typedef NSString * UIActivityType NS_EXTENSIBLE_STRING_ENUM;
@interface UIActivity : NSObject
@property(nonatomic, readonly, nullable) UIActivityType activityType;
// more declarations…
@end

class CopyActivity: UIActivity {
    // Property cannot be an @objc override because its type cannot be represented in Objective-C
    // '@objc' getter for non-'@objc' property
    // adding '@nonobjc' fixes the error, but the method isn't called at runtime
    override var activityType: UIActivityType? {
        return UIActivityType("com.example.Copy")
    }

    // overriding with a non-optional works
    override var activityType: UIActivityType {
        return UIActivityType("com.example.Copy")
    }
}