I think this may be related to covariance (see https://mypy.readthedocs.io/en/stable/generics.html#variance-of-generic-types), like #3119. I think some uses of Dict in type annotations may need to be changed to Mapping (just like some uses of List should be replaced by Sequence). Annotating the type of req_dict as Dict[Any, Awaitable] may work as a workaround.

Thanks for the workaround, but it didn't work for me. However, I've solved the problem by implementing my own version of multi based on native functions:

async def multi_coro(self, coro: Dict[Any, Awaitable], return_exceptions: bool = False) -> Dict[Any, Any]:
    res_list = await asyncio.gather(*coro.values(), return_exceptions=return_exceptions)
    return dict(zip(coro.keys(), res_list))

Since asyncio.gather preserves the order of the coroutines in the result list, this should behave identical to multi (as long as return_exceptions is False). At least it seems to work so far...

Yes, asyncio.gather is very similar to gen.multi (without the dict functionality). Most of the time they're interchangeable but there are some subtle differences (and I'm not sure I know all of them). The main difference has to do with what happens when Future.cancel is called (Tornado's support for cancellation is poor and it tends to just ignore that method; asyncio is better about actually cancelling things although this can cause surprises for code expecting the Tornado semantics).

i'm experiencing this as well when using pyright. I'm seeing errors like this:

  error: Argument of type "list[CoroutineType[Any, Any, dict[str, Any] | str | None]]" cannot be assigned to parameter "children" of type "List[_Yieldable] | Dict[Any, _Yieldable]" in function "multi"
    Type "list[CoroutineType[Any, Any, dict[str, Any] | str | None]]" is not assignable to type "List[_Yieldable] | Dict[Any, _Yieldable]"
      "list[CoroutineType[Any, Any, dict[str, Any] | str | None]]" is not assignable to "List[_Yieldable]"
        Type parameter "_T@list" is invariant, but "CoroutineType[Any, Any, dict[str, Any] | str | None]" is not the same as "_Yieldable"
        Consider switching from "list" to "Sequence" which is covariant
      "list[CoroutineType[Any, Any, dict[str, Any] | str | None]]" is not assignable to "Dict[Any, _Yieldable]" (reportArgumentType)

I suspect that changing the type annotation of gen.multi's children param from Union[List[_Yieldable], Dict[Any, _Yieldable]], to Union[Sequence[_Yieldable], Mapping[Any, _Yieldable]], would make it more usable - but I haven't had a chance to sit down and prove this myself. Just wanted to add this comment in case it's helpful. Thx!