public class C {
    [Obsolete("error", error: true)]
    public int Slice(int i, int j) => 0;
    [Obsolete("error", error: true)]
    public int this[int i] => 0;
    [Obsolete("error", error: true)]
    public int Count => 0;
    public void M() {
        _ = this[^1]; // no error
        _ = this[..]; // no error
    }
}