Enable py::metaclass(PyType_Type)

Author: rwgk

include/pybind11/attr.h

@@ -82,6 +82,7 @@ struct dynamic_attr {};
 struct buffer_protocol {};
 
 /// Annotation which requests that a special metaclass is created for a type
+/// NOTE: pybind11's default metaclass is not compatible with abc.ABCMeta
 struct metaclass {
     handle value;
 
@@ -90,6 +91,12 @@ struct metaclass {
 
     /// Override pybind11's default metaclass
     explicit metaclass(handle value) : value(value) {}
+
+    /// Example usage: py::metaclass(PyType_Type)
+    /// PyType_Type is recommended if compatibility with abc.ABCMeta is required.
+    /// The only potential downside is that static properties behave differently
+    //  (see pybind/pybind11#679 for background).
+    explicit metaclass(PyTypeObject &type_obj) : value(reinterpret_cast<PyObject *>(&type_obj)) {}
 };
 
 /// Specifies a custom callback with signature `void (PyHeapTypeObject*)` that

tests/test_methods_and_attributes.cpp

@@ -367,7 +367,7 @@ TEST_SUBMODULE(methods_and_attributes, m) {
 
     // test_metaclass_override
     struct MetaclassOverride {};
-    py::class_<MetaclassOverride>(m, "MetaclassOverride", py::metaclass((PyObject *) &PyType_Type))
+    py::class_<MetaclassOverride>(m, "MetaclassOverride", py::metaclass(PyType_Type))
         .def_property_readonly_static("readonly", [](const py::object &) { return 1; });
 
     // test_overload_ordering