False positive in @classmethod check for subclasses

[arnimarj]

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

Typeguard version

4.4.4

Python version

3.13.9

What happened?

It seems that the protocol check for @classmethods when a base class implements the protocol, and a base class inherits it, the check fails.

The method is wrongly classified as an instance method.

How can we reproduce the bug?

This shows both a true negative (checking the base class), and a false positive (checking the subclass):

from typing import Protocol, Self

import typeguard


class HasClassFunc(Protocol):
    @classmethod
    def class_func(cls) -> Self: ...


class BaseClass:
    @classmethod
    def class_func(cls) -> Self:
        return cls()


class SubClass(BaseClass):
    pass


@typeguard.typechecked
def my_func(b: type[HasClassFunc]) -> HasClassFunc:
    return b.class_func()


# succeeds
my_func(BaseClass)

# fails
my_func(SubClass)

[arnimarj]

I did some investigation, and the issues seems to be that check_signature_compatible doesn't check base-classes when classifying the attribute.

If the two loops deciding on protocol_type and subject_type loop through the respective MROs, the above example will work (I didn't test sub-classed Protocols though).

The relevant snippet from check_signature_compatible:

    # Check if the protocol-side method is a class method or static method
    for protocol_klass in protocol.mro():
        if attrname in protocol_klass.__dict__:
            descriptor = protocol_klass.__dict__[attrname]
            if isinstance(descriptor, staticmethod):
                protocol_type = "static"
            elif isinstance(descriptor, classmethod):
                protocol_type = "class"

            # stop in first class with this attribute
            break

    # Check if the subject-side method is a class method or static method
    for subject_class in subject.mro():
        if attrname in subject_class.__dict__:
            descriptor = subject_class.__dict__[attrname]
            if isinstance(descriptor, staticmethod):
                subject_type = "static"
            elif isinstance(descriptor, classmethod):
                subject_type = "class"

            # stop in first class with this attribute
            break