Misleading warning message when using ABCMeta instead of Protocol

[noamraph]

Bug Report

To Reproduce

I run this code, expecting it to type-check:

https://mypy-play.net/?mypy=latest&python=3.11&gist=d4c1190de2b786aea78cc801ee2e7829

Expected Behavior

I would expect an error perhaps like this:

main.py:42: error: Argument "key" to "max_if" has incompatible type "Callable[[str], Optional[int]]"; expected "Callable[[str], Optional[S]]" where S is a subclass of "Comparable"  [arg-type]

Actual Behavior

main.py:42: error: Argument "key" to "max_if" has incompatible type "Callable[[str], Optional[int]]"; expected "Callable[[str], None]"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

Note

This was very confusing. While investigating this bug, I found out that I had to define Comparable using typing.Protocol instead of ABCMeta, and then I got the code to type-check.

I guess that since mypy tries to find S which is Comparable, so that Optional[int] would match S | None, and since it recognizes that int isn't a subclass of Comparable, it is left with the None and this is what it shows in the error message. However, this is very confusing.

Your Environment

• Mypy version used: 1.0.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11

[tmke8]

The function isn't really generic in S right? S only appears once.

If you write it like this instead:

def max_if(iterable: Iterable[T], *, key: Callable[[T], Comparable | None]) -> T | None:
    ...

then you get a relatively clear error message:

main.py:42: error: Argument "key" to "max_if" has incompatible type "Callable[[str], Optional[int]]"; expected "Callable[[str], Optional[Comparable]]"  [arg-type]

[noamraph]

@tmke8 I believe that actually the function is generic in S. In my definition, Comparable means that a type is comparable to itself. The key function should return values from a specific type, which is comparable to itself. With the definition you suggested, the key function may return many different types, say int for one item and str for another.

(But even if the definition without another generic is correct, I would still want the error message to be more correct and more helpful.)