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

Beginne damit, das Beispiel Union[str, bytes] mit mypy 0.790 zu reproduzieren, und untersuche anschließend die typeshed-Definitionen für str.contains und die zugehörige Behandlung von Union. Vergleiche die Diagnosen für "zoo" in foo und "zoo" + foo; abgeschlossen ist die Aufgabe, wenn der Containment-Ausdruck den inkompatiblen str/bytes-Fall konsistent mit dem Additionsausdruck meldet.