Expanding to a restricted vocabulary conforming to the protocol

I am trying to create a protocol that allows any objects to be created using JSON NSData.

I am trying to create an extension for a dictionary [String: String] that conforms to this protocol. Unfortunately, for some reason the following code doesn't work:

public protocol InitializableWithData {
    init(data: NSData?) throws 
}


extension Dictionary: InitializableWithData where Key: String, Value: String {
    public init(data: NSData?) {
        self.init()
        // Parse NSData into a [String: String]
    }
}

      

        
        
        
      

    

I am getting the following error:

Extension of type 'Dictionary' with constraints cannot have an inheritance clause

      

        
        
        
      

    

I've also tried:

extension Dictionary: InitializableWithData where Key: NSString, Value: NSString {
    public init(data: NSData?) {
        self.init()
        // Parse NSData into a [String: String]
    }
}

      

        
        
        
      

    

Considering String is a struct but still it doesn't work.