Dustin Spicuzza

Added the gil release, and a test inspired by #2844

This has been updated to only use the special deleter if an alias class is used. I found a place to store the alias indicator that isn't currently covered by...

@cdyson37 yep, this solves the same problem yours does, but takes care of only doing this for aliased shared_ptrs.

> I think the suggested changelog is a bit unclear. It doesn't mention alias classes. Thanks for catching that, I'll update the changelog entry. The alias class thing was added...

Already did that :) https://github.com/pybind/pybind11/pull/2845

Rebased 2845 and this on master to make it easier.

> GIL is global to the whole process and could effectively turn someone's multithreaded code into effectively single threaded. You could use this argument to justify not having trampoline classes....

> Trampolines are very explicit and boilerplate heavy, even with all the macros. You do have to opt-in. It's the implicitness of this solution that doesn't sit right with me....

I've tweaked it slightly and added an additional test for the new conditions. Now, the keep alive feature will only be applied IFF: * Object type has an associated alias...

One of my users has reported that this fix isn't quite there yet, they ran into an issue today where the python object disappeared again -- I wonder if the...