The following code fails with "OSError: [WinError 10022] An invalid argument was supplied".

import asyncio
loop = asyncio.ProactorEventLoop()
asyncio.set_event_loop(loop)
loop.run_until_complete(asyncio.open_connection("::1", 4242))

This is a regression in 3.5.2. 3.5.1 does not have this bug. Connecting to 127.0.0.1 does not cause the problem.

The first offending commit is this one:
https://github.com/python/cpython/commit/03df54d549173e17e1cf9a767199de32a363aa6b

more specifically "return af, type, proto, '', (host, port)". For IPv6, it should be "(host, port, flow info, scope id)" instead of just "(host, port)".

This is still a problem with Python 3.5.3 and 3.6.0.

Any chance someone could look into this bug?

The bug is reproducible on Python 3.7. The connect call always fails when I
try to use connect to any IPv6 address with ProactorEventLoop.

The bug can be bypassed by adding "%0" to the IPv6 address. This will cause
to trigger this if construction:
https://github.com/python/cpython/blob/e76ac9d4ef77b74ea7de768f4de7c38893fad539/Lib/asyncio/base_events.py#L133

As mentioned by Sebastian, it happen because the _ipaddr_info function returns
only the host and the port for IPv6: return af, type, proto, '', (host, port)
https://github.com/python/cpython/blob/e76ac9d4ef77b74ea7de768f4de7c38893fad539/Lib/asyncio/base_events.py#L142

This (host, port) tuple used in ConnectEx call, but it expect 4-element tuple
to connect IPv6: 
https://github.com/python/cpython/blob/55edd0c185ad2d895b5d73e47d67049bc156b654/Modules/overlapped.c#L1090
Instead it tries to connect, using IPv4 and fails.

The bug can be fixed by doing the following:
    if af == socket.AF_INET6:
        return af, type, proto, '', (host, port, 0, 0)
    else:
        return af, type, proto, '', (host, port)

instead of
    return af, type, proto, '', (host, port) 

in
https://github.com/python/cpython/blob/e76ac9d4ef77b74ea7de768f4de7c38893fad539/Lib/asyncio/base_events.py#L142

Somebody please submit a PR so this can be fixed in 3.7.1 and the fix can
be backported to 3.6.7.

> Somebody please submit a PR so this can be fixed in 3.7.1 and the fix can
> be backported to 3.6.7.

Somehow I overlooked this one when I was sifting the issues we needed to fix in 3.7.  I've opened a PR.

New changeset d904c238ca3551750cb97d15d827c3e525970867 by Yury Selivanov in branch 'master':
bpo-27500: Fix static version of getaddrinfo to resolve IPv6 (GH-7993)
https://github.com/python/cpython/commit/d904c238ca3551750cb97d15d827c3e525970867

New changeset 3ed44141216aa1d2d8dd7d170c2dc216a6e718b6 by Miss Islington (bot) in branch '3.7':
bpo-27500: Fix static version of getaddrinfo to resolve IPv6 (GH-7993)
https://github.com/python/cpython/commit/3ed44141216aa1d2d8dd7d170c2dc216a6e718b6

New changeset c00144cd7741a5060662e85717f6559ad46d02aa by Miss Islington (bot) in branch '3.6':
bpo-27500: Fix static version of getaddrinfo to resolve IPv6 (GH-7993)
https://github.com/python/cpython/commit/c00144cd7741a5060662e85717f6559ad46d02aa

Thank you!