Import cycle with type aliasing causes invalid "Invalid type"

[ethanhs]

Running stub checks with typeshed at fb92ee8 on master yielded the following:

[ethanhs@towerofart mypy]$ pytest -k testsamples
============================= test session starts ==============================
platform linux -- Python 3.6.5, pytest-3.5.1, py-1.5.3, pluggy-0.6.0
rootdir: /home/ethanhs/mypy, inifile: pytest.ini
plugins: xdist-1.22.2, forked-0.2, cov-2.5.1
gw0 [2] / gw1 [2] / gw2 [2] / gw3 [2] / gw4 [2] / gw5 [2] / gw6 [2] / gw7 [2]
scheduling tests via LoadScheduling
.F                                                                       [100%]
=================================== FAILURES ===================================
___________________________ SamplesSuite.test_stubs ____________________________
[gw2] linux -- Python 3.6.5 /usr/bin/python
Sample check failed
----------------------------- Captured stdout call -----------------------------
typeshed/third_party/2and3/click/core.pyi:337: error: Invalid type "click.types._ConvertibleType"
typeshed/third_party/2and3/click/core.pyi:416: error: Invalid type "click.types._ConvertibleType"
===================== 1 failed, 1 passed in 13.15 seconds ======================

Ivan pointed out there is an import cycle, and it seems the cycle and type aliasing is giving mypy trouble.

[ethanhs]

Okay I have a minimal repro:

# a.pyi
from b import Parameter

class _ParamType:
    p: Parameter

_ConvertibleType = _ParamType

def convert_type(ty: _ConvertibleType):
    ...

# b.pyi
from a import _ConvertibleType

class Parameter:
    type: _ConvertibleType  # error: Invalid type "a._ConvertibleType

running mypy a.pyi (0.610 and master).

[ilevkivskyi]

Great! It is much easier to fix when there is a repro. I raise priority to high since the problems with import cycles can be quite annoying.