SignificantUsing.cs

using System;

class Lorem {  }
class Ipsum {  }

namespace Dolor
{
    static class Sit
    {
        public static void Amet(this Lorem x) { }
    }
}

namespace Consectetaur
{
    static class Adipisicing
    {
        public static void Amet(this Lorem x) { }
        public static void Amet(this Ipsum x) { }
    }
}

namespace Elit
{
    using Dolor; // Nothing from this namespace whatsoever is referenced below,
                 // but removing this directive introduces a compile-time error.

    using Consectetaur;

    static class Sed
    {
        static void Eiusmod(Action<Lorem> x) { }
        static void Eiusmod(Action<Ipsum> x) { }

        static void Main()
        {
            Eiusmod(x => x.Amet());
        }
    }
}