Confusing error message: Returned type `() -> int` is not assignable to declared return type `() -> int`

[amas127]

Describe the Bug

import typing as tp

def integer_factory() -> int:
    return 1

class A:
    _factory: tp.ClassVar[tp.Callable[[], int]] = integer_factory

    @property
    def factory(self) -> tp.Callable[[], int]:
        return self._factory  # Pyrefly [bad-return]: Returned type `() -> int` is not assignable to declared return type `() -> int`

The title writes itself. A class variable typed as ClassVar obscures pyrefly correctly detect the type when returned with @property descriptor. When ClassVar denotion was removed, the error messages was gone. Even so, ClassVar should make no effects in this scenario and this possibly is a bug.

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeSIEAtsbgE4AuABA6cROgOZOpwvEAddEKGYYYJhwYxOMOgH0wqAMYN6pABQBKJgFoAfJPQNEQpuaZ0YDAK510TAIwjh6ZVF58AgqYcXFKmp0pIj8hADCHnBwAGqodADaDMQRqFAe2LAJCQC6ADRGDDk5TAC8hTJyAarqLhZMAALEdLjEcqxmFmISSjXBGnAwUGA6BmHhaRlZuQVSOb719Va29kyDw4TVQaRCIHkgZFZgUKSEalRQFADETAAKpEcnaxg4BEzKuOiQnHaoDBCfQhCG4AZRgMCYAAsGMk4IgAPTww7iE6EeiceEwdDwzC4ZRweEfL4QH50P4A7FMMD0HgAN1Q0FQmQhRO%2Bv3%2BnyYrQ56DgQPQZAYkM%2BulpcjgFLKTAEIAAzIRHAAmGVCBJyFp0OA5IQ2dDUWiMGCYXSYCBWVQQMVSgDkJPQ9Bg1qE9oYuisAEcbGajboANYwUi6FTKeB8crWgDu8XQTvQIAAvvtApaYAAxaAwChoLB4IhkBNAA

(Only applicable for extension issues) IDE Information

No response

[samwgoldman]

Thanks for the report!

What's happening here is that assigning integer_factory in the class body actually produces a bound method, meaning self._factory will take Self as an argument. If you extend your program with a few lines, you can see:

a = A()
a.factory() # TypeError: integer_factory() takes 0 positional arguments but 1 was given

That 1 positional argument given is the a instance, since A._factory becomes a bound method.

I agree that the error message is super confusing, though, so leaving this open to address the error.

[amas127]

a = A()
a.factory() # TypeError: integer_factory() takes 0 positional arguments but 1 was given

That 1 positional argument given is the a instance, since A._factory becomes a bound method.

Thanks for correction!

[yangdanny97]

For comparison

mypy says Attribute function "_factory" with type "Callable[[], int]" does not accept self argument
pyright says

Cannot access attribute "_factory" for class "A*"
  Could not bind method "<anonymous>" because it is missing a "self" or "cls" parameter

[goutamadwant]

Hi @yangdanny97 can i work on this? thanks!

[yangdanny97]

assigned, thanks

[github-actions]

This issue has someone assigned, but has not had recent activity for more than 2 weeks.

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Thank you for your contributions!

[goutamadwant]

@yangdanny97 I have a fix for this issue on #4081
Let me know if you have any comments or suggestions. thanks!