Mypy reports incompatible types for an optional dataclass field with generic default factory

[sagittarian]

Bug Report

In a dataclass with an optional field with a generic as the default factory, mypy reports that the types are incompatible when there should be no compatibility issue between them.

from typing import List, Optional
from dataclasses import dataclass, field


@dataclass
class A:
    x: Optional[List[int]] = field(default_factory=list)

Expected Behavior

A bare list is compatible with Optional[List[int]], so no error should be reported.

Actual Behavior

$ mypy ./optlist.py 
optlist.py:7: error: Incompatible types in assignment (expression has type "List[_T]", variable has type "Optional[List[int]]")
Found 1 error in 1 file (checked 1 source file)

Your Environment

$ cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04.3 LTS"
$ python --version
Python 3.8.10
$ mypy --version
mypy 0.930

[yoann9344]

def optional_list() -> list | None:
    return list()


@dataclass
class A:
    x: Optional[List[int]] = field(default_factory=optional_list)

Or even more convenient :

from dataclasses import dataclass, field
from typing import List, Optional, TypeVar, Callable


T = TypeVar("T")


def optional(type: type[T]) -> Callable[[], T | None]:
    def factory() -> T | None:
        return type()

    return factory


@dataclass
class A:
    x: Optional[List[int]] = field(default_factory=optional(list))

[Pycz]

Same problem with the default dict factory, which should create an empty dictionary.

from typing import Dict, Any
from dataclasses import dataclass, field


@dataclasses.dataclass
class A:
    data: Dict[str, Any] = dataclasses.field(default_factory=dict)

Error text

error: Incompatible types in assignment (expression has type "Dict[_KT, _VT]", variable has type "Dict[str, Any]")

[Pycz]

Another workaround:

from typing import Dict, Any, cast
from dataclasses import dataclass, field


@dataclasses.dataclass
class A:
    data: Dict[str, Any] = dataclasses.field(default_factory=cast(Dict[str, Any], dict))

[lteich]

I think the workaround above has a mistake in it - it tries to cast a callable into a dictionary. The correct one is:

from typing import Dict, List, Optional, Tuple, Any, cast, Callable
from dataclasses import dataclass, field

@dataclass
class A:
    data: Dict[str, Any] = field(default_factory=cast(Callable[[], Dict[str, Any]], dict))

[ilevkivskyi]

The original examples passes without errors on current master.