I think the root cause of this bug is a bit of confusion.

The "customer-facing" asyncio API, create_connection(), takes two arguments: host and port. The lower-level API that actually deal with connecting sockets, socket.connect() and loop.sock_connect(), takes one argument: the address tuple. These are *not the same thing*, despite an IPv4 address tuple having two elements (host, port), and must not be mixed.

_ensure_resolved() is the function responsible for turning host + port into an address tuple, and it does the right thing, turning host="fe80::1%lo",port=12345 into ('fe80::1', 12345, 0, 1) correctly. The mistake is taking the address tuple and passing it through _ensure_resolved() again, since that's not the correct input type for it: the only correct input type is host + port.

So I think the appropriate fix for this bug is to make sure _ensure_resolved is only called once. In particular, BaseSelectorEventLoop.sock_connect() https://github.com/python/cpython/blob/3bc0ebab17bf5a2c29d2214743c82034f82e6573/Lib/asyncio/selector_events.py#L458 should not call _ensure_resolved(). It might be a good idea to add some comments clarifying that sock_connect() takes an address tuple argument, not host + port, and likewise for sock_connect() on each event loop implementation.