diff --git a/core/lib/Drupal/Core/Form/FormState.php b/core/lib/Drupal/Core/Form/FormState.php
index 79a3abb..c37269a 100644
--- a/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
@@ -1212,8 +1212,8 @@ protected function moduleLoadInclude($module, $type, $name = NULL) {
   /**
    * {@inheritdoc}
    */
-  public function getFormStateForElement(array $element) {
-    return SubFormState::createForFormElement($this, $element);
+  public function getFormStateForSubform(array $element) {
+    return SubFormState::createForSubform($element, $this);
   }
 
 }
diff --git a/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php b/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
index 9361b87..4111b46 100644
--- a/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
+++ b/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
@@ -23,16 +23,6 @@
   protected $decoratedFormState;
 
   /**
-   * Constructs a new instance.
-   *
-   * @param \Drupal\Core\Form\FormStateInterface $decorated_form_state
-   *   The decorated form state.
-   */
-  public function __construct(FormStateInterface $decorated_form_state) {
-    $this->decoratedFormState = $decorated_form_state;
-  }
-
-  /**
    * {@inheritdoc}
    */
   public function setFormState(array $form_state_additions) {
diff --git a/core/lib/Drupal/Core/Form/FormStateInterface.php b/core/lib/Drupal/Core/Form/FormStateInterface.php
index 761e4d2..a396440 100644
--- a/core/lib/Drupal/Core/Form/FormStateInterface.php
+++ b/core/lib/Drupal/Core/Form/FormStateInterface.php
@@ -1099,13 +1099,13 @@ public function addCleanValueKey($key);
   public function cleanValues();
 
   /**
-   * Gets the form state for a form element.
+   * Gets the form state for a subform.
    *
-   * @param mixed[] $element
-   *   The form element for which to get the form state.
+   * @param mixed[] $subform
+   *   The subform for which to get the form state.
    *
    * @return \Drupal\Core\Form\FormStateInterface
    */
-  public function getFormStateForElement(array $element);
+  public function getFormStateForSubform(array $subform);
 
 }
diff --git a/core/lib/Drupal/Core/Form/SubFormState.php b/core/lib/Drupal/Core/Form/SubFormState.php
index dbdb7b9..746dfc7 100644
--- a/core/lib/Drupal/Core/Form/SubFormState.php
+++ b/core/lib/Drupal/Core/Form/SubFormState.php
@@ -18,63 +18,76 @@ class SubFormState extends FormStateDecoratorBase {
   use FormStateValuesTrait;
 
   /**
-   * The root #array_parents value of the subform.
+   * The subform.
    *
-   * @var string[]
+   * @var mixed[]
    */
-  protected $arrayParents = [];
-
-  /**
-   * The root #parents value of the subform.
-   *
-   * @var string[]
-   */
-  protected $parents = [];
+  protected $subform = [];
 
   /**
    * Constructs a new instance.
    *
+   * @param mixed[] $subform
+   *   The subform for which to create a form state.
    * @param \Drupal\Core\Form\FormStateInterface $parent_form_state
    *   The parent form state.
-   * @param string[] $array_parents
-   *   The root #array_parents value of the subset of the form.
-   * @param string[] $parents
-   *   The root #parents value of the subset of the form.
    */
-  public function __construct(FormStateInterface $parent_form_state, array $array_parents, array $parents) {
-    parent::__construct($parent_form_state);
-    $this->arrayParents = $array_parents;
-    $this->parents = $parents;
+  protected function __construct(array &$subform, FormStateInterface $parent_form_state) {
+    $this->decoratedFormState = $parent_form_state;
+    $this->subform = $subform;
   }
 
   /**
-   * Creates a new instance based on a form element.
+   * Creates a new instance for a subform.
    *
+   * @param mixed[] $subform
+   *   The subform for which to create a form state.
    * @param \Drupal\Core\Form\FormStateInterface $parent_form_state
    *   The parent form state.
-   * @param mixed[] $element
-   *   The element to create the sub form state for.
    *
    * @return static
+   */
+  public static function createForSubform(array &$subform, FormStateInterface $parent_form_state) {
+    return new static($subform, $parent_form_state);
+  }
+
+  /**
+   * Gets a subform property value.
+   *
+   * @param string $property
+   *   The property name.
+   *
+   * @return mixed
    *
    * @throws \InvalidArgumentException
-   *   Thrown when the form element does not wrap other form elements, or if it
-   *   does not have the necessary Form API properties assigned.
+   *   Thrown when the requested property does not exist.
    */
-  public static function createForFormElement(FormStateInterface $parent_form_state, array $element) {
-    // Make sure the element contains the required properties.
-    $children = Element::children($element);
-    if (empty($children)) {
-      throw new \InvalidArgumentException('nope');
-    }
-    $required_keys = ['#array_parents', '#parents'];
-    foreach ($required_keys as $required_key) {
-      if (!array_key_exists($required_key, $element) || !is_array($element[$required_key])) {
-        throw new \InvalidArgumentException(sprintf('$element must contain the %s key. Try calling this method from a #process callback instead.', $required_key));
-      }
+  protected function getSubFormProperty($property) {
+    if (!array_key_exists($property, $this->subform)) {
+      throw new \InvalidArgumentException(sprintf('The subform must contain the %s property. Try calling this method from a #process callback instead.', $property));
     }
 
