Some string `in` `Union[bytes, str]` does not produce an error

Given the following code

from typing import Union

foo: Union[str, bytes]  
"zoo" in foo  # no error
"zoo" + foo  # err: Unsupported operand types for + ("str" and "bytes")

I would have expected that mypy complains about str not compatible with bytes just like it does for the + operand.

I've tried the current typeshed version at 3d140160 because it just looked like there was a change here in Oct 2020 after mypy 0.790.

For example it now defines for class(str):

    def __contains__(self, o: Union[str, str]) -> bool: ...  # type: ignore
...
    def __iter__(self) -> Iterator[str]: ...

(__contains__ does not trigger here but its Union[str, str] is weird.)

Your Environment

mypy 0.790
python 3.8
builtin typeshed and @3d140160
Win 10

まず mypy 0.790 で Union[str, bytes] の例を再現し、次に str.contains の typeshed 定義と、関連する Union の処理を調べます。"zoo" in foo と "zoo" + foo の診断を比較し、包含式が加算式と一貫して互換性のない str/bytes のケースを報告すれば完了です。