Default values for generic attributes are unsafe

[ilevkivskyi]

Currently mypy accepts this unsafe code:

from typing import Generic, TypeVar, List
  
T = TypeVar('T')

class B(Generic[T]):
    x: List[T] = []

class C1(B[int]):
    pass

class C2(B[str]):
    pass

C1().x.append(42)
C2().x.append('Hm...')

A potential solution would be to disallow all default values for generic attributes except None. This is however a low priority, since it is a false negative, and a rare corner case.

[andersk]

Same issue as #11538.

We don’t need to go so far as disallowing all defaults other than None. Any default value is fine as long as the default value has a type that can be written without T. For example:

• x: Sequence[T] = [] is fine because [] has type Sequence[Never], which can be written without T, but is convertible to Sequence[T] by covariance. That is, a single [] may simultaneously have type Sequence[T] for all T.
• x: List[T] = [] must be disallowed; no single [] may simultaneously have type List[T] for all T.