Issue #2347711 by yched, undertext: Fixed FieldItemlListInterface::processDefaultValue($default_value) is expected to massage polymorphic data.

core/lib/Drupal/Core/Field/BaseFieldDefinition.php

@@ -387,6 +387,14 @@ class BaseFieldDefinition extends ListDataDefinition implements FieldDefinitionI
     else {
       $value = isset($this->definition['default_value']) ? $this->definition['default_value'] : NULL;
     }
+    // Normalize into the "array keyed by delta" format.
+    if (isset($value) && !is_array($value)) {
+      $properties = $this->getPropertyNames();
+      $property = reset($properties);
+      $value = array(
+        array($property => $value),
+      );
+    }
     // Allow the field type to process default values.
     $field_item_list_class = $this->getClass();
     return $field_item_list_class::processDefaultValue($value, $entity, $this);
@@ -405,8 +413,8 @@ class BaseFieldDefinition extends ListDataDefinition implements FieldDefinitionI
    *     The entity being created.
    *   - \Drupal\Core\Field\FieldDefinitionInterface $definition
    *     The field definition.
-   *   It should return the default value as documented by
-   *   \Drupal\Core\Field\FieldDefinitionInterface::getDefaultValue().
+   *   It should return the default value in the format accepted by the
+   *   setDefaultValue() method.
    *
    * @return $this
    */
@@ -425,8 +433,12 @@ class BaseFieldDefinition extends ListDataDefinition implements FieldDefinitionI
    * any value set here.
    *
    * @param mixed $value
-   *   The default value in the format as returned by
-   *   \Drupal\Core\Field\FieldDefinitionInterface::getDefaultValue().
+   *   The default value for the field. This can be either:
+   *   - a literal, in which case it will be assigned to the first property of
+   *     the first item.
+   *   - a numerically indexed array of items, each item being a property/value
+   *     array.
+   *   - NULL or array() for no default value.
    *
    * @return $this
    */

core/lib/Drupal/Core/Field/FieldConfigBase.php

@@ -340,6 +340,14 @@ abstract class FieldConfigBase extends ConfigEntityBase implements FieldConfigIn
     else {
       $value = $this->default_value;
     }
+    // Normalize into the "array keyed by delta" format.
+    if (isset($value) && !is_array($value)) {
+      $properties = $this->getFieldStorageDefinition()->getPropertyNames();
+      $property = reset($properties);
+      $value = array(
+        array($property => $value),
+      );
+    }
     // Allow the field type to process default values.
     $field_item_list_class = $this->getClass();
     return $field_item_list_class::processDefaultValue($value, $entity, $this);

core/lib/Drupal/Core/Field/FieldDefinitionInterface.php

@@ -175,15 +175,9 @@ interface FieldDefinitionInterface extends ListDataDefinitionInterface {
    * @param \Drupal\Core\Entity\FieldableEntityInterface $entity
    *   The entity for which the default value is generated.
    *
-   * @return mixed
-   *   The default value for the field, as accepted by
-   *   \Drupal\field\Plugin\Core\Entity\FieldItemListInterface::setValue(). This
-   *   can be either:
-   *   - a literal, in which case it will be assigned to the first property of
-   *     the first item.
-   *   - a numerically indexed array of items, each item being a property/value
-   *     array.
-   *   - NULL or array() for no default value.
+   * @return array
+   *   The default value for the field, as a numerically indexed array of items,
+   *   each item being a property/value array (array() for no default value).
    */
   public function getDefaultValue(FieldableEntityInterface $entity);
 

core/lib/Drupal/Core/Field/FieldItemListInterface.php

@@ -242,25 +242,19 @@ interface FieldItemListInterface extends ListInterface, AccessibleInterface {
    * Processes the default value before being applied.
    *
    * Defined or configured default values of a field might need some processing
-   * in order to be a valid value for the field type; e.g., a date field could
-   * process the defined value of 'NOW' to a valid date.
+   * in order to be a valid runtime value for the field type; e.g., a date field
+   * could process the defined value of 'NOW' to a valid date.
    *
-   * @param mixed
-   *   The default value as defined for the field.
+   * @param array
+   *   The unprocessed default value defined for the field, as a numerically
+   *   indexed array of items, each item being an array of property/value pairs.
    * @param \Drupal\Core\Entity\FieldableEntityInterface $entity
    *   The entity for which the default value is generated.
    * @param \Drupal\Core\Field\FieldDefinitionInterface $definition
    *   The definition of the field.
    *
-   * @return mixed
-   *   The default value for the field, as accepted by
-   *   \Drupal\field\Plugin\Core\Entity\FieldItemListInterface::setValue(). This
-   *   can be either:
-   *   - a literal, in which case it will be assigned to the first property of
-   *     the first item.
-   *   - a numerically indexed array of items, each item being a property/value
-   *     array.
-   *   - NULL or array() for no default value.
+   * @return array
+   *   The return default value for the field.
    */
   public static function processDefaultValue($default_value, FieldableEntityInterface $entity, FieldDefinitionInterface $definition);
 

core/tests/Drupal/Tests/Core/Entity/BaseFieldDefinitionTest.php

@@ -161,15 +161,17 @@ class BaseFieldDefinitionTest extends UnitTestCase {
    */
   public function testFieldDefaultValue() {
     $definition = BaseFieldDefinition::create($this->fieldType);
-    $value = $this->randomMachineName();
-    $definition->setDefaultValue($value);
+    $default_value = array(
+      'value' => $this->randomMachineName(),
+    );
+    $definition->setDefaultValue($default_value);
     $entity = $this->getMockBuilder('Drupal\Core\Entity\ContentEntityBase')
       ->disableOriginalConstructor()
       ->getMock();
     // Set the field item list class to be used to avoid requiring the typed
     // data manager to retrieve it.
     $definition->setClass('Drupal\Core\Field\FieldItemList');
-    $this->assertEquals($value, $definition->getDefaultValue($entity));
+    $this->assertEquals($default_value, $definition->getDefaultValue($entity));
   }
 
   /**