[1.16 makes more common] Inference of Any against optional type infers Never

[rggjan]

See the following example:

from typing import Any, Optional, TypeVar

T = TypeVar("T")


def assert_not_none(value: Optional[T]) -> T:
    assert value is not None
    return value


def foo1(a: Optional[Any]) -> int:
    return assert_not_none(a)[3]


def foo2(a: Optional[Any]) -> int:
    assert a is not None
    return a[3]

I would expect that foo1 and foo2 are equivalent. However, running mypy on this file, I get:

test.py:12: error: Value of type <nothing> is not indexable

For some reason, assert_not_none doesn't map Optional[Any] to Any, as it is supposed to (and which would accept the indexing), but instead maps it to <nothing> which throws an error. The second version with asserting that it is not None seems to work fine, though.

[msullivan]

Definitely a bug and a somewhat surprising one too. Probably an issue in constraints.py?

[ilevkivskyi]

The root cause is:

a: Any
def assert_not_none(value: Optional[T]) -> T: ...
reveal_type(assert_not_none(a))  # Revealed type is "Never"

(and same with Optional[Any]) because of loose handling of unions in constraints.py. This is pretty bad IMO, so raising priority to high.

[JukkaL]

I encountered this when preparing an internal codebase for the 1.16 release. The issue wasn't directly related to any change in 1.16, though -- some additional Any | None types are inferred after recent changes.