Swift generic type that conform to protocol cannot be used to refer protocol?

Question

import UIKit

protocol Identifiable
{
}

protocol Storage
{
    func test() -> Data<Identifiable>
}

class DiskStorage<T where T:Identifiable, T:NSCoding>:Storage
{
    func test() -> Data<Identifiable>
    {
       return Data<T>() //error: T is not identical to Identifiable
    }
}

class Data<T where T:Identifiable>
{

}

I thought it would be possible to use generic type that conform protocol in order to call method that reference that same protocol. How to cast it? Tried almost everything, nothing is working. Maybe I understand something wrong...

Any help on this one guys? Thanks a lot

Answer 1

try this

protocol Identifiable
{}

class Data<T where T:Identifiable>
{}

protocol Storage
{
    typealias Element : Identifiable
    func test() -> Data<Element>
}

class DiskStorage<T where T:Identifiable, T:NSCoding>:Storage
{
    func test() -> Data<T>
    {
       return Data<T>()
    }
}

// from REPL
 32> var s = DiskStorage<Foo>()
s: DiskStorage<Foo> = {}
 33> s.test()
$R0: Data<Foo> = {}

---

As I pointed out in this answer, Data<T> have no relationship to Data<Identifiable>. So you can't use Data<T> in place that expecting Data<Identifiable> and hence the compile error.