[PEP 695] Incorrect Variance Computation with Polymorphic Constructor. #19439
Description
Bug Report
In the example below, all 3 classes should be covariant in their generic type. However, mypy seems to infer that only Bar and Baz are covariant, but not Foo. Code sample in pyright playground, mypy playground
from collections.abc import Sequence
from typing import Generic, TypeVar
class Foo[T](Sequence[T]):
@classmethod
def new[T2](cls: "type[Foo[T2]]", arg: list[T2]) -> "Foo[T2]": ...
class Bar[T](Sequence[T]):
@classmethod
def new[T2](cls, arg: list[T2]) -> "Bar[T2]": ...
_T_co = TypeVar("_T_co", covariant=True)
_S = TypeVar("_S")
class Baz(Sequence[_T_co], Generic[_T_co]):
@classmethod
def new(cls: "type[Baz[_S]]", arg: list[_S]) -> "Baz[_S]": ...
def test_foo_covariant(x: Foo[int]) -> Foo[object]:
return x # ❌ got "Foo[int]", expected "Foo[object]"
def test_bar_covariant(x: Bar[int]) -> Bar[object]:
return x
def test_baz_covariant(x: Baz[int]) -> Baz[object]:
return x