Base class attribute type does not provide type context in subclass

[JukkaL]

In this example there is an unnecessary message about needing a type annotation:

from typing import List

class A:
    x: List[str]

class B(A):
    x = []  # Need type annotation

Here we should we able to use the base class type for x as type context for the x initializer.

This is closely related to #3208.

[ilevkivskyi]

#6511 provides another use case for this.

[ashb]

Minimal repro case:

from typing import Iterable

class A:
    a: Iterable[str] = ("a", )

class B(A):
    a = ("a", "b")

class C(B):
    a = ("a", "b", "c")

And the error:

a.py:10: error: Incompatible types in assignment (expression has type "Tuple[str, str, str]", base class "B" defined the type as "Tuple[str, str]")

The error is reported on the assignment in C.

I would have expected that B.a would have the same type as A.a since B.a doesn't have an explicit type.

(This is the case from #6511 copied here in case it is useful)

[ilevkivskyi]

(Raising priority to high since this keeps coming.)

[onlined]

This example from #6766 (comment), which is related to partial list type, doesn't give an error while it should:

class A:
    x = ['a', 'b']

class B(A):
    x = []
    x.append(2)