Generic type inferred wrong; curious difference betweer `aiter(f(x))` and `c = f(x); aiter(c)`

Bug Report

Consider this code (playground):

from typing import TypeVar, Generic, Callable, Coroutine, Any, AsyncIterator

_T = TypeVar("_T")

class StartedToken:
    pass

class ProducerContext(Generic[_T]):
    pass

async def coro_to_aiter(
    coroutine: Callable[[ProducerContext[_T]], Coroutine[Any, Any, None]]
) -> AsyncIterator[_T | StartedToken]:
    yield StartedToken()

async def coro(ctx: ProducerContext[int]) -> None:
    pass

# Split into two lines, no error:
c = coro_to_aiter(coro)  # no error
it = aiter(c)  # no error

# The same on one line -> mypy error:
it2 = aiter(coro_to_aiter(coro))  # error: Argument 1 to "coro_to_aiter" has incompatible type ...

Expected Behavior

I would expect mypy to not error on the it2 definition line. Moreover, regardless of whether mypy can handle these definitions, I would expect to see the same behavior (error or absence of error) in the "Split into two lines" case and the "on one line" case.

Actual Behavior

I get this error on the it2 line, and no errors on other lines:

main.py:24: error: Argument 1 to "coro_to_aiter" has incompatible type "Callable[[ProducerContext[int]], Coroutine[Any, Any, None]]"; expected "Callable[[ProducerContext[int | StartedToken]], Coroutine[Any, Any, None]]" [arg-type]

Your Environment

• Mypy version used: 1.6.1 (playground), current master
• Python version used: 3.11, 3.12

issue の main.py 再現コードから始め、記載されている Python と mypy のバージョンを使用して、分割した aiter(coro_to_aiter(coro)) のケースとネストしたケースを比較します。ジェネリック型推論が各呼び出しをどのように処理するかを追跡し、この不一致に対する集中的な回帰テストを追加または更新します。両方の形式が一貫した期待どおりの診断を受ければ完了です。