Minimal test covering classh_type_casters load_impl Case 2b.

include/pybind11/detail/classh_type_casters.h

@@ -167,7 +167,6 @@ public:
             // we can find an exact match (or, for a simple C++ type, an inherited match); if so, we
             // can safely reinterpret_cast to the relevant pointer.
             else if (bases.size() > 1) {
-                pybind11_fail("classh_type_casters: Case 2b UNTESTED.");
                 for (auto base : bases) {
                     if (no_cpp_mi ? PyType_IsSubtype(base->type, typeinfo->type) : base->type == typeinfo->type) {
                         this_.load_value_and_holder(reinterpret_cast<instance *>(src.ptr())->get_value_and_holder(base));

tests/test_classh_inheritance.cpp

@@ -67,8 +67,8 @@ TEST_SUBMODULE(classh_inheritance, m) {
     m.def("pass_base", pass_base);
     m.def("pass_drvd", pass_drvd);
 
-    py::classh<base1>(m, "base1");
-    py::classh<base2>(m, "base2");
+    py::classh<base1>(m, "base1").def(py::init<>()); // __init__ needed for Python inheritance.
+    py::classh<base2>(m, "base2").def(py::init<>());
     py::classh<drvd2, base1, base2>(m, "drvd2");
 
     m.def("make_drvd2", make_drvd2, py::return_value_policy::take_ownership);

tests/test_classh_inheritance.py

@@ -34,3 +34,16 @@ def test_make_drvd2_up_casts_pass_drvd2():
     assert b2.__class__.__name__ == "drvd2"
     i2 = m.pass_drvd2(b2)
     assert i2 == 3 * 110 + 4 * 120 + 23
+
+
+def test_python_drvd2():
+    class Drvd2(m.base1, m.base2):
+        def __init__(self):
+            m.base1.__init__(self)
+            m.base2.__init__(self)
+
+    d = Drvd2()
+    i1 = m.pass_base1(d)  # load_impl Case 2b
+    assert i1 == 110 + 21
+    i2 = m.pass_base2(d)
+    assert i2 == 120 + 22