title: Make mat-radio-button a required field in a form
body: <p>I am using a form in Angular 6, and I am disabling the submit button on this condition:</p>
  
  <pre><code>&lt;button tabindex="-1" mat-raised-button class="submit-btn" color="primary" type="submit" [disabled]="!EditItemForm.form.valid || !quantityIsValid(newQuantity.value)"&gt;Save&lt;/button&gt;
  </code></pre>
  
  <p>I have 2 <code>mat-radio-button</code> fields inside a <code>mat-radio-group</code>, and I want to disable the submit button if none of the radio buttons are selected.</p>
  
  <p>Here is what I tried:</p>
  
  <pre><code>&lt;form #EditItemForm="ngForm" (ngSubmit)="save(newQuantity.value)"&gt;
      &lt;mat-radio-group&gt;
          &lt;mat-radio-button (click)="setType('Consumable')" value="1" #consumable name="consumable" required&gt;Consumable&lt;/mat-radio-button&gt;
          &lt;mat-radio-button (click)="setType('Returnable')" value="2" #returnable name="returnable" required&gt;Returnable&lt;/mat-radio-button&gt;
      &lt;/mat-radio-group&gt;
  
      &lt;mat-form-field&gt;
          &lt;input matInput [ngModel]="data.quantity" name="newQuantity" #newQuantity placeholder="Quantity"
              type="number" required&gt;
      &lt;/mat-form-field&gt;
  
      &lt;mat-dialog-actions&gt;
          &lt;button tabindex="-1" mat-raised-button [mat-dialog-close]="false"&gt;Cancel&lt;/button&gt;
          &lt;button tabindex="-1" mat-raised-button class="submit-btn" color="primary" type="submit" [disabled]="!EditItemForm.form.valid || !quantityIsValid(newQuantity.value)"&gt;Save&lt;/button&gt;
      &lt;/mat-dialog-actions&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>So I've made the mat-radio-buttons required, I've also tried making the mat-radio-group required but both don't work.</p>
  
  <p>However, if I don't select a radio button, and I type in a quantity, the form will appear to be valid and it will enable the submit button. But, I don't want the submit button to be enabled when there's no radio button selected so how can I do this?</p>
  
  <p>Thanks</p>