false "not async iterable" on use of overload with async iterable

[belm0]

Bug Report

Using typing.overload on an async iterator doesn't behave as I expected:

@overload
async def transitions(self, value: T) -> AsyncIterator[Tuple[T, T]]: ...
@overload
async def transitions(self, predicate: P) -> AsyncIterator[Tuple[T, T]]: ...
async def transitions(self, value_or_predicate):
    # impl with yield ...

if I remove async qualifier on the overloads, it appears to work. But that doesn't seem right...

Expected Behavior

no errors on correct usage

Actual Behavior

error on usage:

error: "Coroutine[Any, Any, AsyncIterator[Tuple[str, str]]]" has no attribute "__aiter__" (not async iterable)  [attr-defined]

Your Environment

• Mypy version used: .812
• Python version used: 3.7.7

[hauntsaninja]

That's kind of unfortunate. Removing the "async" from the overloads is probably your best solution, see #5385 (comment) for some more context

[belm0]

Removing the async qualifier from the overloads seems sure to catch some tools off guard (IDE, etc.).

The agreed-upon solution should be clearly documented in the typing.overload / Protocol docs so that everyone has a standard reference.

[belm0]

Removing the async qualifier now also confuses pylint: pylint-dev/astroid#1015

[dm9pZCAq]

i think this is related to this issue

code:

from abc import ABC, abstractmethod
from typing import Any, AsyncIterator


class MyABC(ABC):
    @abstractmethod
    async def func(self) -> AsyncIterator[Any]: pass


class MyClass(MyABC):
    async def func(self) -> AsyncIterator[Any]:
        yield 1

error:

test.py:11: error: Return type "AsyncIterator[Any]" of "func" incompatible with return type "Coroutine[Any, Any, AsyncIterator[Any]]" in supertype "MyABC"

@hauntsaninja as you can see in code example abstract and actual classes have the same func function signature, so i think #5385 (comment) is not fit into this example

---

• Mypy version used: 0.931
• Python version used: 3.9.9

[hauntsaninja]

They do not have the same type, because yield fundamentally changes what def does, so #5385 still applies. Feel free to replace pass with if False: yield 1. Or remove the async qualifier.

Closing because while mypy's behaviour here is surprising, it is correct, and I don't see a better solution here.