False positive with Union of collections

[shenriotpro]

Hi folks,

import json
from typing import Any


def parse_json(s: str) -> dict[str, Any] | list[dict[str, Any]]:
    return json.loads(s)


def foo() -> list[dict[str, Any]]:
    res = []
    l = parse_json('[{"abc": 123}]')
    assert isinstance(l, list)
    res.extend(l)
    return res


def bar() -> list[dict[str, Any]]:
    l = parse_json('[{"abc": 123}]')
    assert isinstance(l, list)
    return l

if __name__ == '__main__':
    assert foo() == [{'abc': 123}]

I'm not sure why pytype is unhappy with foo (but bar is fine). It looks like a bug to me.

~$ mypy toy.py
Success: no issues found in 1 source file

~$ pytype toy.py
Computing dependencies
Analyzing 1 sources with 0 local dependencies
ninja: Entering directory `.pytype'
[1/1] check toy
FAILED: /home/user/.pytype/pyi/toy.pyi
/usr/bin/python3 -m pytype.main --imports_info /home/user/.pytype/imports/toy.imports --module-name toy --platform linux -V 3.10 -o /home/user/.pytype/pyi/toy.pyi --analyze-annotated --nofail --quick /home/user/toy.py
File "/home/user/toy.py", line 14, in foo: bad return type [bad-return-type]
           Expected: List[Dict[str, Any]]
  Actually returned: List[str]

For more details, see https://google.github.io/pytype/errors.html#bad-return-type
ninja: build stopped: subcommand failed.
Leaving directory '.pytype'

~$ pytype --version
2024.03.19

Cheers.