title: Types have separate declarations of a private property
body: <p>I am learning Angular (which is written in TypeScript) and I stumbled upon this error:</p>
  
  <p><strong>Class 'SnackbarService' incorrectly extends base class 'MatSnackBar'.
  Types have separate declarations of a private property '_overlay'.</strong></p>
  
  <p>when trying to extend <code>MatSnackBar</code> from <code>@angular/material</code>.</p>
  
  <p>This is my code:</p>
  
  <pre class="lang-js prettyprint-override"><code>import { MatSnackBar } from '@angular/material';
  import { Overlay } from '@angular/cdk/overlay';
  import { LiveAnnouncer } from '@angular/cdk/a11y';
  ...
  
  export class SnackbarService extends MatSnackBar {
  
    constructor(
      private _overlay: Overlay, 
      private _liveAnnouncer: LiveAnnouncer,
      ...
    ) {
      super(_overlay, _liveAnnouncer, ...);
      }
    }
  }
  </code></pre>
  
  <p>Any help with any type of explanation on why this happens would be really be appreciated.</p>