`objc_getClassList` crashed with an iOS 16 available class, in device that lower than iOS 16

[kemchenj]

Describe the bug

objc_getClassList crash in the code below:

#if canImport(RoomPlan)

import RoomPlan

@available(iOS 16, *)
class Foo: NSObject {
  var bar: CapturedRoom?
}

#endif

import SwiftUI

@main
struct MetadataAccessorCrashApp: App {
  var body: some Scene {
    WindowGroup {
      Text("")
        .onAppear {
          // Crash here
          let _ = objc_getClassList(nil, 0)
        }
    }
  }
}

Steps To Reproduce
Steps to reproduce the behavior:

1. Clone the repository https://github.com/kemchenj/TypeMetadataAccessorCrash
2. Open MetadataAccessorCrash.xcodeproj project
3. Build and run on any iOS simulator or device that OS version lower than iOS 16 (e.g. iOS 15.5)
4. App will crash when objc_getClassList was called

Expected behavior

Not crash.

Screenshots

Stacktrace:

Environment (please fill out the following information)

• OS: macOS
• Xcode 14.0

Additional context

[theblixguy]

CC @mikeash

[mikeash]

This is, unfortunately, a known problem. The ObjC runtime doesn't know about the limited availability and tries to instantiate the class even though it won't work on the older OS version.

If possible, avoid objc_getClassList and objc_copyClassList. In addition to this problem, they're extremely inefficient and it's usually better to maintain an explicit list of the classes you're interested in.

It's also possible to work around this problem by adding an unused generic parameter to the class. @available(iOS 16, *) class Foo<T>: NSObject will hide it from the ObjC runtime so it will no longer try to instantiate Foo.

[kemchenj]

This is, unfortunately, a known problem. The ObjC runtime doesn't know about the limited availability and tries to instantiate the class even though it won't work on the older OS version.

If possible, avoid objc_getClassList and objc_copyClassList. In addition to this problem, they're extremely inefficient and it's usually better to maintain an explicit list of the classes you're interested in.

It's also possible to work around this problem by adding an unused generic parameter to the class. @available(iOS 16, *) class Foo<T>: NSObject will hide it from the ObjC runtime so it will no longer try to instantiate Foo.

Ok, look likes I should go with the generic parameter way. Thanks for your explanation.