Decorators over `__init__()` make classes unconstructable until after the class is declared, even within functions

[finite-state-machine]

Bug Report

It should be possible, within a function, to construct an instance of a class which is declared globally later in the module. Applying a decorator factory to a class's __init__() causes constructed instances to have type Any, but only before the class declaration¹. (A plain decorator doesn't trigger this issue.)

This probably explains #14519, and may be related to #11293.

To Reproduce

[mypy-play.net]

from __future__ import annotations
from typing import (
        Any,
        Callable,
        TypeVar,
        )
from typing_extensions import (
        assert_type,
        )


Tc = TypeVar('Tc', bound=Callable[..., Any])

def any_decorator_factory() -> Callable[[Tc], Tc]:
    '''this one does nothing
    '''
    def inner(func: Tc) -> Tc:
        return func
    return inner



def function_pre() -> None:
    '''this function is identical to 'function_post()',
       but only this copy generates a mypy error
    '''
    instance = SomeClass()
    assert_type(instance, SomeClass)  # error: type is "Any", not "SomeClass"



class SomeClass:

    @any_decorator_factory()
    def __init__(self):
        pass


def function_post() -> None:
    '''this function is identical to 'function_pre()',
       but only that copy generates a mypy error
    '''
    instance = SomeClass()
    assert_type(instance, SomeClass)  # no error

Expected Behavior

The tool should issue no error for function_pre(); it's just as valid as function_post().

Actual Behavior

Any construction of SomeClass before that type is declared (i.e., before the end of its body) has type Any instead of SomeClass.

Your Environment

• Mypy version used: master 2024-03-12, 1.9.0
• Mypy command-line flags: none necessary
• Mypy configuration options from mypy.ini (and other config files): none necessary
• Python version used: 3.8, 3.11, 3.12

[Edited for clarity, and to simplify the reproduction case by removing vestigial code.]

Footnotes

1. some functions within the class body are affected as well; I haven't narrowed down the exact point constructing the class starts to behave normally, but I can do upon request. ↩