Incorrect inference of type parameter for a generic container

[pkch]

from typing import *

T = TypeVar('T', bound='A')

class X: ...

class Y(X): ...

class A(Iterable[X]):
    def __iter__(self) -> Iterator[X]: ...

class B(A, Iterable[Y]):
    def __iter__(self) -> Iterator[Y]: ...

b = B()
for x in b:
    reveal_type(x)  # 'Y*'

y = list(b)
reveal_type(y)  # 'builtins.list[X*]'

The first reveal_type is correct.
The second is wrong: it should be builtins.list[Y*]. Somehow the presence of A in the bases of B causes this mistake.

In addition, even a workaround doesn't work:

z = List[Y](b)  # Argument 1 to "list" has incompatible type "B"; expected Iterable[Y]
reveal_type(z)  # 'builtins.list[Y*]'

The revealed type is now correct, but the constructor doesn't pass the type check.

[gvanrossum]

Yeah, for and list() are treated completely differently -- for is special-cased in mypy, essentially using structural typing, which finds the __iter__ defined in B, while list() uses nominal typing (until PEP 544 is accepted and implemented). I'm guessing that in the second case the MRO search causes mypy to misfire -- it's probably something like B, A, Iterable[X], Iterable[Y], object. Maybe the MRO calculation needs to be revised to correctly order the two Iterable[] base classes, or merge them into just Iterable[Y]?

[pkch]

The fix seems to be super simple. Unfortunately, I have trouble tweaking the test fixtures to accept the code that runs in production. Once I succeed, I'll submit a PR.