> On Sep 27, 2016, at 5:06 PM, Jordan Rose <jordan_r...@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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