Misleading warning message when using ABCMeta instead of Protocol

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

Start with the linked mypy-play reproduction and compare the diagnostics produced when Comparable uses ABCMeta versus typing.Protocol. Trace the type inference and error-reporting path for the max_if key argument. Done means the ABCMeta case produces a diagnostic that accurately explains the incompatibility rather than showing Callable[[str], None].