(Filling out the following details about bugs will help us solve your issue sooner.)

Reproducible in:

pip freeze | grep slack
python --version
sw_vers && uname -v # or `ver`

The slack_bolt version

slack-bolt==1.18.1
slack_sdk==3.31.0

Python runtime version

Python 3.11.9

OS info

bash: sw_vers: command not found
❯ uname -a
Linux 49d1cffcb728 6.8.4-200.fc39.x86_64 #1 SMP PREEMPT_DYNAMIC Thu Apr  4 20:45:21 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

Steps to reproduce:

(Share the commands to run, source code, and project settings (e.g., setup.py))

from slack_bolt.app.async_app import AsyncApp

class MyCallableMiddleware:

    async def __call__(self, next_) -> None:
        await next_()

app = AsyncApp()
app.middleware(MyCallableMiddleware()) # this raises `ValueError: Async middleware function must be an async function`

Expected result:

AsyncApp.middleware accepts any Callable[..., Awaitable[Any]] object, per signature.

Actual result:

The middleware instance pass the callable check here
https://github.com/slackapi/bolt-python/blob/main/slack_bolt/app/async_app.py#L678
and is thus sent to AsyncCustomMiddleware init here
https://github.com/slackapi/bolt-python/blob/main/slack_bolt/middleware/async_custom_middleware.py#L13
where the signature for func is Callable[..., Awaitable[Any]] (which my middleware instance is).
However, inspect.iscoroutinefunction is used to check func, which does not pass, as it checks for the existence of
CO_COROUTINE flag (see here: CPython source),
despite the instance being both callable and asynchronous.

Requirements

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