I want to use iOS 18’s #Index in an app that still supports iOS 17.

This doesn’t work:

@Model
final class MyModel {
    if #available(iOS 18, *) {
        #Index<MyModel>([.prop1, .prop2])
    }

    var prop1: Bool
    var prop2: Bool
}

How can I conditionally use that feature only on iOS 18?