[swift-evolution] [Proposal draft] Conditional conformances

[Douglas Gregor]

> On Sep 27, 2016, at 5:06 PM, Jordan Rose <jordan_rose at apple.com> wrote:
> 
> Great job thinking this all through (as usual), and I’ll be very happy to have Optional and Array become Equatable. Here’s some of my thoughts on the library evolution aspect of this:
> 
> - Removing a conditional conformance isn’t allowed, obviously.
> - Adding a conditional conformance is just like adding an unconditional conformance—it needs availability info.

Right. The main wrinkle I see here is that, when you add a conditional conformance, you will effectively end up with overlapping conformances when running an old application against a new library. Do you want me to capture these cases in the proposal in a section on “Resilience” or “Library Evolution”, like I’ve tried to capture the effect on ABI Stability? (I think that makes sense)

> - It would be nice™ if making a conditional conformance more general was allowed. Since the plan doesn't allow overlapping conformances, I think this is actually implementable: just don’t put the constraints in the symbol name. I don’t know how to represent the backwards-deploying aspects of this right now, so it probably makes sense to forbid it today, but I think it would be nice if the implementation left the door open.

Yeah. It’s a different set of witness tables that one would need to gather to use the conditional conformance in the newer version of the library vs. in an older version of a library. That’s okay if we leave the witness-table-gathering to the runtime, but not so great if we statically provide the witness tables.


> On that note, what happens here?
> 
> // Module Lib
> public protocol Base {}
> public protocol Sub: Base {}
> public protocol Special: Sub {}
> 
> public struct Impl<T> {}
> extension Impl: Special where T: Special {}
> 
> 
> // Module Client
> import Lib
> 
> extension Impl: Sub where T: Sub {}
> 
> I think this gets rejected because Impl already has a conformance to Sub—the extension in Client, despite being less specialized, shows up too late to actually declare this conformance “better”. Is that correct?

Correct. Impl has a conformance to ‘Sub’ in Lib; Client cannot declare a new one, because it overlaps.  Had all of this code been in one module, it would be well-formed, because the implied conformance to ’Sub’ in the first extension would lose to the explicit conformance to Sub in the second (less-specialized) extension.

	- Doug


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