title: What does "@extend must be used with a %placeholder" mean?
body: <p>While linting the following SASS statements, I get <code>@extend must be used with a %placeholder</code> warning.</p>
  
  <pre><code>.reg-text {
    color: #202226;
    font-family: $font-page;
    font-size: 17px;
    line-height: 25px;
  }
  
  .reg-text-header {
    @extend .reg-text;
    font-weight: 600;
  }
  </code></pre>
  
  <p>What does warning mean and how do I fix it.  Afaik, <code>.extend</code> exists for the purpose of extending classes.</p>