diff --git a/core/lib/Drupal/Core/Form/FormState.php b/core/lib/Drupal/Core/Form/FormState.php
index d0624a6..23cd7b7 100644
--- a/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
@@ -210,6 +210,20 @@ class FormState implements FormStateInterface {
   protected $no_cache;
 
   /**
+   * An associative array of values submitted to the form.
+   *
+   * The validation functions and submit functions use this array for nearly all
+   * their decision making. (Note that #tree determines whether the values are a
+   * flat array or an array whose structure parallels the $form array. See
+   * \Drupal\Core\Render\Element\FormElement for more information.)
+   *
+   * This property is uncacheable.
+   *
+   * @var array
+   */
+  protected $values = array();
+
+  /**
    * An associative array of form value keys to be removed by cleanValues().
    *
    * Any values that are temporary but must still be displayed as values in
@@ -966,6 +980,13 @@ public function setUserInput(array $user_input) {
   /**
    * {@inheritdoc}
    */
+  public function &getValues() {
+    return $this->values;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function setResponse(Response $response) {
     $this->response = $response;
     return $this;
@@ -1237,11 +1258,4 @@ protected function moduleLoadInclude($module, $type, $name = NULL) {
     return \Drupal::moduleHandler()->loadInclude($module, $type, $name);
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  public function getSubFormState(array $element) {
-    return SubFormState::createForSubform($element, $this);
-  }
-
 }
diff --git a/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php b/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
index 6f7c0e3..da24cb1 100644
--- a/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
+++ b/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
@@ -588,7 +588,7 @@ public function setError(array &$element, $message = '') {
    * {@inheritdoc}
    */
   public function clearErrors() {
-    return $this->decoratedFormState->clearErrors();
+    $this->decoratedFormState->clearErrors();
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Form/FormStateInterface.php b/core/lib/Drupal/Core/Form/FormStateInterface.php
index 7d9d80d..798d12c 100644
--- a/core/lib/Drupal/Core/Form/FormStateInterface.php
+++ b/core/lib/Drupal/Core/Form/FormStateInterface.php
@@ -1115,14 +1115,4 @@ public function addCleanValueKey($key);
    */
   public function cleanValues();
 
-  /**
-   * Gets the form state for a subform.
-   *
-   * @param mixed[] $subform
-   *   The subform for which to get the form state.
-   *
-   * @return \Drupal\Core\Form\FormStateInterface
-   */
-  public function getSubFormState(array $subform);
-
 }
diff --git a/core/lib/Drupal/Core/Form/FormStateValuesTrait.php b/core/lib/Drupal/Core/Form/FormStateValuesTrait.php
index 1aa22cd..6626beb 100644
--- a/core/lib/Drupal/Core/Form/FormStateValuesTrait.php
+++ b/core/lib/Drupal/Core/Form/FormStateValuesTrait.php
@@ -19,25 +19,9 @@
 trait FormStateValuesTrait {
 
   /**
-   * An associative array of values submitted to the form.
-   *
-   * The validation functions and submit functions use this array for nearly all
-   * their decision making. (Note that #tree determines whether the values are a
-   * flat array or an array whose structure parallels the $form array. See
-   * \Drupal\Core\Render\Element\FormElement for more information.)
-   *
-   * This property is uncacheable.
-   *
-   * @var array
-   */
-  protected $values = array();
-
-  /**
    * @see \Drupal\Core\Form\FormStateInterface::getValues()
    */
-  public function &getValues() {
-    return $this->values;
-  }
+  abstract public function &getValues();
 
   /**
    * @see \Drupal\Core\Form\FormStateInterface::getValue()
diff --git a/core/lib/Drupal/Core/Form/SubFormState.php b/core/lib/Drupal/Core/Form/SubformState.php
similarity index 82%
rename from core/lib/Drupal/Core/Form/SubFormState.php
rename to core/lib/Drupal/Core/Form/SubformState.php
index 9fe3df6..e54ac21 100644
--- a/core/lib/Drupal/Core/Form/SubFormState.php
+++ b/core/lib/Drupal/Core/Form/SubformState.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Contains \Drupal\Core\Form\SubFormState.
+ * Contains \Drupal\Core\Form\SubformState.
  */
 
 namespace Drupal\Core\Form;
@@ -13,7 +13,7 @@
 /**
  * Stores information about the state of a subform.
  */
-class SubFormState extends FormStateDecoratorBase {
+class SubformState extends FormStateDecoratorBase implements SubformStateInterface {
 
   use FormStateValuesTrait;
 
