title: Bootstrap 4 invalid feedback with input group not displaying
body: <p>I have been looking into Bootstrap 4 - beta, however when using <code>.is-invalid</code> with <code>.input-group</code> it doesn't seem to show up.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" rel="stylesheet" /&gt;
  &lt;div class="form-group"&gt;
    &lt;label for="label"&gt;Label&lt;/label&gt;
    &lt;div class="input-group"&gt;
  
      &lt;div class="input-group-addon"&gt;
        label
      &lt;/div&gt;
      &lt;input type="text" value="" name="label" class="form-control is-invalid"&gt;
    &lt;/div&gt;
  
    &lt;div class="invalid-feedback is-invalid"&gt;
      &lt;strong&gt;Invalid Label&lt;/strong&gt;
    &lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>How are you meant to display an invalid message while using <code>.input-group</code>?</p>
  
  <p>Adding the following CSS works as a workaround, but it seems odd.</p>
  
  <pre><code>.form-group.is-invalid {
      .invalid-feedback {
          display: block;
      }
  }
  </code></pre>