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diff --git a/web/core/lib/Drupal/Core/TypedData/ComputedItemListTrait.php b/web/core/lib/Drupal/Core/TypedData/ComputedItemListTrait.php
new file mode 100644
index 000000000..7a823a474
--- /dev/null
+++ b/web/core/lib/Drupal/Core/TypedData/ComputedItemListTrait.php
@@ -0,0 +1,155 @@
+<?php
+
+namespace Drupal\Core\TypedData;
+
+/**
+ * Provides common functionality for computed item lists.
+ *
+ * @see \Drupal\Core\TypedData\ListInterface
+ * @see \Drupal\Core\TypedData\Plugin\DataType\ItemList
+ * @see \Drupal\Core\Field\FieldItemListInterface
+ * @see \Drupal\Core\Field\FieldItemList
+ *
+ * @ingroup typed_data
+ *
+ * @internal
+ *   This trait has been added in Drupal 8.4.3 as an internal helper and should
+ *   not be used outside of core.
+ */
+trait ComputedItemListTrait {
+
+  /**
+   * Whether the values have already been computed or not.
+   *
+   * @var bool
+   */
+  protected $valueComputed = FALSE;
+
+  /**
+   * Computes the values for an item list.
+   */
+  abstract protected function computeValue();
+
+  /**
+   * Ensures that values are only computed once.
+   */
+  protected function ensureComputedValue() {
+    if ($this->valueComputed === FALSE) {
+      $this->computeValue();
+      $this->valueComputed = TRUE;
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getValue() {
+    $this->ensureComputedValue();
+    return parent::getValue();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setValue($values, $notify = TRUE) {
+    parent::setValue($values, $notify);
+
+    // Make sure that subsequent getter calls do not try to compute the values
+    // again.
+    $this->valueComputed = TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getString() {
+    $this->ensureComputedValue();
+    return parent::getString();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function get($index) {
+    if (!is_numeric($index)) {
+      throw new \InvalidArgumentException('Unable to get a value with a non-numeric delta in a list.');
+    }
+
+    // Unlike the base implementation of
+    // \Drupal\Core\TypedData\ListInterface::get(), we do not add an empty item
+    // automatically because computed item lists need to behave like
+    // non-computed ones. For example, calling isEmpty() on a computed item list
+    // should return TRUE when the values were computed and the item list is
+    // truly empty.
+    // @see \Drupal\Core\TypedData\Plugin\DataType\ItemList::get().
+    $this->ensureComputedValue();
+
+    return isset($this->list[$index]) ? $this->list[$index] : NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function set($index, $value) {
+    $this->ensureComputedValue();
+    return parent::set($index, $value);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function appendItem($value = NULL) {
+    $this->ensureComputedValue();
+    return parent::appendItem($value);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function removeItem($index) {
+    $this->ensureComputedValue();
+    return parent::removeItem($index);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isEmpty() {
+    $this->ensureComputedValue();
+    return parent::isEmpty();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function offsetExists($offset) {
+    $this->ensureComputedValue();
+    return parent::offsetExists($offset);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIterator() {
+    $this->ensureComputedValue();
+    return parent::getIterator();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function count() {
+    $this->ensureComputedValue();
+    return parent::count();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applyDefaultValue($notify = TRUE) {
+    // Default values do not make sense for computed item lists. However, this
+    // method can be overridden if needed.
+    return $this;
+  }
+
+}