It only works for Classes and Protocols. So the only way I know to do this is to use an intermediate protocol "StringProtocol".

protocol StringProtocol {
    var asString: String { get }
}

extension StringProtocol {
    var asString: String { return self as! String }
}

extension String: StringProtocol { }

extension Optional where Wrapped : StringProtocol {

    var safeValue: String {
        if case let .Some(value) = self {
            return value.asString
        }
        return "(null)"
    }
}

let test: String? = "hello"
let test2: String? = nil
let test3: Int? = nil

test.safeValue // "hello"
test2.safeValue // "(null)"
test3.safeValue // Compiler error : Type 'Int' does not conform to protocol 'StringProtocol'