class SwiftVC<View: UIView>: UIViewController, UICollectionViewDelegateFlowLayout {
    var rootView: View? {
        return self.view as? View
    }
}

final class SubSwiftVC: SwiftVC<UIView> {
    func collectionView(_ collectionView: UICollectionView,
                        layout collectionViewLayout: UICollectionViewLayout,
                        sizeForItemAt indexPath: IndexPath) -> CGSize
    {
        return .zero
    }
}

class SwiftVCAdaptor: NSObject {
    @objc func vc() -> UIViewController {
        return SubSwiftVC()
    }
}

    UIViewController *vc = [[SwiftVCAdaptor new] vc];
    if ([vc respondsToSelector:@selector(collectionView:layout:sizeForItemAtIndexPath:)]) {
        // This is not executed.
    }