```
>>> class zero(object):
...     def __index__(self):
...         return 0
... 
>>> z = zero()
>>> import re
>>> p = re.compile('(a)b')
>>> m = p.match('ab')
>>> m.group(0)
'ab'
>>> m.group(z)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: no such group
```

This looks as new feature and can go only in 3.6.

I would still argue that it's a bug. The intention of PEP 357 is that __index__ should be used whenever some object needs to be converted to a Py_ssize_t, which is exactly what you do here.

It would be a bug if it was supported but gave the wrong result.

It has never been supported (the re module predates PEP 357), so it's a new feature.

Attach a patch to add this feature to Py3.6.

What is the use case?

My use case is SageMath: http://trac.sagemath.org/ticket/20750

Interesting. This is very unusual but reasonable use case.

New changeset 0303ab246152 by Serhiy Storchaka in branch 'default':
Issue #27177: Match objects in the re module now support index-like objects
https://hg.python.org/cpython/rev/0303ab246152