Demonstrating that py::cast<std::unique_ptr<Foo>>(foo_py); works with py::classh (it does not build with class_).

Introducing PYBIND11_USE_SMART_HOLDER_AS_DEFAULT macro (tested only undefined; there are many errors with the macro defined).

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.