Giving up on idea to use legacy init_instance only if is_base_of<type_caster_generic, type_caster<T>. There are use cases in the wild that define both a custom type_caster and class_.

Moving get_local_load_function_ptr, init_instance_for_type to smart_holder_type_caster_class_hooks.
Expanding static_assert in py::type::handle_of<> to accommodate smart_holder_type_casters.

Moving define PYBIND11_SMART_HOLDER_TYPE_CASTERS(T) down in the file. NO functional changes. Preparation for follow-up work (to keep that diff smaller).

The user-facing macro is now PYBIND11_SMART_HOLDER_TYPE_CASTERS.

https://github.com/rwgk/rwgk_tbx/commit/a2c2f88174a30f5de80d7d26e0f77c7b60f5fb85

These tests are from experimenting, and for demonstrating UB in pybind11 multiple inheritance handling ("first_base"), to be fixed later.

Removing debugging comments (GET_STACK, GET_INT_STACK). cast.h is identical to current master again, pybind11.h only has the generic_type::initialize(..., &type_caster_generic::local_load) change.

Folding convert_type into lvalue_ref and rvalue_ref paths. Some smart_holder_type_caster_load cleanup.

Adding smart_holder_type_caster_load::loaded_as_shared_ptr, currently bypassing smart_holder shared_ptr tracking completely, but the tests pass and are sanitizer clean.

Pure renaming of function and variable names, for better generalization when convoluting with inheritance. NO functional changes.

Pure renaming of function names in test_classh_inheritance, similar to the systematic approach used in test_class_wip. NO functional changes.

Removing stray isinstance<T>(src): it interferes with the py::module_local feature. Adding missing #includes.