Support for subclass of a metaclassed class without passing the metaclass

[sergeyy32]

Currently, we have a class registered with a metaclass:

struct MetaclassOverride { };
py::class_<MetaclassOverride>(m, "MetaclassOverride", py::metaclass((PyObject *) &PyType_Type)); // or any custom metaclass

We want to register a subclass of this class without explicitly specifying a metaclass

struct ChildClass : MetaclassOverride { };
py::class_<ChildClass, MetaclassOverride>(m, "ChildClass");

What Python returns with this registration:

type(MetaclassOverride)  # returns 'type' or custom metaclass
type(ChildClass)         # returns the built-in pybind11 type

In standard Python, when a subclass is registered without specifying a metaclass, it would automatically inherit the parent’s metaclass. In the current pybind case, however, the subclass gets a different type, which would normally be considered a metaclass conflict in Python.

Do I understand correctly that we would need to carry the metaclass together with the base class everywhere in order to always register subclasses with the same metaclass, or am I mistaken? In general, I wouldn’t want to have to carry the metaclass throughout all my code and be aware of it. I’m not sure whether to treat the current case as a bug, a proposed improvement, or just a feature of how it works.