Distinct overload with 2 type parameters, to solve the problem shown on Eric Lippert's blog (http://ericlippert.com/2013/07/29/a-contravariance-conundrum/)

Distinct2.cs

    public static IEnumerable<T> Distinct<T, U>(this IEnumerable<T> source, IEqualityComparer<U> comparer)
        where T : U
    {
        IEqualityComparer<T> comparer2 = (IEqualityComparer<T>)comparer;
        return Enumerable.Distinct<T>(source, comparer2);
    }