Generics and Protocols issue

Comics.playground

//MARK: Model structs

public struct Comic {
    let id : Int
    let title : String
    let thumbnail : Image
}

public struct Image {
    let path : String
    let imageExtension : String
    
    public func urlPath () -> String {
        return "\(path).\(imageExtension)"
    }
}

//MARK: Generics functions
public class Box<T> {
    public let value: T
    public init(_ value: T) { self.value = value }
}

func sampleComics() -> Box<Comic> {
    
    let testImage = Image(path: "http://imgur.com/erflwm1", imageExtension: "gif")
    let testComic = Comic(id: 0, title: "funny-gif", thumbnail: testImage)
    
    return Box(testComic)
}

//MARK: MVVM stuff
protocol ComicViewModel {
    var displayTitle : String { get }
    var displayThumbnailPath : String { get }
}

extension Comic : ComicViewModel {
    var displayTitle : String {
        return title
    }
    
    var displayThumbnailPath : String {
        return thumbnail.urlPath()
    }
}

func fetchComicsViewModel() -> Box<ComicViewModel> {
    return sampleComics()
}