[PEP 695] Incorrect Variance Computation with Polymorphic Constructor.

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

Start by running the provided example in the mypy playground with the linked Python and mypy versions, then compare its behavior with the pyright playground. Investigate variance inference for the three PEP 695 class definitions and polymorphic constructors; done means mypy accepts Foo[int] where Foo[object] is expected, as it already does for Bar and Baz.