`iter(obj)` incompatible with `obj.__iter__()` if class generic in constrained `TypeVar`

[randolf-scholz]

This happens when using a class whose behavior depends on a constrained TypeVar. It seems that it naively uses the first overload when inferring the type of iter(obj). There's also a false positive when checking compatibility with Iterable, I guess it just tests if some overload matches?

https://mypy-play.net/?mypy=master&python=3.11&gist=bbcb8287aa58a4aceea4063c27c92e2e

from typing import (
    Generic,
    Iterable,
    Iterator,
    Literal,
    TypeAlias,
    TypeVar,
    assert_type,
    overload,
)

T = TypeVar("T")
S: TypeAlias = Literal["slice"]
W: TypeAlias = Literal["window"]
Mode = TypeVar("Mode", S, W)


class Sliding(Generic[T, Mode]):
    data: list[T]
    size: int
    mode: Mode

    @overload
    def __init__(self: "Sliding[T, S]", data: list[T], size: int, mode: S) -> None: ...
    @overload
    def __init__(self: "Sliding[T, W]", data: list[T], size: int, mode: W) -> None: ...
    def __init__(self, data, size, mode):
        self.data = data
        self.size = size
        self.mode = mode

    @overload
    def __iter__(self: "Sliding[T, S]") -> Iterator[slice]: ...
    @overload
    def __iter__(self: "Sliding[T, W]") -> Iterator[list[T]]: ...
    def __iter__(self):
        match self.mode:
            case "slice":
                for i in range(len(self.data) - self.size + 1):
                    yield slice(i, i + self.size)
            case "window":
                for i in range(len(self.data) - self.size + 1):
                    yield self.data[i : i + self.size]
            case _:
                raise TypeError(f"Unknown mode: {self.mode}")


x = Sliding([1, 2, 3, 4, 5], 3, mode="slice")
assert_type(x, Sliding[int, Literal["slice"]])  # ✅
assert_type(x.__iter__(), Iterator[slice])  # ✅
assert_type(iter(x), Iterator[slice])  # ✅
assert_type(list(x), list[slice])  # ✅
x1: Iterable[slice] = x  # ✅
x2: Iterable[list[int]] = x  # ❌ false positive


y = Sliding([1, 2, 3, 4, 5], 3, mode="window")
assert_type(y, Sliding[int, Literal["window"]])  #  ✅
assert_type(y.__iter__(), Iterator[list[int]])  #  ✅
assert_type(iter(y), Iterator[list[int]])  #  ❌ Iterator[slice] (false negative)
assert_type(list(y), list[list[int]])  #  ❌ list[slice]  (false negative)
y1: Iterable[list[int]] = y  # ❌ false positive
y2: Iterable[slice] = y  # ✅

[randolf-scholz]

For reference, pyright is fine with all the assert_type statements and catches the false positives, but complains about "Overloaded implementation is not consistent with signature"...

[randolf-scholz]

Seems this was fixes in 1.12, closing. https://mypy-play.net/?mypy=1.12.0&python=3.13&gist=16484625f51cb1bc0145c60b983369b1