Mix of Iterator Patteren and Pattern Matching

EnumerableHelper.cs

public static IEnumerable<T> AddRangeImmuted<T>(this IEnumerable<T>? source, IEnumerable<T>? items)
{
    return (source, items) switch
    {
        (null, null) => Enumerable.Empty<T>(),
        (_, null) => source,
        (null, _) => items,
        (_, _) => addRangeImmutedIterator(source, items)
    };
    static IEnumerable<T> addRangeImmutedIterator(IEnumerable<T> source, IEnumerable<T> items)
    {
        foreach (var item in source)
        {
            yield return item;
        }
        foreach (var item in items)
        {
            yield return item;
        }
    }
}