@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @NonNull
    public final <R> Observable<R> switchMapSingleDelayError(@NonNull Function<? super T, ? extends SingleSource<? extends R>> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new ObservableSwitchMapSingle<T, R>(this, mapper, true));
    }