bpo-36974: Make tp_call=PyVectorcall_Call work for inherited types (GH-13699)

When inheriting a heap subclass from a vectorcall class that sets
`.tp_call=PyVectorcall_Call` (as recommended in PEP 590), the subclass does
not inherit `_Py_TPFLAGS_HAVE_VECTORCALL`, and thus `PyVectorcall_Call` does
not work for it.

This attempts to solve the issue by:
* always inheriting `tp_vectorcall_offset` unless `tp_call` is overridden
  in the subclass
* inheriting _Py_TPFLAGS_HAVE_VECTORCALL for static types, unless `tp_call`
  is overridden
* making `PyVectorcall_Call` ignore `_Py_TPFLAGS_HAVE_VECTORCALL`

This means it'll be ever more important to only call `PyVectorcall_Call`
on classes that support vectorcall. In `PyVectorcall_Call`'s intended role
as `tp_call` filler, that's not a problem.

1 files changed, 1 insertions, 1 deletions

diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index bf20e81..eed34c9 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -5854,7 +5854,7 @@ MethodDescriptor_vectorcall(PyObject *callable, PyObject *const *args,
 static PyObject *
 MethodDescriptor_new(PyTypeObject* type, PyObject* args, PyObject *kw)
 {
-    MethodDescriptorObject *op = PyObject_New(MethodDescriptorObject, type);
+    MethodDescriptorObject *op = type->tp_alloc(type, 0);
     op->vectorcall = MethodDescriptor_vectorcall;
     return (PyObject *)op;
 }