Incorrect error in factory function on a dataclass

[wkschwartz]

In Python 3.7.0rc1 and mypy 0.620+dev-5dc7454b1acb47dcbb3361c849e6113efcdbaaf2, if I save the following code to dc_test.py

from typing import Type, TypeVar
from dataclasses import dataclass

T = TypeVar('T', bound='A')

@dataclass
class A:
	@classmethod
	def make(cls: Type[T]) -> T:
		return cls()

I get the following mypy output

$ mypy dc_test.py 
dc_test.py:10: error: Incompatible return value type (got "A", expected "T")

If I comment out the @dataclass line, I get no error. I would expect to get no error with or without the @dataclass line.

[gvanrossum]

Confirmed. All I can think of is that something's wrong with how class decorators are implemented in mypy.

[ilevkivskyi]

The type first argument of class methods is patched by plugins for attrs and dataclasses (because __init__ is auto-generated). It looks like this patching is not done carefully (it needs to interact well with bind_self).

[wkschwartz]

Workaround until this issue is fixed

Casting the result to type T silences mypy's false positive error.

-from typing import Type, TypeVar
+from typing import Type, TypeVar, cast
 from dataclasses import dataclass

 T = TypeVar('T', bound='A')

 @dataclass
 class A:
 	@classmethod
 	def make(cls: Type[T]) -> T:
-		return cls()
+		return cast(T, cls())