覆盖扩展中的方法，Swift

Question

Since which version of Swift does the following code no longer build?

import Foundation

@objc class Class: NSObject {
  @objc func str() -> String {
    return "Hello, playground"
  }
}

class Subclass: Class {

}

extension Subclass {
  override func str() -> String {
    return "Hi"
  }
}

From my understanding, previous versions of Swift compiled this code with unexpected results. On Swift 5.1, it no longer builds.

Answer 1

as you can see here ->Extensions documentation you can add new functionality but you can't override methods in extensions.

NOTE

Extensions can add new functionality to a type, but they cannot override existing functionality.

So you can't do it in Swift 5.1

Answer 2

Dynamic modifier

You can do it using the dynamic modifier

@objc class Animal: NSObject {

    @objc dynamic func saySomething() {
        print("I am an Animal")
    }

}

@objc class Dog: Animal { }

extension Dog {

    override func saySomething() {
        print("I am a Dog")
    }

}

Dog().saySomething() // I am a Dog

Tested with Swift 5.1.3