-    return new static($parent_form_state, $element['#parents'], $element['#array_parents']);
+    return $this->subform[$property];
+  }
+
+  /**
+   * Gets the subform's parents.
+   *
+   * @return string[]
+   *   The parent keys (#array_parents).
+   */
+  protected function getSubFormParents() {
+    return $this->getSubFormProperty('#array_parents');
+  }
+
+  /**
+   * Gets the subform state's parents.
+   *
+   * @return string[]
+   *   The parent keys (#parents).
+   */
+  protected function getSubFormStateParents() {
+    return $this->getSubFormProperty('#parents');
   }
 
   /**
@@ -82,12 +95,12 @@ public static function createForFormElement(FormStateInterface $parent_form_stat
    */
   public function &getValues() {
     $exists = NULL;
-    $values = &NestedArray::getValue(parent::getValues(), $this->parents, $exists);
+    $values = &NestedArray::getValue(parent::getValues(), $this->getSubFormStateParents(), $exists);
     if (!$exists) {
       $values = [];
     }
     elseif (!is_array($values)) {
-      throw new \UnexpectedValueException('Drilled down too far! This violates the interface.');
+      throw new \UnexpectedValueException('The form state values do not belong to the subform.');
     }
 
 
@@ -97,8 +110,8 @@ public function &getValues() {
   /**
    * {@inheritdoc}
    */
-  public function getFormStateForElement(array $element) {
-    return static::createForFormElement($this, $element);
+  public function getFormStateForSubform(array $element) {
+    return static::createForSubform($element, $this);
   }
 
 }
diff --git a/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
index 2720a7f..0c3287f 100644
--- a/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -125,7 +125,9 @@ public function form(array $form, FormStateInterface $form_state) {
     $form_state->setTemporaryValue('gathered_contexts', $this->contextRepository->getAvailableContexts());
 
     $form['#tree'] = TRUE;
-    $form['settings'] = $entity->getPlugin()->buildConfigurationForm(array(), $form_state);
+    $form['settings'] = [];
+    $subform_state = $form_state->getFormStateForSubform($form['settings']);
+    $form['settings'] = $entity->getPlugin()->buildConfigurationForm($form['settings'], $subform_state);
     $form['visibility'] = $this->buildVisibilityInterface([], $form_state);
 
     // If creating a new block, calculate a safe default machine name.
@@ -285,7 +287,7 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
 
     // The Block Entity form puts all block plugin form elements in the
     // settings form element, so just pass that to the block for validation.
-    $this->entity->getPlugin()->validateConfigurationForm($form, $form_state->getFormStateForElement($form['settings']));
+    $this->entity->getPlugin()->validateConfigurationForm($form['settings'], $form_state->getFormStateForSubform($form['settings']));
     $this->validateVisibility($form, $form_state);
   }
 
@@ -309,7 +311,7 @@ protected function validateVisibility(array $form, FormStateInterface $form_stat
 
       // Allow the condition to validate the form.
       $condition = $form_state->get(['conditions', $condition_id]);
-      $condition->validateConfigurationForm($form, $form_state->getFormStateForElement($form['visibility'][$condition_id]));
+      $condition->validateConfigurationForm($form['visibility'][$condition_id], $form_state->getFormStateForSubform($form['visibility'][$condition_id]));
     }
   }
 
@@ -322,13 +324,13 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
     $entity = $this->entity;
     // The Block Entity form puts all block plugin form elements in the
     // settings form element, so just pass that to the block for submission.
-    $entity->getPlugin()->submitConfigurationForm($form, $form_state->getFormStateForElement($form['settings']));
+    $entity->getPlugin()->submitConfigurationForm($form['settings'], $form_state->getFormStateForSubform($form['settings']));
 
