Adding test_classh_inheritance, currently failing (passes with class_).

tests/test_classh_inheritance.cpp

+#include "pybind11_tests.h"
+
+#include <pybind11/classh.h>
+
+namespace pybind11_tests {
+namespace classh_inheritance {
+
+struct base {
+    base() : base_id(100) {}
+    virtual ~base() = default;
+    virtual int id() const { return base_id; }
+    int base_id;
+};
+
+struct drvd : base {
+    int id() const override { return 2 * base_id; }
+};
+
+inline drvd *make_drvd() { return new drvd; }
+inline base *make_drvd_up_cast() { return new drvd; }
+
+inline int pass_base(const base *b) { return b->id(); }
+inline int pass_drvd(const drvd *d) { return d->id(); }
+
+} // namespace classh_inheritance
+} // namespace pybind11_tests
+
+PYBIND11_CLASSH_TYPE_CASTERS(pybind11_tests::classh_inheritance::base)
+PYBIND11_CLASSH_TYPE_CASTERS(pybind11_tests::classh_inheritance::drvd)
+
+namespace pybind11_tests {
+namespace classh_inheritance {
+
+TEST_SUBMODULE(classh_inheritance, m) {
+    py::classh<base>(m, "base");
+    py::classh<drvd, base>(m, "drvd");
+
+    m.def("make_drvd", make_drvd, py::return_value_policy::take_ownership);
+    m.def("make_drvd_up_cast", make_drvd_up_cast, py::return_value_policy::take_ownership);
+    m.def("pass_base", pass_base);
+    m.def("pass_drvd", pass_drvd);
+}
+
+} // namespace classh_inheritance
+} // namespace pybind11_tests

tests/test_classh_inheritance.py

+# -*- coding: utf-8 -*-
+
+from pybind11_tests import classh_inheritance as m
+
+
+def test_make_drvd_pass_base():
+    d = m.make_drvd()
+    i = m.pass_base(d)
+    assert i == 200
+
+
+def test_make_drvd_up_cast_pass_drvd():
+    b = m.make_drvd_up_cast()
+    # the base return is down-cast immediately.
+    assert b.__class__.__name__ == "drvd"
+    i = m.pass_drvd(b)
+    assert i == 200