Erroneous "Self argument missing" error when decorators are used

[hvenev]

Bug Report

mypy sometimes erroneously emits a "Self argument missing" error on function definitions inside class definitons. See the example below.

If the function has no paramters, the error is "Method must have at least one argument.". Again, it does not matter what the decorator does to the function.

To Reproduce

import typing

def apply_on_answer[T](f: typing.Callable[[int], T], /) -> T:
    return f(42)

class C:
    @apply_on_answer
    def x(answer: int) -> int:
        return answer

assert type(C.x) is int
assert C.x == 42

Expected Behavior
mypy should emit no errors.

Actual Behavior
mypy emits the following error:

a.py:8: error: Self argument missing for a non-static method (or an invalid type for self)  [misc]

Your Environment

• Mypy version used: cb3bddd
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): ``
• Python version used: 3.13.5

[sterliakov]

Decorators that turn a callable method into a non-callable are exceptionally rare, mypy does not support this usecase. We expect any (decorated or not) method defined in class body to be just that - a method - and check accordingly.

Note that mypy understands that C.x is an integer, even in class body (playground), the only false positive is checking selftype too eagerly. You can safely put an ignore comment there, it should not affect further typechecking.