Open question, with respect to the original code:
https://github.com/pybind/pybind11/blob/76a160070b369f8d82b945c97924227e8b835c94/include/pybind11/pybind11.h#L1510
To me it looks like the exact situation marked as `std::shared_ptr<Good> gp1 = not_so_good.getptr();` here: https://en.cppreference.com/w/cpp/memory/enable_shared_from_this
The comment there is: `// undefined behavior (until C++17) and std::bad_weak_ptr thrown (since C++17)`
Does the existing code have UB pre C++17?

I'll leave handling of enable_shared_from_this for later, as the need arises.