Overriding Enum init?(rawValue: String) to not be optional

The default initializer is failable. It means that if the received parameter does not match a valid enum case it does return nil.

Now you want to do 2 incompatibles things:

1. You want to redefine the default initializer making it not failable. In fact you want a default enum value created when the received param is not valid.
2. Inside your redefined initializer you want to call a failable initializer (which no longer exists) using the same name of the new one.

This is not possible, I the 3 possible solutions as follows:

1. Creating a different init

You define a new not failable initializer with a default value, a different parameter name and inside it you call the default failable initializer.

enum Language: String {
    
    case english = "English", italian = "Italian", french = "French"
    
    init(fromRawValue: String) {
        self = Language(rawValue: fromRawValue) ?? .english
    }
}

2. Redefining the default init

You redefine the default initializer, you make it not failable and you write the full logic inside it.

enum Language: String {
    
    case english = "English", italian = "Italian", french = "French"
    
    init(rawValue: String) {
        switch rawValue {
        case "Italian": self = .italian
        case "French": self = .french
        default: self = .english
        }
    }
}

3. Creating a static func

enum Language: String {

    case english = "English", italian = "Italian", french = "French"

    static func build(rawValue: String) -> Language {
        return Language(rawValue: rawValue) ?? .english
    }
}

Now you can build a Language value writing:

let italian = Language.build(rawValue: "Italian") // Italian
let defaultValue = Language.build(rawValue: "Wrong input") // English