[swift-evolution] [Review] SE-0156: Class and Subtype existentials

[Joe Groff]

> On Feb 28, 2017, at 10:39 PM, David Hart <david at hartbit.com> wrote:
> 
>> 
>> On 28 Feb 2017, at 22:53, Jordan Rose via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Nitpick: 'C<T> & P' is just 'C<T>' in this example. You'd need a refinement of 'P' to make it interesting ('C<T> & Q’).
> 
> Could generic specialisation be disallowed in constraints? I need to think about this.

I don't think there's any added complexity alone in allowing generic base class constraints in existentials, since we already support `<T: C<U>>` as a constraint on a generic parameter. It's the interaction between classes and protocols with associated types that's interesting. You don't even need a generic class:

protocol P { associatedtype T; func foo(_: T) }
class C: P { func foo(_: Int) {} }

protocol Q: P {}

let x: C & Q

-Joe
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