Subclass/intersection issue in Mypy 1.15

[gandhis1]

I wasn't sure if this was an actual bug, so posing a question here first. This code is just an example - it filters using an isinstance call to try to find objects that fit the profile of both Foo and Bar:

from abc import ABC
from collections.abc import Sequence

class Foo(ABC):
    pass

class Bar(ABC):
    pass

class Baz(ABC):
    pass

class FooBar(Foo, Bar):
    pass

class FooBaz(Foo, Baz):
    pass

def my_func(foos: list[Foo]) -> Sequence[Foo]:
    my_list = []
    for foo in foos:
        if not isinstance(foo, Bar):
            continue
        my_list.append(foo)
    return my_list

foos = [FooBar(), FooBaz()]
my_func(foos)

This code does not fail on Mypy 1.14.1:

https://mypy-play.net/?mypy=1.14.1&python=3.12&gist=7b5be66ab0fb3f04d15a99317ef21a7d

But it does fail on Mypy 1.15:

main.py:24: error: Argument 1 to "append" of "list" has incompatible type "__main__.<subclass of "Foo" and "Bar">1"; expected "__main__.<subclass of "Foo" and "Bar">"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

https://mypy-play.net/?mypy=1.15.0&python=3.12&gist=7b5be66ab0fb3f04d15a99317ef21a7d

My question is simple - is this a bug/regression, or this code invalid? And if so, why?

[carljm]

I think this is a bug in the latest mypy. (Which means this would be better as an issue on the mypy repo.) The list type may be invariant, but that just means list[Foo & Bar] is not a subtype of list[Foo], it doesn't mean the parameter of the append method of a list[Foo] shouldn't accept subtypes of Foo.

You can work around it by telling mypy to treat your narrowed foo as a Foo: replace the line my_list.append(foo) with this:

        found: Foo = foo
        my_list.append(found)