Is there any problem that is solved by allowing this example? The asymmetry
with using [...async for...] in the same position (which is not allowed)
worries me. Do you have a real use case where it's clearer to write an
async generator expression instead of a nested async function?

def foo():
    async def inner():
        async for i in ai:
            yield i
    return inner

I would encourage you to think about various ways of nesting async
generator expressions and comprehensions to see if you can poke a hole in
the design.