pickle with prepend/keep_alive extras

[adedner]

Hi.
First of all thanks for the great project, has simplified getting my students to use our C++ code a lot!

I recently started on adding pickling support and ran into two problems (I opened two issues):

Issue:
I have a default pickling method that is automatically registered and would like to override that in certain cases.

Description:
Adding a second def(pybind11::pickling...) does not work since the previous version is sued. Adding pybind11::prepend() leads to unexpected results - the default getstateis used together with the newsetstate. The reason is that in the pickle_factorytheextraargument is only added to thesetter. I can't find a workaround but was wondering if in this case having multiple overloads for setstate/getstatemakes sense? Possibly a newdef(pickle(...))` should remove the first one or always prepend (that was easy enough to do)?

Solution:
At least failing if prepend is used in the extra argument would probably avoid some unexpected behavior.
What worked for me was testing for pybind11:prepend in the Extra variadic template when calling pickle_factory::execute and if present adding that to the __getstate__ registration.
I can provide a MR and possibly a test with that approach if it makes sense.
But as mentioned above perhaps simply removing a previous registration would be better since I believe that overloads would not be called anyway - I'm not that far down the rabbit hole yet to implement this approach.

[gruenich]

I think your request is reasonable. I prefer the way to remove a pickling instead of having multiple overloads.