title: Why can't I pipe a BehaviourSubject?
body: <p>I'm not sure why <code>subscribe</code> works with <code>BehaviourSubject</code> but not <code>pipe</code>.</p>
  
  <p>For example, the code below prints out?</p>
  
  <pre><code>subscribe: 123
  subscribe: 456
  </code></pre>
  
  <p>Why doesn't it also print</p>
  
  <pre><code>pipe: 123
  pipe: 456
  </code></pre>
  
  <p>Code:</p>
  
  <pre><code>import {tap } from 'rxjs/operators';
  import { BehaviorSubject } from 'rxjs';
  
  const subject = new BehaviorSubject(123);
  subject.subscribe(p =&gt; console.log("subscribe: " + p));
  subject.pipe(
    tap(p =&gt; {console.log("pipe:" + p);})
  )
  subject.next(456);
  </code></pre>