[PEP 695] Incorrect Variance Computation with Polymorphic Constructor.

[randolf-scholz]

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

[randolf-scholz]

Simpler repro:

class Foo[T]:
    def new[S](self: "Foo[S]", arg: list[S]) -> "Foo[S]": ...

x: Foo[object] = Foo[int]()

[randolf-scholz]

In

mypy/mypy/expandtype.py

Lines 415 to 420 in eb07c06

	expanded = t.copy_modified(
	arg_types=arg_types,
	ret_type=t.ret_type.accept(self),
	type_guard=(t.type_guard.accept(self) if t.type_guard is not None else None),
	type_is=(t.type_is.accept(self) if t.type_is is not None else None),
	)

def [S] (self: tmp_d.Foo[S`-1], arg: builtins.list[S`-1]) -> tmp_d.Foo[S`-1]

is converted to

def [S] (self: tmp_d.Foo[T`1], arg: builtins.list[T`1]) -> tmp_d.Foo[T`1]

which then causes mypy to infer T as INVARIANT.