Sequence of elements, but “join is unavailable: call the 'joinWithSeparator()'” error

Question

Why do I get the error "join is unavailable: call the joinWithSeparator()" at line 16 (the last line below) when I try to run it on playground? And, how can I fix it?

class Person {
    var firstName: String?
    var lastName: String?
    let gender = "female"

    func fullName() -> String {
        var parts: [String] = []

        if let firstName = self.firstName {
            parts += [firstName]
        }

        if let lastName = self.lastName {
            parts += [lastName]
        }
        return " ".join(parts)
    }
} 

Answer 1

The error message tells you what the problem is, and it tells you how to fix it. Read the error message! Do what the error message says!

return parts.joinWithSeparator(" ")

Answer 2

The right answer to "why"-part of question is:

Because String's method join was removed in Swift 2. So, actually, it is not "unavailable", it just doesn't exist in Swift 2 at all.

SUDDENLY.