     // Submit visibility condition settings.
     foreach ($form_state->getValue('visibility') as $condition_id => $values) {
       // Allow the condition to submit the form.
       $condition = $form_state->get(['conditions', $condition_id]);
-      $condition->submitConfigurationForm($form, $form_state->getFormStateForElement($form['visibility'][$condition_id]));
+      $condition->submitConfigurationForm($form['visibility'][$condition_id], $form_state->getFormStateForSubform($form['visibility'][$condition_id]));
 
       if ($condition instanceof ContextAwarePluginInterface) {
         $context_mapping = isset($values['context_mapping']) ? $values['context_mapping'] : [];
diff --git a/core/modules/image/src/Form/ImageEffectFormBase.php b/core/modules/image/src/Form/ImageEffectFormBase.php
index bc12fb0..538d54f 100644
--- a/core/modules/image/src/Form/ImageEffectFormBase.php
+++ b/core/modules/image/src/Form/ImageEffectFormBase.php
@@ -79,7 +79,9 @@ public function buildForm(array $form, FormStateInterface $form_state, ImageStyl
       '#value' => $this->imageEffect->getPluginId(),
     );
 
-    $form['data'] = $this->imageEffect->buildConfigurationForm(array(), $form_state);
+    $form['data'] = [];
+    $subform_state = $form_state->getFormStateForSubform($form['data']);
+    $form['data'] = $this->imageEffect->buildConfigurationForm($form['data'], $subform_state);
     $form['data']['#tree'] = TRUE;
 
     // Check the URL for a weight, then the image effect, otherwise use default.
@@ -108,7 +110,7 @@ public function buildForm(array $form, FormStateInterface $form_state, ImageStyl
   public function validateForm(array &$form, FormStateInterface $form_state) {
     // The image effect configuration is stored in the 'data' key in the form,
     // pass that through for validation.
-    $this->imageEffect->validateConfigurationForm($form, $form_state->getFormStateForElement($form['data']));
+    $this->imageEffect->validateConfigurationForm($form, $form_state->getFormStateForSubform($form['data']));
   }
 
   /**
@@ -119,7 +121,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
 
     // The image effect configuration is stored in the 'data' key in the form,
     // pass that through for submission.
-    $this->imageEffect->submitConfigurationForm($form, $form_state->getFormStateForElement($form['data']));
+    $this->imageEffect->submitConfigurationForm($form, $form_state->getFormStateForSubform($form['data']));
 
     $this->imageEffect->setWeight($form_state->getValue('weight'));
     if (!$this->imageEffect->getUuid()) {
diff --git a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
index 4fca585..6cd0ded 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
@@ -430,7 +430,7 @@ public function testGetFormStateForElement() {
       'bar' => [],
     ];
 
-    $sub_form_state = $form_state->getFormStateForElement($element);
+    $sub_form_state = $form_state->getFormStateForSubform($element);
     $this->assertNotSame($form_state, $sub_form_state);
     $this->assertInstanceOf(FormStateInterface::class, $sub_form_state);
   }
diff --git a/core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php b/core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php
index 55c060a..429a530 100644
--- a/core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php
@@ -61,7 +61,7 @@ public function testCreateForFormElement() {
       'bar' => [],
     ];
 
-    $sub_form_state = $form_state->getFormStateForElement($element);
+    $sub_form_state = $form_state->getFormStateForSubform($element);
     $this->assertNotSame($form_state, $sub_form_state);
     $this->assertInstanceOf(FormStateInterface::class, $sub_form_state);
   }
@@ -78,7 +78,7 @@ public function testCreateForFormElementMissingArrayParents() {
       'bar' => [],
     ];
 
-    $form_state->getFormStateForElement($element);
+    $form_state->getFormStateForSubform($element);
   }
 
   /**
@@ -94,7 +94,7 @@ public function testCreateForFormElementMissingParents() {
       'bar' => [],
     ];
 
-    $form_state->getFormStateForElement($element);
+    $form_state->getFormStateForSubform($element);
   }
 
   /**
@@ -107,7 +107,7 @@ public function testGetValues($parents, $expected) {
     $form_state->setValues($this->initialValues);
 
     $element = NestedArray::getValue($this->element, $parents);
-    $sub_form_state = $form_state->getFormStateForElement($element);
+    $sub_form_state = $form_state->getFormStateForSubform($element);
     $sub_values = $sub_form_state->getValues();
     $this->assertSame($expected, $sub_values);
   }
@@ -155,7 +155,7 @@ public function testGetValue($parents, $key, $expected, $default = NULL) {
     $form_state->setValues($this->initialValues);
 
     $element = NestedArray::getValue($this->element, $parents);
-    $sub_form_state = $form_state->getFormStateForElement($element);
+    $sub_form_state = $form_state->getFormStateForSubform($element);
     $sub_values = $sub_form_state->getValue($key, $default);
     $this->assertSame($expected, $sub_values);
   }
@@ -201,7 +201,7 @@ public function testSetValues($parents, $new_values, $expected) {
     $form_state->setValues($this->initialValues);
 
     $element = NestedArray::getValue($this->element, $parents);
-    $sub_form_state = $form_state->getFormStateForElement($element);
+    $sub_form_state = $form_state->getFormStateForSubform($element);
     $sub_form_state->setValues($new_values);
     $this->assertSame($expected, $form_state->getValues());
   }