Non-ReadOnly TypedDict fields should behave invariantly in subtyping

[JelleZijlstra]

Describe the Bug

from typing import Any, Dict

from typing_extensions import ReadOnly, TypedDict

class TD(TypedDict):
    a: int

class TD2(TypedDict):
    a: ReadOnly[int]

class TD3(TypedDict):
    a: bool

def want_td(td: TD) -> None:
    pass

def capybara(
    td: TD, td2: TD2, td3: TD3
) -> None:
    want_td(td2)  # E: incompatible_argument
    want_td(td3)  # E: incompatible_argument

Both of the want_td calls should be rejected. A TypedDict with a ReadOnly[X] key is not assignable to one with an equivalent X key, because the latter allows assignment. For similar reasons, all (non-ReadOnly) items should be treated as invariant in assignability checks.

Sandbox Link

https://pyrefly.org/sandbox/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoCCKcANFACJIDGMAULaJLIqmgPoCmAHjBygM5IwAzNlz4AShwCGAEwDyKADakoAFUQdZlGvSpLp-fuvIAKDQi06YASgBctKE6jS7mFHVr7DxteQBM5pra1LYOzi5uUnKKKgDaqDAAunoGRiYAzEGWITT2js6uUABGYGBK9LIcwFAA7tIebDCyps1ufjZQALQAfFAAcsIc4c4IPpXVUFTSCHDF0iDSpgVObSZkzf7tARuyGdsZtJ29A0MjTvWNza2y-p1QAMRQAKJuqFSQYzBIxUocbAs0ABXCB8OgRS4wJotZoZe5PV7uD7YaTfX7-QEgsG0IA

(Only applicable for extension issues) IDE Information

No response

[yangdanny97]

This should (hopefully) be a fairly quick fix in is_subset_eq, i'll leave it around for another week or two in case a new contributor wants to take a stab at it, otherwise I'll pick it up myself.

[connernilsen]

@yangdanny97 For the good first issue tag, are there any pointers we could give for where to start looking at the changes that need to be made?

[yangdanny97]

@connernilsen the change would involve adding an is_subset_eq in the opposite direction for non-readonly fields here:

https://github.com/facebook/pyrefly/blob/08fa989ff8e4a0a781f51d03dedc33c090c62d20/pyrefly/lib/solver/subset.rs#L723