[swift-evolution] Allow protocol vars to match derived types

[Joe Groff]

> On Mar 7, 2016, at 8:12 AM, Mark Anders via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Consider the following (you can paste it in a Playground to see the error):
> 
> class Node { }
> class Containable : Node{}
> 
> protocol Refers {
>     var to : Node {get}
> }
> 
> class Link : Refers {
>     var to : Node
>     init(n : Node) {
>         to = n
>     }
> }
> 
> class Contains : Refers {
>     var to : Containable
>     init(c : Containable) {
>         to = c
>     }
> }
> 
> This currently does not work because it seems that to adopt a protocol, the type of protocol var must match exactly.
> 
> It would be great if objects could be said to adopt a protocol if the type of the var is the type or a derived type.  
> This would allow me to treat the structure in a type safe way (i.e. only a Containable can have a Contains relationship), 
> while me to have a set of Refers and iterate through each Node.
> 
> Is there a reason why the type must match exactly?  Or could protocols be enhanced to to allow matching
> derived types, similar to assignment and func parameter rules?  

No fundamental reason, this just isn't something we've had time to implement. It should be straightforward to support.

-Joe
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