Inheritance breaks mypy type checking with optional attributes if Generics are involved

Inheritance breaks mypy type checking with optional attributes if Generics are involved:

from typing import Generic, Optional, TypeVar

import attr

T = TypeVar("T")


@attr.mutable()
class Parent(Generic[T]): # Removing `Generic` here and `[float]` below doesn't show `mypy` issues anymore
    run_type: Optional[int] = None


@attr.mutable()
class Child(Parent[float]):
    pass


Parent(
    run_type =None,
)

Child(
    run_type =None, # error: Argument "run_type" to "Child" has incompatible type "None"; expected "int"  [arg-type]
)

There is an error displayed which is wrong. Doing the same without any Generic involved works as expected.

Not quite sure if this is a mypy issue or an attrs typing issue.

Start by running the supplied Python reproducer with mypy and compare the Parent and Child constructor diagnostics. Trace whether the incorrect inference comes from attrs-generated typing or mypy's generic inheritance handling; done means the reproducer accepts Child(run_type=None) without regressing the non-generic case.