@@ -22,7 +22,7 @@ class SubFormState extends FormStateDecoratorBase {
    *
    * @var mixed[]
    */
-  protected $subform = [];
+  protected $subform;
 
   /**
    * Constructs a new instance.
@@ -62,7 +62,7 @@ public static function createForSubform(array &$subform, FormStateInterface $par
    * @throws \InvalidArgumentException
    *   Thrown when the requested property does not exist.
    */
-  protected function getSubFormProperty($property) {
+  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.', $property));
     }
@@ -77,7 +77,7 @@ protected function getSubFormProperty($property) {
    *   The parent keys (#array_parents).
    */
   protected function getArrayParents() {
-    return $this->getSubFormProperty('#array_parents');
+    return $this->getSubformProperty('#array_parents');
   }
 
   /**
@@ -87,7 +87,7 @@ protected function getArrayParents() {
    *   The parent keys (#parents).
    */
   protected function getParents() {
-    return $this->getSubFormProperty('#parents');
+    return $this->getSubformProperty('#parents');
   }
 
   /**
@@ -103,15 +103,14 @@ public function &getValues() {
       throw new \UnexpectedValueException('The form state values do not belong to the subform.');
     }
 
-
     return $values;
   }
 
   /**
    * {@inheritdoc}
    */
-  public function getSubFormState(array $element) {
-    return static::createForSubform($element, $this);
+  public function getCompleteFormState() {
+    return $this->decoratedFormState instanceof SubformStateInterface ? $this->decoratedFormState->getCompleteFormState() : $this->decoratedFormState;
   }
 
 }
diff --git a/core/lib/Drupal/Core/Form/SubformStateInterface.php b/core/lib/Drupal/Core/Form/SubformStateInterface.php
new file mode 100644
index 0000000..f837a21
--- /dev/null
+++ b/core/lib/Drupal/Core/Form/SubformStateInterface.php
@@ -0,0 +1,27 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Form\SubformStateInterface.
+ */
+
+namespace Drupal\Core\Form;
+
+
+/**
+ * Stores information about the state of a subform.
+ */
+interface SubformStateInterface extends FormStateInterface {
+
+  /**
+   * Gets the complete form state.
+   *
+   * @return \Drupal\Core\Form\FormStateInterface
+   *
+   * @deprecated Deprecated as of Drupal 8.1.x. Scheduled for removal before
+   *   Drupal 9.0.0. Subforms should not depend on another form's form state,
+   *   and any dependencies should be passed on through the forms' API methods.
+   */
+  public function getCompleteFormState();
+
+}
diff --git a/core/lib/Drupal/Core/Plugin/PluginFormInterface.php b/core/lib/Drupal/Core/Plugin/PluginFormInterface.php
index 84b48d3..7e1fbba 100644
--- a/core/lib/Drupal/Core/Plugin/PluginFormInterface.php
+++ b/core/lib/Drupal/Core/Plugin/PluginFormInterface.php
@@ -34,7 +34,7 @@
    * @param \Drupal\Core\Form\FormStateInterface $form_state
    *   The current state of the form. Calling code should pass on a sub form
    *   state created through
-   *   \Drupal\Core\Form\FormStateInterface::getSubFormState().
+   *   \Drupal\Core\Form\SubformState::createForSubform().
    *
    * @return array
    *   The form structure.
@@ -50,7 +50,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
    * @param \Drupal\Core\Form\FormStateInterface $form_state
    *   The current state of the form. Calling code should pass on a sub form
    *   state created through
-   *   \Drupal\Core\Form\FormStateInterface::getSubFormState().
+   *   \Drupal\Core\Form\SubformState::createForSubform().
    */
   public function validateConfigurationForm(array &$form, FormStateInterface $form_state);
 
@@ -63,7 +63,7 @@ public function validateConfigurationForm(array &$form, FormStateInterface $form
    * @param \Drupal\Core\Form\FormStateInterface $form_state
    *   The current state of the form. Calling code should pass on a sub form
    *   state created through
-   *   \Drupal\Core\Form\FormStateInterface::getSubFormState().
+   *   \Drupal\Core\Form\SubformState::createForSubform().
    */
   public function submitConfigurationForm(array &$form, FormStateInterface $form_state);
 
diff --git a/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
index 4da2df7..d4daa95 100644
--- a/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -14,6 +14,7 @@
 use Drupal\Core\Extension\ThemeHandlerInterface;
 use Drupal\Core\Form\FormState;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\SubformState;
 use Drupal\Core\Language\LanguageManagerInterface;
 use Drupal\Core\Plugin\ContextAwarePluginInterface;
 use Drupal\Core\Plugin\Context\ContextRepositoryInterface;
@@ -126,7 +127,7 @@ public function form(array $form, FormStateInterface $form_state) {
 
