title: Access ngForm of parent in child
body: <p>I have a structure like this:</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;form #myForm="ngForm"&gt;
    &lt;div class=row&gt;
      &lt;app-section-a [myForm]="myForm"&gt;&lt;/app-section-a&gt;
  </code></pre>
  
  <p>and my child component:</p>
  
  <pre class="lang-js prettyprint-override"><code>@Component({
  ...
    viewProviders: [ { provide: ControlContainer, useExisting: NgForm } ]
  })
  // ...
  @Input() myForm: ElementRef;
  </code></pre>
  
  <pre class="lang-html prettyprint-override"><code>&lt;input ...&gt;
  &lt;button type=submit [disabled]="!myForm.valid"&gt;
  </code></pre>
  
  <p>and, this works perfectly to ensure that all the components in the form are counted for validity. The thing is, I don't think I should need to pass the reference to ngForm directly as an input. How do I skip that input and still make this work?</p>
  
  <p>a detail: happens to be an angular 5 project.</p>