title: Why is there no forward declaration in concepts c++?
body: <p>When I try this example:</p>
  <pre class="lang-cpp prettyprint-override"><code>template &lt;typename T&gt;
  concept only_int = std::same_as&lt;T, int&gt;;
  
  int add_ints(only_int auto&amp;&amp;... args) {
      return (std::forward&lt;decltype(args)&gt;(args) + ... + 0);
  }
  </code></pre>
  <p>It works... but when I only declare it like this:</p>
  <pre class="lang-cpp prettyprint-override"><code>template &lt;typename T&gt;
  concept only_int;
  
  ...
  
  // defined later on...
  </code></pre>
  <p>It would throw compilation errors.</p>
  <p>Is this a missing feature? or it is intended to leave like this?</p>