     $form['#tree'] = TRUE;
     $form['settings'] = [];
-    $subform_state = $form_state->getSubFormState($form['settings']);
+    $subform_state = SubformState::createForSubform($form['settings'], $form_state);
     $form['settings'] = $entity->getPlugin()->buildConfigurationForm($form['settings'], $subform_state);
     $form['visibility'] = $this->buildVisibilityInterface([], $form_state);
 
@@ -287,7 +288,7 @@ 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['settings'], $form_state->getSubFormState($form['settings']));
+    $this->entity->getPlugin()->validateConfigurationForm($form['settings'], SubformState::createForSubform($form['settings'], $form_state));
     $this->validateVisibility($form, $form_state);
   }
 
@@ -311,7 +312,7 @@ 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['visibility'][$condition_id], $form_state->getSubFormState($form['visibility'][$condition_id]));
+      $condition->validateConfigurationForm($form['visibility'][$condition_id], SubformState::createForSubform($form['visibility'][$condition_id], $form_state));
     }
   }
 
@@ -324,7 +325,7 @@ 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.
-    $sub_form_state = $form_state->getSubFormState($form['settings']);
+    $sub_form_state = SubformState::createForSubform($form['settings'], $form_state);
     // Call the plugin submit handler.
     $entity->getPlugin()->submitConfigurationForm($form['settings'], $sub_form_state);
     $block = $entity->getPlugin();
@@ -338,7 +339,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
     foreach ($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['visibility'][$condition_id], $form_state->getSubFormState($form['visibility'][$condition_id]));
+      $condition->submitConfigurationForm($form['visibility'][$condition_id], SubformState::createForSubform($form['visibility'][$condition_id], $form_state));
 
       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 def903f..24542c4 100644
--- a/core/modules/image/src/Form/ImageEffectFormBase.php
+++ b/core/modules/image/src/Form/ImageEffectFormBase.php
@@ -9,7 +9,7 @@
 
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\Form\SubFormState;
+use Drupal\Core\Form\SubformState;
 use Drupal\image\ConfigurableImageEffectInterface;
 use Drupal\image\ImageStyleInterface;
 use Drupal\Component\Plugin\Exception\PluginNotFoundException;
@@ -79,7 +79,7 @@ public function buildForm(array $form, FormStateInterface $form_state, ImageStyl
     );
 
     $form['data'] = [];
-    $subform_state = $form_state->getSubFormState($form['data']);
+    $subform_state = SubformState::createForSubform($form['data'], $form_state);
     $form['data'] = $this->imageEffect->buildConfigurationForm($form['data'], $subform_state);
     $form['data']['#tree'] = TRUE;
 
@@ -109,7 +109,7 @@ 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['data'], $form_state->getSubFormState($form['data']));
+    $this->imageEffect->validateConfigurationForm($form['data'], SubformState::createForSubform($form['data'], $form_state));
   }
 
   /**
@@ -120,7 +120,7 @@ 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['data'], $form_state->getSubFormState($form['data']));
+    $this->imageEffect->submitConfigurationForm($form['data'], SubformState::createForSubform($form['data'], $form_state));
 
     $this->imageEffect->setWeight($form_state->getValue('weight'));
     if (!$this->imageEffect->getUuid()) {
diff --git a/core/tests/Drupal/Tests/Core/Form/FormStateDecoratorBaseTest.php b/core/tests/Drupal/Tests/Core/Form/FormStateDecoratorBaseTest.php
index d707661..ff69778 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormStateDecoratorBaseTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormStateDecoratorBaseTest.php
@@ -1058,10 +1058,14 @@ public function testGetError() {
    * @covers ::getErrors
    */
   public function testGetErrors() {
+    $errors = [
+      'foo' => 'bar',
+    ];
     $this->decoratedFormState->expects($this->once())
-      ->method('getErrors');
+      ->method('getErrors')
+      ->willReturn($errors);
 
-    $this->sut->getErrors();
+    $this->assertSame($errors, $this->sut->getErrors());
   }
 
   /**
@@ -1223,4 +1227,4 @@ public function testCleanValues() {
     $this->assertSame($this->sut, $this->sut->cleanValues());
   }
 
-}
\ No newline at end of file
+}
diff --git a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
index 71d1097..51b1a91 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
@@ -420,19 +420,16 @@ public function testCleanValues($form_state) {
   }
 
