title: Why can't I constrain a concept
body: <p>It's not allowed to constrain a concept like this:</p>
  
  <pre><code>template&lt;typename T&gt;
  concept A = expr_1;
  
  template&lt;A T&gt;
  concept B = expr_2;   // error
  </code></pre>
  
  <p>However, it seems the same result can always be achieved like this:</p>
  
  <pre><code>template&lt;typename T&gt;
  concept A = expr_1;
  
  template&lt;typename T&gt;
  concept B = A&lt;T&gt; &amp;&amp; expr_2;   // ok
  </code></pre>
  
  <p>What's the reason for not allowing the first version, if it can always be rewritten as the second?</p>