Extension method to group the elements by key in source sequence

EnumerableExtension.cs

public static class EnumerableExtension
{
    public static IEnumerable<IGrouping<TSource, TSource>> GroupBy<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> keyFinder)
    {
        if (source is null)
            throw new ArgumentNullException(nameof(source));
        if (keyFinder is null)
            throw new ArgumentNullException(nameof(keyFinder));

        TSource key = default;

        return source
            .Select(x =>
            {
                if (x is null)
                    return (isValid: false, default);

                if (keyFinder.Invoke(x))
                    key = x;

                return (isValid: (key is not null), value: x);
            })
            .Where(x => x.isValid)
            .GroupBy(_ => key, x => x.value);
    }
}