[swift-evolution] Subclass Existentials

[jaden.geller at gmail.com]

It's pretty bizarre that conforming to `protocol P: Q {}` requires an explicit conformance to Q when Q is a class but not when Q is a protocol. It's the same syntax, but conformance is only "inherited" for protocols.

> On Feb 1, 2017, at 3:47 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> 
> Sent from my iPad
> 
>> On Feb 1, 2017, at 5:11 PM, David Sweeris via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 
>>> On Feb 1, 2017, at 3:01 PM, David Hart via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> 
>>> On 1 Feb 2017, at 22:54, Douglas Gregor <dgregor at apple.com> wrote:
>>> 
>>>> 
>>>>> On Jan 29, 2017, at 8:44 PM, Slava Pestov via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> 
>>>>> This is a nice generalization of the existing protocol composition syntax. Implementation might get a little tricky — this touches a lot of things, such as inheritance clauses, constraints in generic signatures, and casts. It would require thorough testing.
>>>>> 
>>>>> There are also a few odd corner cases to sort out:
>>>>> 
>>>>> typealias T = SomeClass & SomeProtocol
>>>>> 
>>>>> class C : T { // should we allow this? probably yes
>>>>> }
>>>>> 
>>>>> protocol P : T { // should we allow this? probably no
>>>>> }
>>>> 
>>>> IIRC, we already allow the latter where T is a typealias for SomeProtocol1 & SomeProtocol2. Why not allow it generally?
>>> 
>>> Well what would it mean? A protocol can't inherit or conform to a class.
>> 
>> That anything which conforms to that protocol must subclass SomeClass?
> 
> Yes, this is exactly what I would expect.  I was calling it a supertype requirement earlier in the thread.
> 
>> 
>> - Dave Sweeris
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