Add.Overloads.Collection.cs

using System;
using System.Collections.Generic;

static class Program
{
  static void Add(IReadOnlyCollection<object> items) { }
  static void Add(IEnumerable<IReadOnlyCollection<object>> items) { }

  static void Main() {
    var list = new List<object[]>();
    Add(list); // Which one called?
  }
}