Mypy: `issubclass()` doesn't narrow down types of type variables

[ambv]

Example:

from typing import *

TypeT = TypeVar("TypeT", bound=type)

class Base:
    field: str = "Hey"

class C1:
    def method(self, other: type) -> str:
        if issubclass(other, Base):
            # reveal_type(other) == Type[Base]
            return other.field
        return "Hi"

class C2(Generic[TypeT]):
    def method(self, other: TypeT) -> str:
        if issubclass(other, Base):
            # reveal_type(other) == TypeT`1
            return other.field  # mypy error: "TypeT" has no attribute "field"
        return "Hi"

Actual behavior as of 0.740 included in the comments above. Expected behavior would be to not raise an error on the penultimate line.

[ilevkivskyi]

Looks like an issue in checker.py (the logic around issubclass() is pretty ad-hoc there). One needs to add an elif for type variables in find_isinstance_check() there.

This might potentially need also an adjustment in meet.py either in narrow_declared_type() or in is_overlapping_types() or in meet_types().

[TH3CHARLie]

@ilevkivskyi I'd like to take this

[ilevkivskyi]

I'd like to take this

OK, go ahead.