Decorating __init__() with a class decorator is not supported

[s417-lama]

Bug Report

I want to define a decorator as a class to decorate another class's methods. However, when __init__() constructor is decorated with that class, mypy reports Unsupported decorated constructor type error.

(For reference) Stackoverflow question I posted: https://stackoverflow.com/questions/79374167/mypy-unsupported-decorated-constructor-type-error-when-decorating-init-w

To Reproduce

from types import MethodType
from typing import TypeVar, Callable, Type, ParamSpec, Generic

T = TypeVar("T")
P = ParamSpec("P")
R = TypeVar("R")

class method_decorator(Generic[P, R]):
    def __init__(self, method: Callable[P, R]):
        self.method = method

    def __get__(self, instance: T | None, cls: Type[T]) -> Callable[..., R]:
        return self if instance is None else MethodType(self, instance)

    def __call__(self, *args: P.args, **kwargs: P.kwargs) -> R:
        return self.method(*args, **kwargs)

class Foo:
    @method_decorator # error: Unsupported decorated constructor type  [misc]
    def __init__(self, x: int):
        self.x = x

    @method_decorator
    def mul(self, a: int) -> int:
        return self.x * a

def gen_foo(x: int) -> Foo:
    return Foo(x) # error: Returning Any from function declared to return "Foo"  [no-any-return]

foo = gen_foo(2)
print(foo.mul(3))

Actual Behavior

$ uv run mypy --strict test.py
test.py:19: error: Unsupported decorated constructor type  [misc]
test.py:28: error: Returning Any from function declared to return "Foo"  [no-any-return]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.13.0
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): default
• Python version used: 3.11.9