Literal key `in` narrowing for TypedDict w/o extra items

[yangdanny97]

Describe the Bug

from typing import *

class TD(TypedDict):
    x: int

def test(td: TD):
    if "y" in td:
        td["y"] # unexpected error

Python: 3.12

If we explicitly check the key exists we should not give this error. Naively we could narrow to Any, but this would apply to the whole dict, and would interact weirdly when there is a union

from typing import *

class TD(TypedDict):
    x: int

class TD2(TypedDict):
    x: int
    y: int

def test(td: TD | TD2):
    if "y" in td:
        td["y"] # should this be Any or int | Any ?

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAEALqcROgOZ0Q3G6UN0AqADroRAYyio4cOgBUAIgApZzGJnkQxDAJSIRdA3XyJO6BiJGYYYRvAaKGmEwt37DEG0JCkvpxk7dDQ0cAbS8fEABdEAAaEDJKayhSQgZaKAoAYjoABVJEsGS6NCw8fDoxXHRINgBXSlQGCCrCEWyAZRgYOgALBgZiOEQAemGEpJTeNmGYdGHMXDE4YcrqiDqGpqrhujBeOlQAN1RoVGxYCqqa%2Bsbm9DpcYi30OFb0MgYeqoBaQ5hKOB3OgAXjoXgAzIQAIwAJi8IhAAF84qgtBA-gAxaAwCglHAEEjkRFAA

(Only applicable for extension issues) IDE Information

No response

[rchen152]

Open typed dicts (ones that don't have closed= or extra_items=) are approximated as having extra items ReadOnly[object] (https://typing.python.org/en/latest/spec/glossary.html#term-open), so IMO it would make sense to give td["y"] a type of object in the above examples.

[grievejia]

Would Any be a better choice in practice than object (at least in default mode), given that the former is better at controlling cascading false positives?

That said, silently introducing an Any to the program without an error/warning feels like undesirable if user wants very strict type checking.

[yangdanny97]

I originally thought Any would be better because hasattr-narrowing introduces a new attribute w/ type Any, and this check feels conceptually similar to hasattr

But we do treat TypedDicts as Mapping[str, object] in a lot of places already

[yangdanny97]

the first example behaves correctly now but the second one does not