title: RxJs pipe and lettable operator `map`: 'this' context of type 'void' is not assignable to method's 'this' of type 'Observable<{}>'
body: <p>I have this very basic example that uses lettable operator <code>map</code> with <code>pipe</code> from <code>rxjs@5.5</code>:</p>
  
  <pre><code>import { map } from 'rxjs/operator/map';
  
  let o = of(1, 2, 3, 4).pipe(
      map((v) =&gt; v * 2)
  );
  </code></pre>
  
  <p>But it produces the error <code>Error:(34, 5) TS2684:The 'this' context of type 'void' is not assignable to method's 'this' of type 'Observable&lt;{}&gt;'.</code> What's the problem here?</p>