title: Why can't concept refinement use the terse syntax
body: <p>When refining concepts, the way it is consistently done in the standard is to fully write out the concept being refined. For instance, in <a href="http://eel.is/c++draft/concepts.integral" rel="noreferrer">[concepts.integral]</a>, <code>SignedIntegral</code> refines <code>Integral</code> like so:</p>
  
  <pre><code>template&lt;class T&gt;
    concept Integral = is_integral_v&lt;T&gt;;
  template&lt;class T&gt;
    concept SignedIntegral = Integral&lt;T&gt; &amp;&amp; is_signed_v&lt;T&gt;;
  </code></pre>
  
  <p>Why can't the refined concept be written as:</p>
  
  <pre><code>template&lt;Integral T&gt;
    concept SignedIntegral2 = is_signed_v&lt;T&gt;;
  </code></pre>
  
  <p><code>SignedIntegral2</code> seems to have the same obvious meaning of <code>SignedIntegral</code>, but it doesn't even compile on clang. Is there a reason for this?</p>