@davedelong mis-read the proposal. Post is collapsed for posterity.
But that wouldn't be legal in today's language, as you'd be satisfying a conformance to a public protocol (Foo) with an internal definition. That, as I said at the end of the pitch, is a different feature, and it raises some issues that our proposal does not. We're not prepared to try to work out those issues, nor are we sure that if they were worked out, our use case would be covered.