[Python-Dev] PEP 590: vectorcall without tp_call

Hello, I have one implementation question about vectorcall which is not specified in PEP 590: what should happen if a type implements vectorcall (i.e. _Py_TPFLAGS_HAVE_VECTORCALL is set) but doesn't set tp_call (i.e. tp_call == NULL)? I see the following possibilities: 1. Ignore this problem/assume that it won't happen. This would be bad, since callable(obj) would be False even though obj() would succeed. 2. Raise SystemError. 3. Automatically set tp_call to PyVectorcall_Call. I would vote for 3 since it's the most user-friendly option. There is also no way how it could be wrong: it ensures that tp_call and vectorcall are consistent. Any opinions? Jeroen.