Incorrect widening of `T | None` with unions to `builtins.object` in generic functions

Bug Report

Generic function with signature (fun: Callable[[], T | None]) -> T | None has its type incorrectly widened to builtins.object when T is a union type

To Reproduce

import typing

T = typing.TypeVar("T")


def gen_fun_one(fun: typing.Callable[[], T | int]) -> T | int:
    return fun()


def gen_fun_two(fun: typing.Callable[[], T | None]) -> T | None:
    return fun()


def foo() -> int | str | None:
    pass


typing.reveal_type(gen_fun_one(foo))  # Union[builtins.str, None, builtins.int]
typing.reveal_type(gen_fun_two(foo))  # builtins.object

Gist URL: https://gist.github.com/mypy-play/f2401bd493520f4c72cad4a416195e19
Playground URL: https://mypy-play.net/?mypy=latest&python=3.12&gist=f2401bd493520f4c72cad4a416195e19

Expected Behavior

Both gen_fun_one(foo) and gen_fun_two(foo) are of type str | int | None

Actual Behavior

Function gen_fun_two(foo) is of type builtins.object

Your Environment

• Mypy version used: 1.9.0
• Python version used: 3.12
• Default Mypy playground settings

まず、リンク先の mypy playground にある提供済みの Python 再現コードを実行し、gen_fun_one と gen_fun_two の reveal_type の結果を比較します。2 回目の呼び出しに関係するジェネリックな union の推論を追跡します。報告されたケースのカバレッジを含め、builtins.object ではなく str | int | None と推論されれば作業完了です。