"Cannot determine type" on inner class inheritance

[mthuurne]

I run mypy on the following code, with no arguments other than the file name:

class InnerBase:
    pass

class OuterBase:
    class Inner(InnerBase):
        pass

class OuterMixin:
    class Inner(InnerBase):
        pass

class Outer(OuterBase, OuterMixin):
    pass

The output that I get is as follows:

inner.py:12: error: Cannot determine type of 'Inner' in base class 'OuterBase'
inner.py:12: error: Cannot determine type of 'Inner' in base class 'OuterMixin'

Line 12 is the line that starts with class Outer.

I don't understand why the types cannot be determined: Type[OuterBase.Inner] and Type[OuterMixin.Inner] respectively would be the types for the inner classes, I think.

Note that the errors go away when Outer inherits only from OuterBase or only from OuterMixin.

I'm using mypy 0.641 on Python 3.6.5.

[ilevkivskyi]

Duplicate of #2871

(As a side note this is still technically not type safe, this is like you would a have multiple inheritance with inner: str and inner: int in two bases.)

[mthuurne]

Sorry for opening a duplicate; neither the manual search I did nor the GitHub "related issues" beta function turned up the existing report.

This minimal test case leaves out some details that the production code uses. Even so, I think it's type-safe as long as the Outer class will only use Inner according to the InnerBase interface.