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

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?