mypy@0.920 regression: Argument 1 to "get" of "Mapping" has incompatible type

I got a new regression from the latest release on a real project: https://github.com/wemake-services/wemake-python-styleguide/blob/master/wemake_python_styleguide/logic/arguments/function_args.py#L125-L126

Error:

wemake_python_styleguide/logic/arguments/function_args.py:126: error: Argument 1 to "get" of "Mapping" has incompatible type "Optional[Any]"; expected "str"

Simplier repro:

from typing import Mapping, Any, Optional

x: Optional[Any]
m: Mapping[str, int]

m.get(x)
# error: Argument 1 to "get" of "Mapping" has incompatible type "Optional[Any]"; expected "str"

So, what do you think: is this a valid error? Because it will work at runtime with no problem. And since it has Any part in it, sometimes it can even be str. So, in my app it was working as expected in all cases: if x is str and exists in m - then fine. If not - then just return None.

I am openning it here, because it looks like a typeshed issue, rather than a mypy issue.

Comienza con la reproducción mínima en Python y compara su resultado con mypy 0.920 con el tipado de Mapping.get utilizado por typeshed. Después inspecciona las líneas 125-126 de wemake_python_styleguide/logic/arguments/function_args.py y determina si el diagnóstico informado refleja un stub incorrecto o un tipado válido; se considera terminado cuando se haya alcanzado una resolución documentada y, si es necesario, el cambio correspondiente en typeshed.