title: Why does Nullable!(Nullable!int) refuse to compile?
body: <p>The following code refuses to compile:</p>
  
  <pre><code>Nullable!(Nullable!int) nni = Nullable!(Nullable!int)(10);
  </code></pre>
  
  <p>With this error message:</p>
  
  <pre><code>Error: inout method nullable.Nullable!(Nullable!(immutable(int))).Nullable.this is not callable using a mutable object
  </code></pre>
  
  <p>Why?</p>