[swift-users] odd `==` `!=` behavior with class that inherits `NSObject`

[Zhao Xin]

Sample 1: both `==` and `!=` is true.

import Foundation


class Foo:NSObject {

    let name:String


    init(name:String) {

        self.name = name

    }


    public static func ==(lhs: Foo, rhs: Foo) -> Bool {

        guard type(of:lhs) == type(of:rhs) else { return false }

        return lhs.name == rhs.name

    }

}


let a = Foo(name: "bar")

let b = Foo(name: "bar")


print(a == b) // true

print(a != b) // true

Sample 2: Add above code to a do-block, behavior changes to expect

do {

    class Foo:NSObject {

        let name:String



        init(name:String) {

            self.name = name

        }



        public static func ==(lhs: Foo, rhs: Foo) -> Bool {

            guard type(of:lhs) == type(of:rhs) else { return false }

            return lhs.name == rhs.name

        }

    }



    let a = Foo(name: "bar")

    let b = Foo(name: "bar")



    print(a == b) // false

    print(a != b) // true

}

Sample 3: A little investigation shows that `==` didn't call NSObject's `
func isEqual(_ object: Any?) -> Bool` but `!=` did.

class Foo:NSObject {

    let name:String



    init(name:String) {

        self.name = name

    }



    public static func ==(lhs: Foo, rhs: Foo) -> Bool {

        guard type(of:lhs) == type(of:rhs) else { return false }

        return lhs.name == rhs.name

    }



    override func isEqual(to object: Any?) -> Bool {

        print("111")

        return super.isEqual(to: object)

    }



    override func isEqual(_ object: Any?) -> Bool {

        print("2222")

        return super.isEqual(object)

    }

}


let a = Foo(name: "bar")

let b = Foo(name: "bar")


print(a == b) // true

print(a != b) // 2222, true

print(!(a == b)) // false


So I am wondering what is the future? Will we keep on using `isEqual(_
 object: Any?)` with class that inherits `NSObject`, or we are trying to
drop it?

Xcode  8.3.1 (8E1000a), 3.1 (swiftlang-802.0.51 clang-802.0.41), macOS 10.12.4
(16E195)

Zhaoxin
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