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inherit_slots() uses:
 /* Inherit tp_vectorcall_offset only if tp_call is not overridden */
 if (!type->tp_call) {
 COPYSLOT(tp_vectorcall_offset);
 }
PyType_Ready() contains the following assertion:
 /* Consistency checks for PEP 590:
 * - Py_TPFLAGS_METHOD_DESCRIPTOR requires tp_descr_get
 * - _Py_TPFLAGS_HAVE_VECTORCALL requires tp_call and
 * tp_vectorcall_offset > 0
 * To avoid mistakes, we require this before inheriting.
 */
 if (type->tp_flags & Py_TPFLAGS_METHOD_DESCRIPTOR) {
 _PyObject_ASSERT((PyObject *)type, type->tp_descr_get != NULL);
 }
 if (type->tp_flags & _Py_TPFLAGS_HAVE_VECTORCALL) {
 _PyObject_ASSERT((PyObject *)type, type->tp_vectorcall_offset > 0);
 _PyObject_ASSERT((PyObject *)type, type->tp_call != NULL);
 }
I understand that tp_vectorcall_offset=0 is fine and the expected value for a type which doesn't have the flag _Py_TPFLAGS_HAVE_VECTORCALL.