   /**
-   * @covers ::getSubFormState
+   * @covers ::setValues
+   * @covers ::getValues
    */
-  public function testGetSubFormState() {
-    $form_state = new FormState();
-    $element = [
-      '#parents' => ['foo'],
-      '#array_parents' => ['foo'],
-      'bar' => [],
+  public function testGetValues() {
+    $values = [
+      'foo' => 'bar',
     ];
-
-    $sub_form_state = $form_state->getSubFormState($element);
-    $this->assertNotSame($form_state, $sub_form_state);
-    $this->assertInstanceOf(FormStateInterface::class, $sub_form_state);
+    $form_state = new FormState();
+    $form_state->setValues($values);
+    $this->assertSame($values, $form_state->getValues());
   }
 
 }
diff --git a/core/tests/Drupal/Tests/Core/Form/FormStateValuesTraitTest.php b/core/tests/Drupal/Tests/Core/Form/FormStateValuesTraitTest.php
index 822bafb..acc3507 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormStateValuesTraitTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormStateValuesTraitTest.php
@@ -23,54 +23,63 @@ class FormStateValuesTraitTest extends UnitTestCase {
    * @covers ::setValueForElement
    */
   public function testSetValueForElement() {
-    $element = array(
-      '#parents' => array(
+    $element = [
+      '#parents' => [
         'foo',
         'bar',
-      ),
-    );
+      ],
+    ];
     $value = $this->randomMachineName();
 
     $form_state = new FormStateValuesTraitStub();
     $form_state->setValueForElement($element, $value);
-    $expected = array(
-      'foo' => array(
+    $expected = [
+      'foo' => [
         'bar' => $value,
-      ),
-    );
+      ],
+    ];
     $this->assertSame($expected, $form_state->getValues());
   }
 
   /**
    * @covers ::getValue
    *
-   * @dataProvider providerTestGetValue
+   * @dataProvider providerGetValue
    */
   public function testGetValue($key, $expected, $default = NULL) {
     $form_state = (new FormStateValuesTraitStub())->setValues([
       'foo' => 'one',
-      'bar' => array(
+      'bar' => [
         'baz' => 'two',
-      ),
+      ],
     ]);
     $this->assertSame($expected, $form_state->getValue($key, $default));
   }
 
-  public function providerTestGetValue() {
-    $data = array();
-    $data[] = array(
+  /**
+   * Provides data to self::testGetValue().
+   *
+   * @return array[]
+   *   Items are arrays of two items:
+   *   - The key for which to get the value (string)
+   *   - The expected value (mixed).
+   *   - The default value (mixed).
+   */
+  public function providerGetValue() {
+    $data = [];
+    $data[] = [
       'foo', 'one',
-    );
-    $data[] = array(
-      array('bar', 'baz'), 'two',
-    );
-    $data[] = array(
-      array('foo', 'bar', 'baz'), NULL,
-    );
-    $data[] = array(
+    ];
+    $data[] = [
+      ['bar', 'baz'], 'two',
+    ];
+    $data[] = [
+      ['foo', 'bar', 'baz'], NULL,
+    ];
+    $data[] = [
       'baz', 'baz', 'baz',
-    );
-    $data[] = array(
+    ];
+    $data[] = [
       NULL,
       [
         'foo' => 'one',
@@ -78,7 +87,7 @@ public function providerTestGetValue() {
           'baz' => 'two',
         ],
       ],
-    );
+    ];
     return $data;
   }
 
@@ -105,7 +114,7 @@ public function testGetValueModifyReturn() {
   /**
    * @covers ::setValue
    *
-   * @dataProvider providerTestSetValue
+   * @dataProvider providerSetValue
    */
   public function testSetValue($key, $value, $expected) {
     $form_state = (new FormStateValuesTraitStub())->setValues([
@@ -115,31 +124,40 @@ public function testSetValue($key, $value, $expected) {
     $this->assertSame($expected, $form_state->getValues());
   }
 
-  public function providerTestSetValue() {
-    $data = array();
-    $data[] = array(
-      'foo', 'one', array('bar' => 'wrong', 'foo' => 'one'),
-    );
-    $data[] = array(
-      array('bar', 'baz'), 'two', array('bar' => array('baz' => 'two')),
-    );
-    $data[] = array(
-      array('foo', 'bar', 'baz'), NULL, array('bar' => 'wrong', 'foo' => array('bar' => array('baz' => NULL))),
-    );
+  /**
+   * Provides data to self::testSetValue().
+   *
+   * @return array[]
+   *   Items are arrays of two items:
+   *   - The key for which to set a new value (string)
+   *   - The new value to set (mixed).
+   *   - The expected form state values after setting the new value (mixed[]).
+   */
+  public function providerSetValue() {
+    $data = [];
+    $data[] = [
+      'foo', 'one', ['bar' => 'wrong', 'foo' => 'one'],
+    ];
+    $data[] = [
+      ['bar', 'baz'], 'two', ['bar' => ['baz' => 'two']],
+    ];
+    $data[] = [
+      ['foo', 'bar', 'baz'], NULL, ['bar' => 'wrong', 'foo' => ['bar' => ['baz' => NULL]]],
+    ];
     return $data;
   }
 
