Incorrect type reification on optional union attribute in conditional

[elprans]

Repro:

bug.py:

from __future__ import annotations

from typing import *


class B:
 pass


class A:
 name: Optional[Union[str, B]]


def foo(a: A):
 if isinstance(a.name, str):
 pass
 elif isinstance(a.name, B):
 pass
 else:
 raise AssertionError

 bar(a.name)


def bar(arg: Union[str, B]):
 pass

mypy incorrectly reports an error:

bug.py:22: error: Argument 1 to "bar" has incompatible type "object"; expected "Union[str, B]"

Interestingly, when a.name is aliased with another name, the issue disappers, i.e. the following is checked without an issue:

ok.py:

from __future__ import annotations

from typing import *


class B:
 pass


class A:
 name: Optional[Union[str, B]]


def foo(a: A):
 a_name = a.name

 if isinstance(a_name, str):
 pass
 elif isinstance(a_name, B):
 pass
 else:
 raise AssertionError

 bar(a_name)


def bar(arg: Union[str, B]):
 pass

[ilevkivskyi]

I confirm this. This honestly looks like a mystery, it looks like mypy applies a join instead of a union for attribute expressions, but understanding why would require some investigation.

[hauntsaninja]

This was fixed by #18538