Mypy errors out with attrs, generics and inheritance #15658

Bug Report

When checking python code that uses attrs dataclasses, Generic classes and inheritance, mypy fails to infer the actual type of a TypeVar.

Swapping attrs for std dataclasses removes the error.

Simplifying inheritance chain also removes the error.

I suspect this might be an issue in the mypy-attr plugin.

To Reproduce
can't put a playground link since it doesn't support attrs sorry.

import abc
from typing import Generic, TypeVar

from attr import frozen


# define a hierarchy of State types
class BaseState(metaclass=abc.ABCMeta):
    ...


T_State = TypeVar("T_State", bound=BaseState, covariant=True)


@frozen()
class JumpState(BaseState):
    start: int


# define a hierarchy of signals


@frozen()
class AbstractBaseSignal(metaclass=abc.ABCMeta):
    pass


T_Signal = TypeVar("T_Signal", bound=AbstractBaseSignal)


@frozen()
class SignalA(Generic[T_State], AbstractBaseSignal, metaclass=abc.ABCMeta):
    state: T_State


@frozen()
class SignalAA(SignalA[T_State], metaclass=abc.ABCMeta):
    pass


# this signal specializes the generic type to a well defined JumpState type
class SignalAAA(SignalAA[JumpState]):
    pass


def foo(s: SignalAAA) -> None:
    reveal_type(s)  # SignalAAA
    reveal_type(s.state)  # T_State instead of JumpState
    reveal_type(s.state.start)  # start is not found on T_State

if you swap all the frozen decorators for dataclass it suddendly works, s.state is correctly inferred as a JumpState.

if you remove the class SignalAA and have SignalAAA inherit directly from SignalA (i.e. short-circuit the inheritance chain) it works correctly as well.

Expected Behavior

I would expect this code to typecheck, especially since it does with standard dataclasses. I suspect a problem with the mypy-attrs plugin.

Actual Behavior

mypy output on this example:

type_test.py:47: note: Revealed type is "type_test.SignalAAA"
type_test.py:49: note: Revealed type is "T_State`1"
type_test.py:50: error: "T_State" has no attribute "start"  [attr-defined]
type_test.py:50: note: Revealed type is "Any"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: issue confirmed at least starting from 0.991 to latest version. It was working fine with version 0.910.
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
python_version = 3.9
warn_unused_configs = True
warn_unused_ignores = True
warn_return_any = True
disallow_untyped_defs = True
disallow_incomplete_defs = True
follow_imports = silent
ignore_missing_imports = True

• Python version used: 3.9
• attrs package version: 23.1.0 (bug reproduced in 22.1.0 as well)

The bug was observed first while updating our mypy version from 0.910 to 0.991.