Return type AsyncIterable not correctly deduced for abstract methods

[MaxG87]

Bug Report

I am trying to write an abstract class (Protocol or abc.ABC) with a method that yields values. This concept works well for def foo(self, args: list[T]) -> Iterable[T]: ..., but breaks down for async def foo(self, args: list[T]) -> AsyncIterable[T]: .... If yield is added to the body of the async method, the return type is correctly deduced.

In order to make the issue self documenting, I add a snippet from the playground example:

import abc
import typing as t


class MyABC(abc.ABC):
    # Putting `yield` instead of `...` in the body would cure the error.
    @abc.abstractmethod
    async def get_strings(self, strings: list[str]) -> t.AsyncIterable[str]: ...


class MyImplementation(MyABC):
    async def get_strings(self, strings: list[str]) -> t.AsyncIterable[str]:
        for string in strings:
            yield string

The error is error: Return type "AsyncIterable[str]" of "get_strings" incompatible with return type "Coroutine[Any, Any, AsyncIterable[str]]" in supertype "MyABC" [override]

To Reproduce
https://mypy-play.net/?mypy=latest&python=3.12&gist=86f6de6b39e17f87e2c4e46b01aaebc2

Expected Behavior

No error, even without yield.

Actual Behavior

The error is error: Return type "AsyncIterable[str]" of "get_strings" incompatible with return type "Coroutine[Any, Any, AsyncIterable[str]]" in supertype "MyABC" [override]

Your Environment
The error is reproduced on mypy 1.10 on the Playground.

edit: I found #5385 which describes a very similar problem, maybe the same even. However, I am not too convinced with the proposed solution. I think it should be possible to write a Protocol or abc.ABC just the way I would write a normal function.

[MaxG87]

Using the remark in #5385, a "fixed" protocol would look like this:

import abc
import typing as t


class MyABC(abc.ABC):
     @abc.abstractmethod
    def get_strings(self, strings: list[str]) -> t.AsyncIterable[str]: ...


class MyImplementation(MyABC):
    async def get_strings(self, strings: list[str]) -> t.AsyncIterable[str]:
        for string in strings:
            yield string

(playground)

I still find this unfortunate, though. However, maybe this is a rough edge that has to be accepted.

[JelleZijlstra]

As you say, a duplicate of #5385.