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.

Inizia con la riproduzione Python minima e confronta il risultato con mypy 0.920 con la tipizzazione di Mapping.get utilizzata da typeshed. Poi esamina le righe 125-126 di wemake_python_styleguide/logic/arguments/function_args.py e determina se la diagnosi segnalata riflette uno stub errato o una tipizzazione valida; il lavoro è completato quando si è raggiunta una risoluzione documentata e, se necessario, la modifica corrispondente a typeshed.