diff --git a/core/lib/Drupal/Core/Entity/Field/FieldItemList.php b/core/lib/Drupal/Core/Entity/Field/FieldItemList.php
index cf4e3aa..9a6a4d4 100644
--- a/core/lib/Drupal/Core/Entity/Field/FieldItemList.php
+++ b/core/lib/Drupal/Core/Entity/Field/FieldItemList.php
@@ -80,7 +80,7 @@ public function getLangcode() {
    * {@inheritdoc}
    */
   public function getFieldDefinition() {
-    return new FieldDefinition($this->definition);
+    return $this->definition;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Entity/Plugin/DataType/FieldItemList.php b/core/lib/Drupal/Core/Entity/Plugin/DataType/FieldItemList.php
index 9a8225b..b3ce264 100644
--- a/core/lib/Drupal/Core/Entity/Plugin/DataType/FieldItemList.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/DataType/FieldItemList.php
@@ -9,6 +9,7 @@
 
 use Drupal\Core\TypedData\Annotation\DataType;
 use Drupal\Core\Annotation\Translation;
+use Drupal\Core\Entity\Field\FieldItemList as ExistingFieldItemList;
 
 /**
  * Defines a data type plugin for entity fields, i.e. the list of field items.
@@ -21,6 +22,6 @@
  *   class = "\Drupal\Core\Entity\Field\Field"
  * )
  */
-class FieldItemList extends \Drupal\Core\Entity\Field\FieldItemList {
+class FieldItemList extends ExistingFieldItemList {
 
 }
diff --git a/core/lib/Drupal/Core/TypedData/ComplexDataInterface.php b/core/lib/Drupal/Core/TypedData/ComplexDataInterface.php
index 8acfaa3..2e3d0e7 100644
--- a/core/lib/Drupal/Core/TypedData/ComplexDataInterface.php
+++ b/core/lib/Drupal/Core/TypedData/ComplexDataInterface.php
@@ -105,10 +105,6 @@ public function getPropertyDefinition($name);
   /**
    * Gets an array of property definitions of contained properties.
    *
-   * @param array $definition
-   *   The definition of the container's property, e.g. the definition of an
-   *   entity reference property.
-   *
    * @return array
    *   An array of property definitions of contained properties, keyed by
    *   property name.
diff --git a/core/lib/Drupal/Core/TypedData/DataDefinition.php b/core/lib/Drupal/Core/TypedData/DataDefinition.php
index 316e06a..86d50d4 100644
--- a/core/lib/Drupal/Core/TypedData/DataDefinition.php
+++ b/core/lib/Drupal/Core/TypedData/DataDefinition.php
@@ -96,7 +96,7 @@ public function setDescription($description) {
    * {@inheritdoc}
    */
   public function isList() {
-    return $this instanceof ListDefinitionInterface;
+    return ($this instanceof ListDefinitionInterface);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/TypedData/ListDefinitionInterface.php b/core/lib/Drupal/Core/TypedData/ListDefinitionInterface.php
index 0bf8aaf..c07d60e 100644
--- a/core/lib/Drupal/Core/TypedData/ListDefinitionInterface.php
+++ b/core/lib/Drupal/Core/TypedData/ListDefinitionInterface.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Contains \Drupal\Core\TypedData\DataDefinitionInterface.
+ * Contains \Drupal\Core\TypedData\ListDefinitionInterface.
  */
 
 namespace Drupal\Core\TypedData;
diff --git a/core/modules/datetime/lib/Drupal/datetime/DateTimeComputed.php b/core/modules/datetime/lib/Drupal/datetime/DateTimeComputed.php
index c0f6123..d6954b9 100644
--- a/core/modules/datetime/lib/Drupal/datetime/DateTimeComputed.php
+++ b/core/modules/datetime/lib/Drupal/datetime/DateTimeComputed.php
@@ -29,7 +29,7 @@ class DateTimeComputed extends TypedData {
   /**
    * {@inheritdoc}
    */
-  public function __construct(array $definition, $name = NULL, TypedDataInterface $parent = NULL) {
+  public function __construct($definition, $name = NULL, TypedDataInterface $parent = NULL) {
     parent::__construct($definition, $name, $parent);
     if (!isset($definition['settings']['date source'])) {
       throw new \InvalidArgumentException("The definition's 'date source' key has to specify the name of the date property to be computed.");