Adding lists of subtypes together

[alanhdu]

Using mypy==0.620 and Python 3.6.6, the following code:

from typing import List, Optional

x: List[Optional[int]] = [1, 2, 3]
[None] + x

fails with:

a.py:4: error: Unsupported operand types for + ("List[None]" and "List[Optional[int]]")

This is presumably because lists are invariant w.r.t their generic parameter, and the declration requires a List of the same type

typeshed/stdlib/3/builtins.pyi

Line 619 in 4e40b03

def __add__(self, x: List[_T]) -> List[_T]: ...

.

I don't know what the right solution here is -- can we just replace the x: List[_T] with x: List[_T_co] and make it covariant? Or doe we need another typevar _U = TypeVar("U", bound=_T) or something?

[JelleZijlstra]

This is close to python/mypy#5492. I found a fix in #2404.

[wy193777]

#2404 has been reverted. Do we have a solution for this issue?

[tamuhey]

+1 from microsoft/pyright#1777