Extending swift optional for default values

Question

I'm trying to extend Swift's Optional type with default values. Providing empty values in API requests should raise an exception. I've done this for the String type, but I can't achieve the same result with the Integer type:

extension Optional where Wrapped == String {

    var unwrappedValue: String {

        get {
            switch self {

            case .some(let value):
                return value

            case .none:
                return ""
            }
        }
    }
}

The Integer version is throwing the following Error:

Protocol 'Integer' can only be used as a generic constraint because it has Self or associated type requirements

extension Optional where Wrapped == Integer {

    var unwrappedValue: Integer {

        get {
            switch self {

            case .some(let value):
                return value

            case .none:
                return 0
            }
        }
    }
}

Answer 1

If you use this for a lot of Types you might want to consider the following addition to the answer of Leo Dabus:

protocol Defaultable {
    static var defaultValue: Self { get }
}

extension Optional where Wrapped: Defaultable {
    var unwrappedValue: Wrapped { return self ?? Wrapped.defaultValue }
}

This way you can extend your types very easily:

extension Int: Defaultable {
    static var defaultValue: Int { return 0 }
}

extension String: Defaultable {
    static var defaultValue: String { return "" }
}

extension Array: Defaultable {
    static var defaultValue: Array<Element> { return [] }
}

And usage goes like this:

let optionalInt: Int? = 10 // Optional(10)
let unwrappedInt = optionalInt.unwrappedValue // 10

let optionalString: String? = "Hello" // Optional("Hello")
let unwrappedString = optionalString.unwrappedValue // "Hello"

let optionalArray: [Int]? = nil // nil
let unwrappedArray = optionalArray.unwrappedValue // []