Issue #1942902 by Dave Reid: Fixed Placeholder not supported in telephone default widget.

core/modules/telephone/lib/Drupal/telephone/Plugin/field/widget/TelephoneDefaultWidget.php

@@ -20,11 +20,27 @@
  *   label = @Translation("Telephone number"),
  *   field_types = {
  *     "telephone"
+ *   },
+ *   settings = {
+ *     "placeholder" = ""
  *   }
  * )
  */
 class TelephoneDefaultWidget extends WidgetBase {
 
+  /**
+   * Implements Drupal\field\Plugin\Type\Widget\WidgetInterface::settingsForm().
+   */
+  public function settingsForm(array $form, array &$form_state) {
+    $element['placeholder'] = array(
+      '#type' => 'textfield',
+      '#title' => t('Placeholder'),
+      '#default_value' => $this->getSetting('placeholder'),
+      '#description' => t('Text that will be shown inside the field until a value is entered. This hint is usually a sample value or a brief description of the expected format.'),
+    );
+    return $element;
+  }
+
   /**
    * Implements \Drupal\field\Plugin\Type\Widget\WidgetInterface::formElement().
    */
@@ -32,6 +48,7 @@ public function formElement(array $items, $delta, array $element, $langcode, arr
     $element['value'] = $element + array(
       '#type' => 'tel',
       '#default_value' => isset($items[$delta]['value']) ? $items[$delta]['value'] : NULL,
+      '#placeholder' => $this->getSetting('placeholder'),
     );
     return $element;
   }

core/modules/telephone/lib/Drupal/telephone/Tests/TelephoneFieldTest.php

@@ -64,6 +64,12 @@ function testTelephoneField() {
       'label' => 'Telephone Number',
       'entity_type' => 'node',
       'bundle' => 'article',
+      'widget' => array(
+        'type' => 'telephone_default',
+        'settings' => array(
+          'placeholder' => '123-456-7890',
+        ),
+      ),
     );
     field_create_instance($instance);
 
@@ -74,16 +80,21 @@ function testTelephoneField() {
       ))
       ->save();
 
+    // Display creation form.
+    $this->drupalGet('node/add/article');
+    $this->assertFieldByName("field_telephone[und][0][value]", '', 'Widget found.');
+    $this->assertRaw('placeholder="123-456-7890"');
+
     // Test basic entery of telephone field.
     $edit = array(
       "title" => $this->randomName(),
       "field_telephone[und][0][value]" => "123456789",
     );
 
-    $this->drupalPost('node/add/article', $edit, t('Save'));
+    $this->drupalPost(NULL, $edit, t('Save'));
     $this->assertRaw('<a href="tel:123456789">', 'A telephone link is provided on the article node page.');
 
-    // Add number with a space in it. Need to ensure it is stripped.
+    // Add number with a space in it. Need to ensure it is stripped on output.
     $edit = array(
       "title" => $this->randomName(),
       "field_telephone[und][0][value]" => "1234 56789",