cannot apply @asynccontextmanager to a function that uses TypeVarTuple

[ZeeD]

I think this is a bug in mypy.
mypy-play.net

I want to properly annotate a generic async context manager to preserve the types of the *args in the returned value.

Ts = TypeVarTuple('Ts')

@asynccontextmanager
async def fun(*args: *Ts) -> AsyncIterator[tuple[*Ts]]:
    ...

but this snippet generate the following error

main.py:11: error: Argument 1 to "asynccontextmanager" has incompatible type "Callable[[VarArg(*Ts)], AsyncIterator[tuple[*Ts]]]"; expected "Callable[[VarArg(Never), KwArg(Never)], AsyncIterator[Never]]"  [arg-type]

The function types - without the @asynccontextmanager - seems to work correctly

[sterliakov]

Fun fact: don't use empty body for testing this, asynccontextmanager seems to reveal many independent bugs...

from contextlib import asynccontextmanager
from typing import AsyncIterator, TypeVarTuple

Ts = TypeVarTuple('Ts')

@asynccontextmanager  # E: Argument 1 to "asynccontextmanager" has incompatible type "Callable[[VarArg(*Ts)], AsyncIterator[tuple[*Ts]]]"; expected "Callable[[VarArg(Never), KwArg(Never)], AsyncIterator[Never]]"  [arg-type]
async def fun(*args: *Ts) -> AsyncIterator[tuple[*Ts]]:
    yield args
    

@asynccontextmanager  # E: Argument 1 to "asynccontextmanager" has incompatible type "Callable[[int], Coroutine[Any, Any, AsyncIterator[int]]]"; expected "Callable[[int], AsyncIterator[Never]]"  [arg-type]
async def int_bad(arg: int) -> AsyncIterator[int]:  # E: Missing return statement  [empty-body]
    ...


@asynccontextmanager
async def int_good(arg: int) -> AsyncIterator[int]:
    yield arg

Compare int_bad and int_good. missing-body error apparently breaks unnesting double "async-ness" of async def ... -> AsyncIterator[...].

Playground

[sterliakov]

#17340 seems to be related.