Accidental usage of IEnumerable.Contains rather than HashSet<T>.Contains

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace GenericConsoleTest
{
	class Base {}

	class Derived : Base {}

	internal class Program
	{
		private static void Main(string[] args)
		{
			var hashset = new HashSet<Derived> ();
			var derived = new Derived();
			hashset.Add(derived);

			// I call HashTable.Contains
			Console.WriteLine (hashset.Contains (derived));
			// I call Enumerable<Base>.Contains
			// In .Net pre-4.0 this causes compiler error
			Console.WriteLine (hashset.Contains ((Base)derived));
		}
	}
}