its all-the-more worthwhile > Can you add test(s) for this?

So far the answer is "no".  i haven't been able to find a case that fails in master but works in this PR. How did you come across the behavior in the whatsnew? > How did you come across the behavior in the whatsnew?

Thats the whatsnew note I forgot to include in #29641. Ah OK. So not necessarily tied to the rest of the PR? > So not necessarily tied to the rest of the PR?

Right.  ditto for the comment in core.apply thanks