(🐞) incompatible `Callable`s are incorrectly joined into `builtins.function`

from typing import Callable, _T as T
from types import FunctionType

class C:
    def __init__(self, i: int=1): ...

def join(t1: T, t2: T) -> T: ...

c1: Callable[[], object] = C
c2: Callable[[int], object] = C
result = join(c1, c2)
reveal_type(result)  # Revealed type is "builtins.function"
print(isinstance(result, FunctionType)  # False

this is not an instance of builtins.function, it is a type[C]. it should be joined into a union, or something else, either object or Callable[..., object] or something

the same defect is observed with a non-function callable instance:

class C:
    def __call__(self, i: int = 1): ...

• reraised from #17017 as it wasn't resolved

まず、提供された mypy の再現コードを実行し、明らかになった型と実行時の値を比較します。builtins.function を生成する原因となっている callable の結合と型推論の経路を追跡し、互換性のない callable 型に対する意図した結果を確定したうえで、クラスの例と callable インスタンスの例の両方に対するカバレッジを追加します。