How do I extend this dictionary?

ExtendThisDictionaryPlayground.swift

import Foundation

protocol ComponentObject {
    var name: String { get }
}

private var _idPrev = 0;
private func defaultName(baseName: String) -> String {
    return baseName + String(++_idPrev)
}

class Thing : ComponentObject {
    let name: String
    
    init(name: String? = nil) {
        self.name = name ?? defaultName("thing")
    }
}

class OtherThing : ComponentObject {
    let name: String
    
    init(name: String? = nil) {
        self.name = name ?? defaultName("other")
    }
}

let w = Thing()
w.name

let m = OtherThing()
m.name

var d = [String: ComponentObject]()
d[w.name] = w
d[m.name] = m

let c = d["other2"]!

c.name

When I write the extension like this:

public extension Dictionary where
    Key: StringLiteralConvertible, Value: ComponentObject {
    public func getByPath(path: Key) -> Value? {
        // TODO: Split the path and descend into components...
        return self[path]
    }
}

I get an error when I call getByPath:

let c2 = d.getByPath("other2")! // Error: Using ComponentObject as a concrete type conforming to protocol 'ComponentObject' is not supported.

When I make ComponentObject a class instead of a protocol everything works.

However, Apple states that "Because it is a type, you can use a protocol in many places where other types are allowed" here and shows examples of using protocols as the value in a collection here. So, what am I doing wrong?

The answers were found and documented over at StackOverflow.com