[Attrs] Invalid typing of init arguments in subclasses of generics #5744

from typing import Generic, List, TypeVar
import attr

T_BASE_ITEM = TypeVar("T_BASE_ITEM", bound="BaseItem")


class BaseItem(object):
    pass


class ConcreteItem(BaseItem):
    pass


@attr.s
class Container(Generic[T_BASE_ITEM]):
    items = attr.ib(type=List[T_BASE_ITEM], factory=list)


@attr.s
class ConcreteContainer(Container[ConcreteItem]):
    pass


container1 = ConcreteContainer()
container1.items.append(ConcreteItem())
container1.items.extend([ConcreteItem()])
container1.items = [ConcreteItem()]

container2 = ConcreteContainer(items=[ConcreteItem()])

List item 0 has incompatible type "ConcreteItem"; expected "T_BASE_ITEM"

@attr.s
class ItemWrapper(Generic[T_BASE_ITEM]):
    item = attr.ib(type=T_BASE_ITEM)

'T_BASE_ITEM' is a type variable and only valid in type context