How to add an optional string extension?

Question

You can create a String extension like so:

extension String {
   func someFunc -> Bool { ... }
}

but what if you want it to apply to opt

Answer 1

extension Optional where Wrapped == String {
var isNil: Bool {
    return self == nil
}

The above answer(written by @Vlad Hatko) works fine but in swift 4 there are some issues, so I changed it to this.