My solution (without looking) to http://msmvps.com/blogs/jon_skeet/archive/2010/11/02/evil-code-overload-resolution-workaround.aspx

OverloadResolution.cs

using System;

namespace OverloadResolutionChallenge
{
    class Program : One
    {
        static void Foo<T>(T? t = default(T?)) where T : struct
        {
            Console.WriteLine("struct");
        }

        static void Foo<T>(IClass<T> t = null) where T : class
        {
            Console.WriteLine("class");
        }

        interface IClass<T> where T : class
        {
        }
        static void Main(string[] args)
        {
            Foo<int>();
            Foo<string>();
            Foo<int?>();
        }
    }

    class One
    {
        protected static void Foo<T>()
        {
            Console.WriteLine("nullable");
        }
    }
}