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.

最小限の Python 再現コードから始め、その結果を mypy 0.920 での typeshed における Mapping.get の型付けと比較します。次に wemake_python_styleguide/logic/arguments/function_args.py の 125-126 行目を調べ、報告された診断が誤った stub を反映しているのか、妥当な型付けを反映しているのかを判断します。必要に応じて typeshed に対応する変更を行い、文書化された解決に到達すれば完了です。