Incorrect inference of type parameter for a generic container

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

Start by running the supplied minimal reproduction through mypy and compare the two reveal_type results. Trace generic Iterable inference for B and list's constructor checking, then add regression coverage for the expected list[Y*] result and for accepting B as Iterable[Y].

Written by the indexing model from the issue text.

Description

bug 
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.

Dominant language
Python
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Merged PRs (30d)
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