Add info about protocols with superclass requirement [SE-0156]

[valeriyvan]

Add info about protocols with superclass requirement.

Fixes: #445
Fixes: rdar://171309018

[amartini51]

Thanks for taking the time to write up this addition to the documentation! Overall, I think these changes look good, but we should work through a couple minor questions.

This looks like the correct place to discuss this in the guide. Adding a code listing is good also, to match the existing tone of the docs in this chapter, and to show the syntax in an approachable way.

Can we find a less contrived example? The Renderer class is good — it feels real, which is like how other examples in the guide are written — but the code doesn't actually do anything.

What updates should we make to the reference? I don't see this syntax discussed in Protocol Declaration. If you'd like, I can write a draft of the changes there, or I can defer to you.

As a minor processes question, was this syntax new in SE-0156? I see discussion in that proposal of protocol composition, but not of this being newly allowed. Because of the similarities between conformance to a protocol and subclassing a superclass, I'm guessing this syntax might predate the Swift Evolution process. If it's not from that proposal, let's remove the SE number from the PR name.

@hborla Can you help identify a technical reviewer for this PR?

CC @invalidname who will be working on Swift docs when I'm out of the office starting sometime next month.

[valeriyvan]

As a minor processes question, was this syntax new in SE-0156? I see discussion in that proposal of protocol composition, but not of this being newly allowed. Because of the similarities between conformance to a protocol and subclassing a superclass, I'm guessing this syntax might predate the Swift Evolution process. If it's not from that proposal, let's remove the SE number from the PR name.

SE-0156 didn't introduce new syntax but new semantics allowing existential to have specific class requirements: https://github.com/swiftlang/swift-evolution/blob/main/proposals/0156-subclass-existentials.md#proposed-solution.

[valeriyvan]

What updates should we make to the reference? I don't see this syntax discussed in Protocol Declaration. If you'd like, I can write a draft of the changes there, or I can defer to you.

I have applied minimal changes into Declarations.md as well. Please review.

[valeriyvan]

Can we find a less contrived example? The Renderer class is good — it feels real, which is like how other examples in the guide are written — but the code doesn't actually do anything.

Updated example. Please have a look.

[valeriyvan]

@amartini51 please review recent changes

[amartini51]

Thanks for your patience while I was out on new-parent leave! This revised code listing looks good now. I'll follow up again to get a technical reviewer to look at these changes.

I pushed two minor/mechanical changes to your PR, and noted one paragraph in the reference that I don't think needs to be added.

[amartini51]

I don't think you need this paragraph. The reference doesn't follow the three-part "intro, code listing, walkthrough" format that the guide uses. The reference generally doesn't link to the guide, except in places where only the chapter in the guide has the full information.