namespace System.Collections.Immutable
{
    public static class ImmutableDictionary
    {
        public static ImmutableDictionary<TKey, TValue>! ToImmutableDictionary<TSource, TKey, TValue>(this IEnumerable<TSource>! source, Func<TSource, TKey>! keySelector, Func<TSource, TValue>! elementSelector)
            where TKey, notnull;
    }
}