`asyncio.gather` is sometimes erroneously type-checked

[Tinche]

Bug Report

I'm adding typing to a large codebase, and have noticed weird asyncio.gather behavior.

To Reproduce

Here's a self-contained test:

from asyncio import gather


async def returns_dict() -> dict:
    return {}


async def returns_int() -> int:
    return 1


async def test():
    tasks = (returns_dict(), returns_int())
    reveal_type(await gather(*tasks))  # note: Revealed type is "Any"

Expected Behavior

The revealed type should be: note: Revealed type is "Tuple[builtins.dict*[Any, Any], builtins.int*]"

Actual Behavior

The correct return type is not recognized.

If the example is slightly changed:

from asyncio import gather


async def returns_dict() -> dict:
    return {}


async def returns_int() -> int:
    return 1


async def test():
    reveal_type(await gather(returns_dict(), returns_int()))

it works properly.

Your Environment

• Mypy version used: 0.902
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): allow_redefinition, check_untyped_defs, namespace_packages, warn_unreachable
• Python version used: 3.9.5
• Operating system and version: macOs Big Sur

[JelleZijlstra]

This is because it doesn't know which overload to pick in https://github.com/python/typeshed/blob/master/stdlib/asyncio/tasks.pyi#L48. I'm not sure there's a good solution until we have PEP 646/TypeVarTuple.

I checked and it does infer the type of tasks correctly: main.py:14: note: Revealed type is 'Tuple[typing.Coroutine[Any, Any, builtins.dict[Any, Any]], typing.Coroutine[Any, Any, builtins.int]]'

[Tinche]

Thanks for the comment. It's a little strange since it seems all the information needed is present. Why doesn't it know which overload to pick?

[JelleZijlstra]

True, maybe it should be able to do it since we know the *args are a tuple of known size.