[asukaminato0721]

Summary

Fixes #3362

Recognizes f = classmethod(f) during binding and types cls as type[Self], preserves subclass-aware return inference and class-attribute initialization, avoids triggering when classmethod is shadowed.

Test Plan

add test

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D118597715. (Because this pull request was imported automatically, there will not be any future comments.)

[stroxler]

This needs a rebase before we can merge it, since it has build failures.

In addition, there are some behavior issues:

1. Wrong-scope lookup: an inner class’s f = classmethod(f) can mark an unrelated outer-class method. Outer().f() then incorrectly infers type[Outer] instead of Outer.
2. Aliases change retroactively: saving g = f before wrapping f also converts g’s inferred behavior to a classmethod, although it remains an instance method at runtime.
3. Decorator composition breaks: the flag changes the undecorated function’s receiver before existing decorators run. A valid adapter from an instance-taking function to a class-taking callable gains a false-positive argument error.
4. Overloads become inconsistent: only the implementation is marked, producing new invalid-overload errors on otherwise valid overload declarations.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