title: Partial Specialization of Alias Templates
body: <p>Partial specializations of alias templates are not permitted:</p>
  
  <p>For example, trying to be creative, yields this error in clang:</p>
  
  <pre><code>template &lt;typename T&gt;
  using unwrapped_future_t = T;
  
  template &lt;typename T&gt;
  using unwrapped_future_t&lt;future&lt;T&gt;&gt; = typename future&lt;T&gt;::value_type;
                          ^~~~~~~~~~~
  &gt; error: partial specialization of alias templates is not permitted
  </code></pre>
  
  <p>Why is this not permitted?</p>