title: Co/Contravariance? Tuple<string, IEnumerable<string>> not satisfied with List<string>
body: <p>I cant wrap my head around why I cant call a method with this signature:</p>
  
  <pre><code>public void Test(Tuple&lt;int, IEnumerable&lt;string&gt;&gt; x);
  </code></pre>
  
  <p>like that:</p>
  
  <pre><code>Test(Tuple.Create(4, new List&lt;string&gt;()))
  </code></pre>
  
  <p>I think this has to do with co/contravariance but thats just a guess. Can somebody share his cleverness?</p>