How to extend a function of a protocol in swift 3?

I have following code:

protocol Protocol_1 {
    var prop_1: String { get set }
    var prop_2: Int { get set }
}

protocol Protocol_2 {
    var prop_3: Double { get set }
    var prop_4: Bool { get set }
}

extension Protocol_1 {
    mutating func set<T>(value: T, forKey key: String)
    {
        switch key {
        case "prop_1":
            if value is String {
                prop_1 = value as! String
            }

        case "prop_2":
            if value is Int {
                prop_2 = value as! Int
            }

        default:
            break
        }
    }
}

extension Protocol_2 {
    mutating func set<T>(value: T, forKey key: String)
    {
        switch key {
        case "prop_3":
            if value is Double {
                prop_3 = value as! Double
            }

        case "prop_4":
            if value is Bool {
                prop_4 = value as! Bool
            }

        default:
            break
        }
    }
}

struct MyStruct : Protocol_1, Protocol_2 {
    var prop_1: String
    var prop_2: Int
    var prop_3: Double
    var prop_4: Bool
}

var myStruct = MyStruct(prop_1: "hello", prop_2: 0, prop_3: 3.5, prop_4: true)
myStruct.set(value: "bye", forKey: "prop_1")

Now the playground gives me an error because it is not clear what set function should be called. Playground execution failed: error: ProtocolsPlayground.playground:59:1: error: ambiguous use of 'set(value:forKey:)' myStruct.set(value: "bye", forKey: "prop_1")

This is clear but how can I achieve such function extension or is there a work around? Especially if the Protocol_1 is not editable.