Support for async comprehensions is broken

[gvanrossum]

Here's a simple example:

from typing import AsyncIterable

class bar(AsyncIterable[int]):
    def __aiter__(self):
        self.count = 3
        return self
    async def __anext__(self):
        self.count -= 1
        if self.count >= 0:
            return self.count
        raise StopAsyncIteration

async def foo() -> None:
    x = [i async for i in bar()]
    for i in x:
        print(i)

def main():
    a = foo()
    try:
        print(a.__await__().send(None))
    except StopIteration as e:
        print("stop with value =", e.value)

main()

When run with Python 3 this prints

2
1
0
stop with value = None

However when type-checked with mypy we get an error on the async comprehension in foo():

__tmp__.py:14: error: Iterable expected
__tmp__.py:14: error: "bar" has no attribute "__iter__"; maybe "__aiter__"?

(Note the irony in the error message. :-)

This makes me think that mypy doesn't check the async flag on the comprehension's AST node. If we replace foo() with

async def foo() -> None:
    async for i in bar():
        print(i)

the output is the same and mypy accepts it, strengthening my diagnosis.