When Py_TPFLAGS_BASETYPE and Py_TPFLAGS_DISALLOW_INSTANTIATION flags are set on extension type tp_flags field, attempt to create type subclass instance fails:

>>> import _foo
>>> class Bar(_foo.Foo): pass
... 
>>> Bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create 'Bar' instances

According to documentation, it shouldn't happen:
https://docs.python.org/3/c-api/typeobj.html#Py_TPFLAGS_DISALLOW_INSTANTIATION

Extension code example:
_foo.c.txt

Tested on 3.10.3 and 3.12.0a0.

• PR: gh-96746: Docs: Clear up Py_TPFLAGS_DISALLOW_INSTANTIATION inheritance #99002

• PR: [3.11] gh-96746: Docs: Clear up Py_TPFLAGS_DISALLOW_INSTANTIATION inheritance (GH-99002) #99212

• PR: [3.10] gh-96746: Docs: Clear up Py_TPFLAGS_DISALLOW_INSTANTIATION inheritance (GH-99002) #99213