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I just noticed that I hadn't used the real "types.coroutine" in my Py3.5 tests when reporting back in issue 24017. When I pass a Cython generator through it, I get
"""
Traceback (most recent call last):
 File "tests/run/test_coroutines_pep492.pyx", line 245, in test_coroutines_pep492.CoroutineTest.test_func_5 (test_coroutines_pep492.c:13445)
 for el in bar():
 File "/opt/python3.5/lib/python3.5/types.py", line 197, in wrapped
 'non-coroutine: {!r}'.format(coro))
TypeError: callable wrapped with types.coroutine() returned non-coroutine: <generator object at 0x7f178c458898>
"""
This is actually obvious, given that the sole purpose of the decorator is to turn something that is a Generator and *not* a Coroutine into something that is a Coroutine, as a means for the user to say "but I know better". So checking for the return value being a Coroutine is wrong. Instead, it should check that it's a Generator and if it's not an Awaitable, wrap it as a self-returning Awaitable. That's more or less what my proposed implementation in issue 24017 did:
 class types_coroutine(object):
 def __init__(self, gen):
 self._gen = gen
 class as_coroutine(object):
 def __init__(self, gen):
 self._gen = gen
 self.send = gen.send
 self.throw = gen.throw
 self.close = gen.close
 def __await__(self):
 return self._gen
 def __call__(self, *args, **kwargs):
 return self.as_coroutine(self._gen(*args, **kwargs))