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.

Start with the minimal Python reproduction and compare its result under mypy 0.920 with the Mapping.get typing used by typeshed. Then inspect lines 125-126 of wemake_python_styleguide/logic/arguments/function_args.py and determine whether the reported diagnostic reflects an incorrect stub or valid typing; done means reaching a documented resolution and corresponding typeshed change if needed.