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I'm not sure where exactly the error lies, but issue 27128 broke iter() for Argument Clinic class methods. The following works in Python 3.5, but not in Python 3.6:
from datetime import datetime
from asyncio import Task
next(iter(datetime.now, None))
next(iter(Task.all_tasks, None))
In 3.6 StopIteration is raised:
>>> next(iter(datetime.now, None))
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration
>>> next(iter(Task.all_tasks, None))
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration
(In 3.5 a `datetime.datetime` and `set` object are produced, respectively)
The only thing these two methods have in common is that they are class methods with no arguments, parsed out by the Argument Clinic generated code (so using _PyArg_Parser).
What appears to have changed is that iter() was switched from using PyObject_Call to _PyObject_FastCall, see https://github.com/python/cpython/commit/99ee9c70a73ec2f3db68785821a9f2867c3f637f 

Forgot to addthis: this bug was found via https://stackoverflow.com/questions/44283540/iter-not-working-with-datetime-now 

It works in 3.5 and 3.7, but raises a StopIteration in 3.6.

Oh, it's my fault: it's a bug coming from FASTCALL optimizations. The strange thing is that the bug wasn't catched by the giant Python test suite :-(
I knew that _PyStack_UnpackDict() has a bug in Python 3.6, but I completely forgot to fix it :-(
https://github.com/python/cpython/pull/1886 fixes the bug, but has not test yet.

next(iter(datetime.now, None)) can be turned into a nice test.

New changeset f0ff849adc6b4a01f9d1f08d9ad0f1511ff84541 by Victor Stinner in branch '3.6':
bpo-30524: Fix _PyStack_UnpackDict() (#1886)
https://github.com/python/cpython/commit/f0ff849adc6b4a01f9d1f08d9ad0f1511ff84541

New changeset 3b5cf85edc188345668f987c824a2acb338a7816 by Victor Stinner in branch 'master':
bpo-30524: Write unit tests for FASTCALL (#2022)
https://github.com/python/cpython/commit/3b5cf85edc188345668f987c824a2acb338a7816

New changeset b7577456c430283f8b7ec4e914b701cb943cc69b by Victor Stinner in branch '3.6':
bpo-30524: Write unit tests for FASTCALL (#2022) (#2030)
https://github.com/python/cpython/commit/b7577456c430283f8b7ec4e914b701cb943cc69b

Sorry for the regression, sadly, it wasn't catch before by any test. I added a lot of new tests, so we should cover more cases. Oh, and the bug has been fixed in 3.6 :-)

bpo-30473 has been marked as a duplicate of this bug.