Protocols with associated protocols?

[Karl]

There was a thread about this a while ago:

[Pitch] Generalized supertype constraints

One of the most frequent frustrations I encounter when writing generic code in Swift is the requirement that supertype constraints be concrete. When I mentioned this on Twitter (https://twitter.com/anandabits/status/929958479598534656\) Doug Gregor mentioned that this feature is smaller and mostly straightforward to design and implement (https://twitter.com/dgregor79/status/929975472779288576\). I currently have a PR open to add the high-level description of this feature found below to the gen…

I'd love to see this happen.