From List[Optional[X]] to List[X] with assert

[Alberth289346]

The trimmed down version of the issue is as follows:

from typing import List, Optional

xs: List[Optional[int]] = [1]
assert all(x is not None for x in xs)
ys: List[int] = xs    # Line 5

gives error:

ex.py:5: error: Incompatible types in assignment
         (expression has type "List[Optional[int]]",
            variable has type "List[int]")

I expected it to work, as it is an extension of the working

x: Optional[int] = 1
assert x is not None
y: int = x    # Works

except I have a list of possibly integer rather than one.

I can likely solve the issue by copying the list one item at a time and doing the check, but xs is a proper result already. I already had the assert all(...) in my original use-case.

mypy 0.670
python 3.7.7

[JelleZijlstra]

See #4573.

[Alberth289346]

List being invariant. Good point, I didn't think of that. Thanks.