`bad-unpacking` error when unpacking a TypedDict that inherits another TypedDict

[macintacos]

Describe the Bug

I have the following minimal reproducer:

import typing_extensions as te


class InheritFromMe(te.TypedDict):
    foo: bool


class TestBadUnpackingError(InheritFromMe):
    bar: bool


unpack_this: InheritFromMe = {"foo": True}
test1: TestBadUnpackingError = {"bar": True, **unpack_this}
test2: TestBadUnpackingError = {"bar": True, "foo": True}

print(f"{ test1 = }")
print(f"{ test2 = }")

The output of the program is:

$ python test.py
test1 = {'bar': True, 'foo': True}
test2 = {'bar': True, 'foo': True}

They're functionally equivalent, as far as I can tell. However, pyrefly reports the first as a bad-unpacking error:

$ pyrefly check test.py
ERROR Unpacked `TypedDict[InheritFromMe]` is not assignable to `TypedDict[TestBadUnpackingError]` [bad-unpacking]
  --> /Users/macinburrito/GitLocal/Work/semaphore/test.py:13:48
   |
13 | test1: TestBadUnpackingError = {"bar": True, **unpack_this}
   |                                                ^^^^^^^^^^^
   |
 INFO 1 error

I think that this shouldn't be a bad-unpacking error, but perhaps I'm missing some nuance of the type system here?

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeSIAxAAQDqMUAxrgLYzUAuunAFuwAqkATjDBRS1AMoYcBADS8IcakuqpqxVAHMYAHXQ0A7nxHVSuAK7VGGaoaEQOMBXB6pTM6rHfo1eCxycpMQQ6FoqvoIcPLjohPqUCdQAEjAeHtaWxLDUYDCoHBYiyrhgispoWHj4iEkAtNQA4jCBIqhQdRwQbEHE7Ix8jADWoeF5MDiow-XUAKpw7CIAbvlQAPocwTAAFACUnNyhcH2MgaF5QiKYvfAzMPh9DmzogYaOPLzsAJIAIgCiuXyhWKvAKdnYcAsxGIuCEgW2MEIWkIagCuGYLGyLWc1C0uE6%2BMwolCjggsQUMRWQgULUYuxmtAcTjUWGoQgskVI0VimSJ1EwEBEp3EETMliE1GwQlwhgWQn0%2Bm6MLhvVGa3uTnQcDJWrUyicCvQ%2BkYUFQcGUX3QJkcADFpSwALI7JyEAAqW0wPwgp12tV81ADYFwuEQkuDUENxtN5uorvgHAAQqhMLN0JphqM-pdYdtLdaOHbWE7ffoAwHsO5Q3hcBGjXWOemhhseEpQ3m0rb7U7qABeajAXTgYOD0Ou9kwAC%2B%2BiccA4AEZR-Gkym01MRmEs9KJX2ByAK-KkLHxwoAFQnhtr5tKKcveMAJkXs%2BXqcbmez2-7g-3I6PFhxg6DXAfzHP8b30YgHBebYwEHYBOHjOde2oKcQHpNNII4aDYPg2c7yQlC0P0ZZVg2LZthnec0OI9pSL6cj7zQkA5BAAJoDgEhyEQKg5i6KBHAkMAOVOHU4ENIkyiDIQWAKNZ0AsFhsDSbYagiDh9jqAA%2BahZyEP0yzZFoil8GCQAAOXkxTdP7fAUP0JiQDIEQxFIQguBYKAKBoQQnJFSpZHwTJ0EgLQigKHV4gMKQYHYHgOA4Yg4EQAB6JLHNEcRCFhLQkpgdAkswdE4CS5ggogEKhDC2IktyWE1CWVBoFQbAchK4LQq6HlcGIDqtQishuXQOoqW1Hk%2B0HABmQg5zvQc7InZipi6FYbWgGAKD86oOJACcgA

(Only applicable for extension issues) IDE Information

No response

[yangdanny97]

Thanks for reporting! Given that mypy and pyright do not raise this error, this is likely a bug.

[yangdanny97]

Actually, given that Hacktoberfest is still going, I'll put this up-for-grabs to see if anyone in the community wants to try :)

The code pointer for matching dict literals with typed dicts is here:

https://github.com/facebook/pyrefly/blob/main/pyrefly/lib/alt/class/typed_dict.rs#L66

Tests should be added to https://github.com/facebook/pyrefly/blob/main/pyrefly/lib/test/typed_dict.rs

[rchen152]

The pyrefly team discussed this a bit, and we realized that unpacking one TypedDict into another is technically unsafe (example). To make it safe, you'd want to add closed=True to the definition of InheritFromMe. (Note that closed is a new feature; to get it in Python versions before 3.15, you need to import TypedDict from typing_extensions.)

Since mypy and pyright do allow this unsafe pattern, we're planning to allow it by default but add an opt-in error to flag it. The error will likely be on by default in a future strict mode.

[rchen152]

Thanks @michaelcortese for submitting a PR to fix the default behavior. I'll take on the remaining work outlined in my previous comment.