Need some help

protocols.swift

//: Playground - noun: a place where people can play

import Cocoa

enum Error : ErrorType {
    case NIY
}

protocol LangValue {
    mutating func set(object object: LangValue, forKey key: String) throws
}

extension Int : LangValue {}

extension LangValue {
    mutating func set(object object: LangValue, forKey key: String) throws { throw Error.NIY }
}


extension Dictionary : LangValue {}

//extension Dictionary {
//    mutating func set(object object: Value, forKey key: Key) throws {
//        //self[key] = object
//    }
//}

protocol StringType : Hashable {
    var characters: String.CharacterView { get }
}
extension String : StringType {}

extension Dictionary where Key : StringType, Value : LangValue {
    mutating func set(object object: Value, forKey key: Key) throws {
        self[key] = object
    }
}

var dict = Dictionary<String, LangValue>()
do { try dict.set(object: 12, forKey: "Why?") } catch { print("\(error)") }

// QUESTION: Why is the LangValue.set(object,forKey) version called instead of
// the Dictionary.set(object,forKey) version called? String is a StringType and
// LangValue is a LangValue. Shouldn't that be the right one called?