static int
 CType_Type_traverse(PyObject *self, visitproc visit, void *arg)
 {
+    ctypes_state *st = GLOBAL_STATE();
+    StgInfo *info;
+    if (PyStgInfo_FromType(st, self, &info) < 0) {
+        return -1;
+    }
+    if (info) {
+        Py_VISIT(info->proto);    // GC-related
+        Py_VISIT(info->argtypes);
+        Py_VISIT(info->converters);
+        Py_VISIT(info->restype);
+        Py_VISIT(info->checker);
+    }
     Py_VISIT(Py_TYPE(self));
-    return 0;
+    return PyType_Type.tp_traverse(self, visit, arg);
 }

 static int
 CType_Type_clear(PyObject *self)
 {
     ctypes_state *st = GLOBAL_STATE();
     StgInfo *info;
     if (PyStgInfo_FromType(st, self, &info) < 0) {
         return -1;
     }
     if (info) {
         Py_CLEAR(info->proto);    // GC-related
         Py_CLEAR(info->argtypes);
         Py_CLEAR(info->converters);
         Py_CLEAR(info->restype);
         Py_CLEAR(info->checker);
     }
-    return 0;
+    return PyType_Type.tp_clear(self);
 }

 static PyType_Slot foo_type_slots[] = {
     ...
-    {Py_tp_traverse, CDataType_traverse},
-    {Py_tp_clear, CDataType_clear},
     ...
 };

 PyType_Spec foo_type_spec = {
     ...
-    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC |
+    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE |
     ...
 };