mypy master fails in typeshed CI

[JelleZijlstra]

  File "/home/travis/virtualenv/python3.5.2/lib/python3.5/site-packages/mypy/nodes.py", line 2275, in SymbolTableNode
    def type(self) -> Optional['mypy.types.Type']:
  File "/opt/python/3.5.2/lib/python3.5/typing.py", line 649, in __getitem__
    return Union[arg, type(None)]
  File "/opt/python/3.5.2/lib/python3.5/typing.py", line 552, in __getitem__
    dict(self.__dict__), parameters, _root=True)
  File "/opt/python/3.5.2/lib/python3.5/typing.py", line 512, in __new__
    for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
  File "/opt/python/3.5.2/lib/python3.5/typing.py", line 512, in <genexpr>
    for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
  File "/opt/python/3.5.2/lib/python3.5/typing.py", line 190, in __subclasscheck__
    self._eval_type(globalns, localns)
  File "/opt/python/3.5.2/lib/python3.5/typing.py", line 177, in _eval_type
    eval(self.__forward_code__, globalns, localns),
  File "<string>", line 1, in <module>
AttributeError: module 'mypy' has no attribute 'types'

From https://travis-ci.org/python/typeshed/jobs/227972982

Presumably we need to use 'Optional[mypy.types.Type]' instead? Not sure how this wasn't caught by any of mypy's own tests.

[gvanrossum]

Just traded this back to Ivan's recent Optional cleanup. More to follow.

[ilevkivskyi]

This is definitely due to Optional['mypy.types.Type']. But this (eager union simplification using forward reference evaluation) was fixed several month ago in typing. Do we need to support also older versions of typing? If yes, then I can make a small PR replacing this with 'Optional[mypy.types.Type]'.

[gvanrossum]

Yes, we need to support whatever typing.py is shipped with Python 3.5.1. #ilevkivskyi how soon can you have this fixed? Otherwise I'll just revert #3292 (the culprit).

[ilevkivskyi]

I can do this within 30 minutes.

[ilevkivskyi]

@JelleZijlstra @gvanrossum Here is PR #3303 that should fix this. Could you please verify that the crash is fixed?

Btw, maybe we need to add tests with different versions of typing to mypy CI? It is quite easy to miss something like this in future.

[JelleZijlstra]

Thanks Ivan!

As for preventing this in the future, the bug apparently doesn't appear with mypy 3.5.1, because that's what mypy's CI uses. Typeshed's CI uses the default Travis version of 3.5, which is 3.5.2.

[gvanrossum]

the bug apparently doesn't appear with mypy 3.5.1, because that's what mypy's CI uses

Incorrect. The repro I have also uses 3.5.1. As I said above, I suspect that the bug depends on import order within mypy itself and our Travis runs use runtests.py which imports some other modules so the import order problem is avoided.

[ilevkivskyi]

I just checked when exactly Union was fixed in typing: the problem exists in both 3.5.1 and 3.5.2, but is fixed in 3.5.3 and in 3.6.1.