using System;

using System.Linq;

using System.Collections.Generic;

​

public interface IA { }

public interface IB : IA { }

public class CIA : IA { }

public class CIAD : CIA { }

public class CIB : IB { }

public class CIBD : CIB { }

​

public static class Test

{

    public static IList<T> Something<T>(IEnumerable<T> foo) where T : IA

    {

        var bar = foo.ToList();

​

        // All those calls are legal

        Something2(new List<IA>());

        Something2(new List<IB>());

        Something2(new List<CIA>());

        Something2(new List<CIAD>());

        Something2(new List<CIB>());

        Something2(new List<CIBD>());

        Something2(bar.Cast<IA>());

​

        // This call is illegal

        Something2(bar);

​

        return bar;

    }

​

    private static void Something2(IEnumerable<IA> foo)

    {

    }

}

​

public class Program

{

    public static void Main()

    {

​

    }

}