Workaround:

provide an implementation in the Protocol Definition that yield an instance of the correct type:

from collections.abc import AsyncIterator
from contextlib import asynccontextmanager
from typing import Protocol


class P(Protocol):
    @asynccontextmanager
    async def f(self) -> AsyncIterator[None]: 
        """Create a context but yield nothing."""
        yield None  # Workaround for https://github.com/python/mypy/issues/17996

You should use a def instead of an async def. Due to the way async generators are detected by CPython, an async def without a yield has a different type.

This unfortunate edge case comes up pretty regularly; we should add it to https://mypy.readthedocs.io/en/stable/common_issues.html.

@JelleZijlstra Thanks, using a def without async in the Protocol method definition works indeed, even when concrete implementations use async def.

It's pretty surprising, having to bend the Prototype definition away from what will get implemented, and in effect making the Prototype less representative of what is expected.

If the error is expected to stay, then having it mentioned in Common Issues, as you suggest, is a good idea. Not only is it the natural place to find such information; it's also a respectable and high-ranking source that will hopefully improve web search results.

To help with the latter, I suggest also including the error message in the common issues document.

There's reasonable docs for this in https://mypy.readthedocs.io/en/stable/more_types.html#typing-async-await

A PR that mentions the error message in "Common issues" and links to the above section would be welcome