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main.py

from __future__ import annotations

from contextlib import suppress
from typing import Any, ClassVar
from typing_extensions import reveal_type


class Foo: ...
class SubFoo(Foo):
    def bar(self) -> int: return 5


class A:
    clsvar: ClassVar[Foo] = Foo()
    
    def foo(self: A, x: int) -> int:
        return x

class SubA1(A):
    num: int = 42
    
    def __init__(self, num: int) -> None: ...
    
    def foo(self: SubA1, x: int) -> int:
        return self.num + x


class SubA2(A):
    clsvar: ClassVar[SubFoo] = SubFoo()

    def foo(self: SubA2, x: int) -> int:
        return self.clsvar.bar() + x


def factory_one() -> type[A]: return SubA1
def factory_two() -> type[A]: return SubA2

def main() -> None:
    
    main_foo = factory_one().foo
    reveal_type(main_foo)  # Revealed as "(self: A, x: int) -> int"
    
    # Bound method problem
    with suppress(AttributeError):
        main_foo(A(), 2)  # Type checks, but runtime error
        print("Won't print")
    
    my_cls = factory_one()  # This is checked as "__main__.A" but it is "__main__.SubA1"
    
    # Constructor problem
    with suppress(TypeError):
        my_instance = my_cls()  # Type checks, but runtime error
        print("Won't print")
    
    my_cls_two = factory_two()
    reveal_type(my_cls_two.clsvar)  # Revealed type is "__main__.Foo"
    my_cls_two.clsvar = Foo()
    
    # ClassVar problem
    with suppress(AttributeError):
        my_cls_two().foo(41)
        print("Won't print")
        
        # Granted, instances have the same problem, but I think it is
        # unresonable to 'fix' this
    
main()