confusing behavior with TypeVars and multiple inheritance checks in 0.770

Hi, I was just testing out 0.770 and I'm getting a surprising result:

from typing import TypeVar

class Foo(object):
    pass

FooOrStr = TypeVar('FooOrStr', Foo, str)

def doit(reset: bool, arg: FooOrStr) -> FooOrStr:
    if reset:
        if isinstance(arg, Foo):
            reveal_type(arg)
            arg = Foo()
        elif isinstance(arg, str):
            reveal_type(arg)
            arg = ''
        else:
            raise TypeError()
    return arg

test.py:12: note: Revealed type is 'test.Foo*'
test.py:12: note: Revealed type is 'test.<subclass of "str" and "Foo">'
test.py:13: error: Incompatible types in assignment (expression has type "Foo", variable has type "str")  [assignment]
test.py:15: note: Revealed type is 'builtins.str*'

Based on the latest blog post about 0.770 I suspect this is by design, but what can I do to work around it (short of adding some artificial method to make Foo incompatible with str)?

It would be good to add something to the docs about this, because I think it could be a fairly common trap. Also, I think adjusting the error message to clarify the relationship to multiple inheritance would help, e.g. variable has type Foo and 'test.<subclass of "str" and "Foo">'.

Thanks!

Commencez par exécuter mypy 0.770 ou l’exemple de TypeVar et d’héritage multiple de l’issue afin de reproduire les types révélés et l’erreur d’affectation. Lisez ensuite la documentation pertinente sur TypeVar et le rétrécissement des types, ainsi que les tests de diagnostic, puis documentez le comportement, une solution de contournement et une formulation plus claire de la relation entre Foo et le type de sous-classe inféré.