Extending [String:T] - Compiles but fails at linking radar raised rdar://22028034

DictionaryProtocol.swift

public protocol StringyType {
    var string:String { get }
    init(_ string:String)
}
extension String : StringyType {
    public var string:String{ return self }
    public init(_ string:String) {
        self = string
    }
}

public protocol DictionaryType {
    typealias Key
    typealias Value
    typealias Element = (Key, Value)
    subscript(key: Key) -> Value? { get set }
}

extension DictionaryType where Key:StringyType {
    public var one:Value? {
        return self[Key("One")]
    }
}
// This is the line that actually triggers the linker failure. Accidentally ommittted from original verison of Gist.
extension Dictionary : DictionaryType {}

swiftc run result

Undefined symbols for architecture x86_64:
  "__TFVSs10Dictionarym9subscriptFq_GSqq0__", referenced from:
      __TTWu0_Rq_Ss8Hashable_GVSs10Dictionaryq_q0__18DictionaryProtocol14DictionaryTypeS1_FS2_m9subscriptFqq_S2_3KeyGSqqq_S2_5Value_ in DictionaryProtocol-417391.o
ld: symbol(s) not found for architecture x86_64
<unknown>:0: error: linker command failed with exit code 1 (use -v to see invocation)