Demo:

>>> from abc import ABC
>>> issubclass(1, ABC)
Segmentation fault (core dumped)

The stack trace is attached.

Before reimplementation of abc in C, the result was confusing too:

Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:54:40) [MSC v.1900 64 bit (AMD64)]
 on win32
>>> from abc import ABC
>>> issubclass(1, ABC)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "abc.py", line 230, in __subclasscheck__
  File "_weakrefset.py", line 84, in add
TypeError: cannot create weak reference to 'int' object

In debug mode, the following assertion fails:

python: ./Modules/_abc.c:642: _abc__abc_subclasscheck_impl: Assertion `((((((PyObject*)(mro))->ob_type))->tp_flags & ((1UL << 26))) != 0)' failed.

Is there any sense in accepting non-types as the first argument of issubclass()? I would add a check just in issubclass().

> Is there any sense in accepting non-types as the first argument of issubclass()?

No, though it is not (clearly) documented. The docs mention TypeError, but only for the second argument if my reading is correct.

In practice, issubclass() raises a TypeError if the first argument is not a class object:

>>> issubclass(1, int)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: issubclass() arg 1 must be a class

Though, as I mentioned above, behavior for ABCs was always weird.

I've also checked that ABC.register() doesn't allow non-classes (and PEP 3119 mentions that).

Looking at PyObject_IsSubclass in Objects/abstract.c, the only case in which its check_class() could be avoided is if there is a custom __subclasscheck__:

>>> class M(type):
...   def __subclasscheck__(cls, c):
...     return c == 1 or super().__subclasscheck__(c)
...
>>> class A(metaclass=M):
...   pass
...
>>> issubclass(1, A)
True

If there is no need to support such weird __subclasscheck__, check_class() could be called earlier.

Note, however, that check_class() treats anything having __bases__ as a class, so moving the check alone is not enough to avoid the crash in all cases.

Actually, the behaviour when __suclasscheck__ returns True for non-class objects may be used by some code. Even typing module did this, I tried to remove as much as possible of this, but I think there may be few such situations left.

Therefore, the patch by Inada-san (that makes C behaviour follow Python behaviour) looks reasonable to me.

OK, making a new implementation behave as the old one is fine with me too.

BTW, do TypeErrors related to weak references deserve any treatment? Isn't it a kind of coincidence that the error raised due to usage of WeakSet in issubclass(obj, ABC) is what we expect? (Sorry, I'm not familiar with WeakSets).

Could you please show examples with __suclasscheck__ returning True for non-class objects?

> BTW, do TypeErrors related to weak references deserve any treatment? Isn't it a kind of coincidence that the error raised due to usage of WeakSet in issubclass(obj, ABC) is what we expect? (Sorry, I'm not familiar with WeakSets).

Sorry, I can't get what is your point.
I don't want to change ABC behavior for now.  I want to make C implementation consistent with Python implementation, except some (unrealistic) corner cases.

New changeset fc7df0e664198cb05cafd972f190a18ca422989c by INADA Naoki in branch 'master':
bpo-32999: Fix ABC.__subclasscheck__ crash (GH-6002)
https://github.com/python/cpython/commit/fc7df0e664198cb05cafd972f190a18ca422989c

New changeset d824b4e4afbe9cf02310e39b14402fb2aa271f8f by Miss Islington (bot) in branch '3.7':
bpo-32999: Fix ABC.__subclasscheck__ crash (GH-6002)
https://github.com/python/cpython/commit/d824b4e4afbe9cf02310e39b14402fb2aa271f8f

@inada.naoki: I don't question your change. My point is the same as in #33018 (I've discovered that PR only after I commented). The error message is misleading, and it's just a coincidence that a TypeError and not some other error is raised when abc attempts to add a non-type object to a WeakSet.

@serhiy.storchaka: Note that an example that you requested is unlikely to be related to ABC and probably is more like my artificial __subclasscheck__ example. So, making changes just for ABC as suggested in #33018 might make sense.

Sorry for status update, this was due to a concurrent modification.

Serhiy, for example `issubclass(typing.MutableMapping, typing.Mapping)` returns `True` while neither of those two are actual class objects. These relationships are kept mostly so that `typing.*` can be used as a drop-in replacement for `collections.abc.*` (plus backwards compatibility).

New changeset f757b72b2524ce3451d2269f0b8a9f0593a7b27f by INADA Naoki in branch 'master':
bpo-32999: Revert GH-6002 (fc7df0e6) (GH-6189)
https://github.com/python/cpython/commit/f757b72b2524ce3451d2269f0b8a9f0593a7b27f

New changeset 5d8bb5d07be2a9205e7059090f0ac5360d36b217 by Ivan Levkivskyi (Miss Islington (bot)) in branch '3.7':
bpo-32999: Revert GH-6002 (fc7df0e6) (GH-6189) (GH-6190)
https://github.com/python/cpython/commit/5d8bb5d07be2a9205e7059090f0ac5360d36b217

New changeset c65bf3fe4a2bde424b79e350f36b7aaa3f6476f6 by INADA Naoki in branch 'master':
bpo-32999: ast: Convert useless check to assert (GH-6197)
https://github.com/python/cpython/commit/c65bf3fe4a2bde424b79e350f36b7aaa3f6476f6

New changeset c71edab15d023360388da8360700d419b5f48c81 by Miss Islington (bot) in branch '3.7':
bpo-32999: ast: Convert useless check to assert (GH-6197)
https://github.com/python/cpython/commit/c71edab15d023360388da8360700d419b5f48c81