Swift classes extending NSObject overriding ==

OptionalEquatable.swift

// The following shows that Equatable for NSObject is not necessarily safe
// for Swift classes that extend NSObject and override Equatable.
// Reference: http://natashatherobot.com/swift-equatable-with-optionals/

class TitleNSObject: NSObject {
    var title: String = ""
    
    init(title: String) {
        super.init()
        self.title = title
    }
}

// NSObject is already declared to be Equatable
//extension Title: Equatable {}

func ==(lhs: TitleNSObject, rhs: TitleNSObject) -> Bool {
    return lhs.title == rhs.title
}

let t = TitleNSObject(title: "a")
let tOptional: TitleNSObject? = TitleNSObject(title: "a")

t == tOptional // false

tOptional == t // false

t == tOptional! // true

tOptional! == t // true

struct Title {
    let title: String
}

extension Title: Equatable {}

func == (lhs: Title, rhs: Title) -> Bool {
    return lhs.title == rhs.title
}

let t2 = Title(title: "a")
let t2Optional: Title? = Title(title: "a")

t2 == t2Optional // true
t2Optional == t2 // true