Hello, i'm trying to fix a bug in django type stubs, and i believe i've stubled upon bug in mypy.

Django has following syntax for defining foreign keys:

class Book(models.Model):
    author = models.ForeignKey(Author, null=True)

This means author should be Author | None.

Right now django-types are using @override for __init__ with differently typed self parameter:

https://github.com/sbdchd/django-types/blob/90fc353b7d05ec600a4807d8b29343a6538692b1/django-stubs/db/models/fields/related.pyi#L293

@overload
def __init__(
        self: ForeignKey[_M],
       ...
       null: Literal[False],
...
@overload
def __init__(
        self: ForeignKey[Optional[_M]],
        ...
       null: Literal[True],
...

And it works for mypy, but doesn't work for pyright. In mypy it works becouse it uses undocumented feature of typing self, as i discovered here:

• Invalid typing for __init__ in Generic classes microsoft/pyright#2909 (comment)

Proper, and most concise way of doing this, would be overloading __new__ with args/kwargs and bool: Literal[...], and leaving full annotation with all arguments typed in __init__ (which i attempted here sbdchd/django-types#86).

But, when we try to do this: mypy would ignore overrides in __new__ and will always use non-optional case.

To Reproduce

To reproduce this without much clutter, we can use following code:

# related_field_minimal.pyi
from typing import Generic, Literal, Optional, Type, TypeVar, overload

class Model: ...

_M = TypeVar("_M", bound=Optional[Model])

class ForeignKey(Generic[_M]):
    @overload
    def __new__(
        cls,
        to: Type[_M],
        null: Literal[False] = ...,
    ) -> ForeignKey[_M]: ...
    @overload
    def __new__(
        cls,
        to: Type[_M],
        null: Literal[True],
    ) -> ForeignKey[Optional[_M]]: ...
    def __init__(self, to: Type[_M], null: bool) -> None: ...

# models.py
from typing import Optional
from related_field_minimal import ForeignKey, Model


class Test(Model):
    pass


class BookModel(Model):
    required_field = ForeignKey(to=Test, null=False)
    optional_f1 = ForeignKey(to=Test, null=True)
    optional_f2 = ForeignKey[Test](to=Test, null=True)
    optional_f3 = ForeignKey[Optional[Test]](to=Test, null=True)

reveal_type(BookModel.required_field)
reveal_type(BookModel.optional_f1)
reveal_type(BookModel.optional_f2)
reveal_type(BookModel.optional_f3)

When there is no __init__ in ForeignKey, inferred types are correct, both in mypy and pyright.

models.py:15: note: Revealed type is "related_field_minimal.ForeignKey[models.Test]"
models.py:16: note: Revealed type is "related_field_minimal.ForeignKey[Union[models.Test, None]]"
models.py:17: note: Revealed type is "related_field_minimal.ForeignKey[Union[models.Test, None]]"
models.py:18: note: Revealed type is "related_field_minimal.ForeignKey[Union[models.Test, None]]"

As soon as we add __init__ to our type definition in related_field_minimal.pyi, it becomes incorrect in mypy:

models.py:15: note: Revealed type is "related_field_minimal.ForeignKey[models.Test]"
models.py:16: note: Revealed type is "related_field_minimal.ForeignKey[models.Test]"
models.py:17: note: Revealed type is "related_field_minimal.ForeignKey[models.Test]"
models.py:18: note: Revealed type is "related_field_minimal.ForeignKey[Union[models.Test, None]]"

(pyright still handles this case correctly).

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10