False negative with a constrained generic type in the presence of `Any`

[russelldavis]

from typing import Any, AnyStr
def foo(arg1: AnyStr, arg2: AnyStr) -> None: ...
bar: Any = ""
foo(1, "") # Error correctly reported
foo(b"", bar) # No error (correct)
foo(1, bar) # No error (false negative)

In the presence of Any, I would expect mypy to allow any of the values of the constraint (in this case, str or bytes), but not any arbitrary type.

Playground link: https://mypy-play.net/?mypy=latest&python=3.8&flags=strict&gist=6970b49fa90d7767b0ef26c9f977e182

[ilevkivskyi]

Hm, interesting, likely this is caused by the fact that Any is a subtype of every type and mypy infers Any as the value for AnyStr (as a join of int and Any), and them when checking whether it is a valid value for AnyStr it concludes that it is.

It may actually be not easy to fix, since we want foo(bar, bar) to work. A while ago I proposed to make join(int, Any) to return int, but people were not happy about this since it may cause what some may see as a false positives in other places.