Overview

This is a verification xcode project. I witnessed the strange behavior of the app. (only iOS13)

1. Specify the protocol (protocol HogeProtocol) inherited by the subclass as the Generic Argument of super class (class Base)

protocol HogeProtocol {}
enum Hoge: HogeProtocol {}
final class Base<Hoge: HogeProtocol> {}

final class Object1<Hoge: HogeProtocol>:  Base<Object1>, HogeProtocol {}
final class Object2<Hoge: HogeProtocol>:  Base<Object1> {}
final class Object3<Hoge: HogeProtocol>:  Base<Object1> {}
.
.
final class Object1000<Hoge: HogeProtocol>:  Base<Object1> {}

2. Initialize Object1

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        _ = Object1()
    }
}

3. The following error is output from a certain number or more, and app crashes.

type metadata accessor for xxx

GIF

Get Started

1. Clone
2. Open VerificationApp.xcodeproj
3. Run (maybe clash:bomb:)

Result

The verification results in various environments are as follows.

Environment

• Xcode
  • Xcode11.1(Swift5.1)
  • Xcode11.2.1(Swift5.1.3)
  • Xcode11.3(Swift5.1.3)
• Build Configuration
  • Debug
  • Release
• Simulator
  • iOS12.2
  • iOS13.3
• Subclass Count
  • 100
  • 1000

iOS12

Subclass count is 100

Xcode Version	Debug	Release
Xcode11.1	✅	✅
Xcode11.2.1	✅	✅
Xcode11.3	✅	✅

Subclass count is 1000

Xcode Version	Debug	Release
Xcode11.1	✅	✅
Xcode11.2.1	✅	✅
Xcode11.3	✅	✅

iOS13

Subclass count is 100

Xcode Version	Debug	Release
Xcode11.1	✅	✅
Xcode11.2.1	✅	✅
Xcode11.3	✅	✅

Subclass count is 1000

Xcode Version	Debug	Release
Xcode11.1	❌	✅
Xcode11.2.1	❌	✅
Xcode11.3	❌	✅

Conclusion

This bug occurs when building iOS 13 and debug.:thinking:

(+ when you create a lot of subclasses like class Object)

Reference

• https://gist.github.com/marty-suzuki/22969715bfa01196f2a47349bf45b4c4 (@marty-suzuki)