[InvincibleRMC]

Attempt to address #15529

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

aiohttp (https://github.com/aio-libs/aiohttp)
- aiohttp/worker.py:76:26: error: Too few arguments  [call-arg]
+ aiohttp/worker.py:73:14: error: Left operand of "or" is always false  [redundant-expr]
- aiohttp/worker.py:76:26: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-call-arg for more info
+ aiohttp/worker.py:73:14: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-redundant-expr for more info
- aiohttp/worker.py:76:26: note: "wsgi" is considered instance variable, to make it class variable use ClassVar[...]
+ aiohttp/worker.py:76:13: error: Statement is unreachable  [unreachable]
+ aiohttp/worker.py:88:9: error: Statement is unreachable  [unreachable]

discord.py (https://github.com/Rapptz/discord.py)
- discord/ext/commands/core.py:2373: error: "Callable[[Command[Any, Any, Any] | Callable[..., Coroutine[Any, Any, Any]]], Command[Any, Any, Any] | Callable[..., Coroutine[Any, Any, Any]]]" has no attribute "predicate"  [attr-defined]
- discord/ext/commands/core.py:2448: error: "Callable[[Command[Any, Any, Any] | Callable[..., Coroutine[Any, Any, Any]]], Command[Any, Any, Any] | Callable[..., Coroutine[Any, Any, Any]]]" has no attribute "predicate"  [attr-defined]

[JelleZijlstra]

This is incorrect because it does not return true for all callables that return a CoroutineType.

[linuxdaemon]

Perhaps a new type needs to be added to represent async functions, because with the current typing system there is no way to type

def foo(a) -> Awaitable[int]:
    return asyncio.Future()

as being distinct from

async def foo(a) -> int:
    return 5

and no way to distinguish them at runtime other than iscoroutinefunction, which can't be accurately typed as the annotations indicate a narrowing on the return type, while the logic of iscoroutinefunction is using flags on the function's code object.

Thus, you can't know definitively if a function is asynchronous in the current system.