title: How to stop mat-autocomplete to take custom user input values apart from given options?
body: <p>I am using mat-auto complete component from material.angular.io. The default behavior is user can input any value as well as it gives options to choose from. Also you can add your input to chosen values. 
  You can check example here. 
  <a href="https://stackblitz.com/angular/ngmvgralayd?file=app%2Fautocomplete-simple-" rel="noreferrer">https://stackblitz.com/angular/ngmvgralayd?file=app%2Fautocomplete-simple-</a>
  example.html</p>
  
  <p>here is the code I am using for generating auto complete input field. </p>
  
  <pre><code>&lt;form class="example-form"&gt;
    &lt;mat-form-field class="example-full-width"&gt;
      &lt;input type="text" placeholder="Pick one" aria-label="Number" matInput [formControl]="myControl" [matAutocomplete]="auto" disabled="true"&gt;
      &lt;mat-autocomplete #auto="matAutocomplete"&gt;
        &lt;mat-option *ngFor="let option of options" [value]="option"&gt;
          {{ option }}
        &lt;/mat-option&gt;
      &lt;/mat-autocomplete&gt;
    &lt;/mat-form-field&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>But I want the form field to take only values from the given option and want to prevent from entering any values by users apart from given option. How to achieve this? It is like select input with auto complete feature. </p>