The new boostedblob error is related to this line here, and is because asyncio.create_task is annotated as requiring a coroutine following #6779, and this PR would change the definition of AsyncIterator.__anext__ such that it wouldn't necessarily be guaranteed to return a coroutine anymore.

Cc. @sobolevn, since this partially reverts your PR 🙂

Would SupportsAnext need fixing? https://github.com/python/typeshed/blob/master/stdlib/_typeshed/__init__.pyi#L36

currently it requires a coroutine - but any Awaitable is supported (it gets wrapped with anext_awaitable with a default)

Would SupportsAnext need fixing? https://github.com/python/typeshed/blob/master/stdlib/_typeshed/__init__.pyi#L36

Good spot. contextlib._SupportsAclose as well.

anext is actually slightly odd as it returns whatever __anext__ returns - eg an Awaitable or even not an awaitable!

Python 3.10.2 (main, Jan 15 2022, 18:02:07) [GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class Foo:
...     def __anext__(self):
...         return "hello"
... 
>>> anext(Foo())
'hello'
>>> 

so the type annotation for builtins.anext should look more like this:

class SupportsAnyAnext(Protocol[_T_co]):
    def __anext__(self) -> _T_co: ...

class SupportsAwaitableAnext(Protcol[_T_co]):
    def __anext__(self) -> Awaitable[_T_co]: ...

@overload
def anext(__i: SupportsAnyAnext[_T]) -> _T: ...
@overload
async def anext(__i: SupportsAwaitableAnext[_T], default: _VT) -> _T | _VT: ...

so that anext doesn't incorrectly promote an AsyncIterable.__next__ Awaitable into a Coroutine

anext is actually slightly odd as it returns whatever __anext__ returns - eg an Awaitable or even not an awaitable!

Sure, but I'd imagine that if __anext__ returns something that isn't awaitable, it's almost certainly indicative of a programming error. So surely we'd be doing the developer a favour by raising an error if they try to call anext on an object that has an __anext__ method that doesn't return an awaitable?

my point was more about preserving the return type of the .__anext__ call:

_Awaitable_T_co = TypeVar("_Awaitable_T_co", bound=Awaitable, covariant=True)  # bound isn't strictly true - anext supports any return value

class SupportsAnext(Protocol[_Awaitable_T_co]):
    def __anext__(self) -> _Awaitable_T_co: ...

class SupportsAwaitableAnext(Protocol[_T_co]):
    def __anext__(self) -> Awaitable[_T_co]: ...

_Awaitable_T = TypeVar("_Awaitable_T", bound=Awaitable)

@overload
def anext(__i: SupportsAnext[_Awaitable_T]) -> _Awaitable_T: ...
@overload
async def anext(__i: SupportsAwaitableAnext[_T], default: _VT) -> _T | _VT: ...

my point was more about preserving the return type of the .__anext__ call:

It's frying my brain somewhat, but I think I understand. Thanks for the explanation!

Better now @graingert?

Thanks for the review @graingert! (I'll leave this open for a while to give other maintainers a chance to review.)

(Planning to merge this in a day or so, unless any other maintainers want to review, since it's sort of blocking python/mypy#12321)

Diff from mypy_primer, showing the effect of this PR on open source code:

tornado (https://github.com/tornadoweb/tornado)
- tornado/gen.py:429: error: Incompatible return value type (got "WaitIterator", expected "AsyncIterator[Any]")
- tornado/gen.py:429: note: Following member(s) of "WaitIterator" have conflicts:
- tornado/gen.py:429: note:     Expected:
- tornado/gen.py:429: note:         def __anext__(self) -> Coroutine[Any, Any, Any]
- tornado/gen.py:429: note:     Got:
- tornado/gen.py:429: note:         def __anext__(self) -> Future[Any]

boostedblob (https://github.com/hauntsaninja/boostedblob)
+ boostedblob/boost.py:547: error: Need type annotation for "task"
+ boostedblob/boost.py:547: error: Argument 1 to "create_task" has incompatible type "Awaitable[T]"; expected "Union[Generator[Any, None, <nothing>], Coroutine[Any, Any, <nothing>]]"