Incorrect invalid-return-type for constrained generics in async functions

[Apakottur]

Summary

Consider the following code:

# a.py

from typing import Any, Generic, TypeVar


T = TypeVar("T", bound=tuple[Any, ...])


class Select(Generic[T]):
    pass


def first[T](v: T) -> Select[tuple[T]]:
    raise NotImplementedError


async def second[T](query: Select[tuple[T]]) -> T:
    raise NotImplementedError


async def variant_one() -> int:
    a = await second(first(123))
    return a


async def variant_two() -> int:
    return await second(first(123))

Running ty check a.py gives the following error:

error[invalid-return-type]: Return type does not match returned value
  --> a.py:26:28
   |
26 | async def variant_two() -> int:
   |                            --- Expected `int` because of return type
27 |     return await second(first(123))
   |            ^^^^^^^^^^^^^^^^^^^^^^^^ expected `int`, found `int | None`
   |
info: element `None` of union `int | None` is not assignable to `int`

Found 1 diagnostic

The functions variant_one and variant_two do the exact same thing, but returning the value directly leads to a type error. mypy and pyright both accept this entire file.

This code snippet is based on a real life scenario with SQLAlchemy, figured it's better to not have any dependencies.

Version

0.0.40

[carljm]

Thanks for the report! I recorded the fully generalized form of this in #3596. That bug may require deeper work to fix, but I think we can fix this likely more-common async case by extending some existing special-casing of Generator.