title: compilation problem/error while using an rxjs switchmap
body: <p>Hi i am getting an error in my code . I have an angular 5 formGroup and i am trying to use the pipe operation and switchMap inside.</p>
  
  <p>However it give me an error. The following is my code snippet.</p>
  
  <pre><code>this.formGroup.valueChanges.pipe(
    switchMap(
        (formValue) =&gt; {
          console.log(formValue);
        }
  
    )
  
  ).subscribe();
  </code></pre>
  
  <p>the error i is as below</p>
  
  <pre><code>Argument of type '(formValue: any) =&gt; void' is not assignable to parameter of type '(value: any, index: number) =&gt; ObservableInput&lt;{}&gt;'.
    Type 'void' is not assignable to type 'ObservableInput&lt;{}&gt;'.ts(2345)
  </code></pre>
  
  <p>really appreciate any help
  thank you </p>