   /**
    * @covers ::hasValue
    *
-   * @dataProvider providerTestHasValue
+   * @dataProvider providerHasValue
    */
   public function testHasValue($key, $expected) {
     $form_state = (new FormStateValuesTraitStub())->setValues([
       'foo' => 'one',
-      'bar' => array(
+      'bar' => [
         'baz' => 'two',
-      ),
+      ],
       'true' => TRUE,
       'false' => FALSE,
       'null' => NULL,
@@ -147,40 +165,48 @@ public function testHasValue($key, $expected) {
     $this->assertSame($expected, $form_state->hasValue($key));
   }
 
-  public function providerTestHasValue() {
-    $data = array();
-    $data[] = array(
+  /**
+   * Provides data to self::testHasValue().
+   * 
+   * @return array[]
+   *   Items are arrays of two items:
+   *   - The key to check for in the form state (string)
+   *   - Whether the form state has an item with that key (bool).
+   */
+  public function providerHasValue() {
+    $data = [];
+    $data[] = [
       'foo', TRUE,
-    );
-    $data[] = array(
-      array('bar', 'baz'), TRUE,
-    );
-    $data[] = array(
-      array('foo', 'bar', 'baz'), FALSE,
-    );
-    $data[] = array(
+    ];
+    $data[] = [
+      ['bar', 'baz'], TRUE,
+    ];
+    $data[] = [
+      ['foo', 'bar', 'baz'], FALSE,
+    ];
+    $data[] = [
       'true', TRUE,
-    );
-    $data[] = array(
+    ];
+    $data[] = [
       'false', TRUE,
-    );
-    $data[] = array(
+    ];
+    $data[] = [
       'null', FALSE,
-    );
+    ];
     return $data;
   }
 
   /**
    * @covers ::isValueEmpty
    *
-   * @dataProvider providerTestIsValueEmpty
+   * @dataProvider providerIsValueEmpty
    */
   public function testIsValueEmpty($key, $expected) {
     $form_state = (new FormStateValuesTraitStub())->setValues([
       'foo' => 'one',
-      'bar' => array(
+      'bar' => [
         'baz' => 'two',
-      ),
+      ],
       'true' => TRUE,
       'false' => FALSE,
       'null' => NULL,
@@ -188,31 +214,54 @@ public function testIsValueEmpty($key, $expected) {
     $this->assertSame($expected, $form_state->isValueEmpty($key));
   }
 
-  public function providerTestIsValueEmpty() {
-    $data = array();
-    $data[] = array(
+  /**
+   * Provides data to self::testIsValueEmpty().
+   *
+   * @return array[]
+   *   Items are arrays of two items:
+   *   - The key to check for in the form state (string)
+   *   - Whether the value is empty or not (bool).
+   */
+  public function providerIsValueEmpty() {
+    $data = [];
+    $data[] = [
       'foo', FALSE,
-    );
-    $data[] = array(
-      array('bar', 'baz'), FALSE,
-    );
-    $data[] = array(
-      array('foo', 'bar', 'baz'), TRUE,
-    );
-    $data[] = array(
+    ];
+    $data[] = [
+      ['bar', 'baz'], FALSE,
+    ];
+    $data[] = [
+      ['foo', 'bar', 'baz'], TRUE,
+    ];
+    $data[] = [
       'true', FALSE,
-    );
-    $data[] = array(
+    ];
+    $data[] = [
       'false', TRUE,
-    );
-    $data[] = array(
+    ];
+    $data[] = [
       'null', TRUE,
-    );
+    ];
     return $data;
   }
 
 }
 
 class FormStateValuesTraitStub {
+
   use FormStateValuesTrait;
+
+  /**
+   * The submitted form values.
+   *
+   * @var mixed[]
+   */
+  protected $values = [];
+
+  /**
+   * {@inheritdoc}
+   */
+  public function &getValues() {
+    return $this->values;
+  }
 }
diff --git a/core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php b/core/tests/Drupal/Tests/Core/Form/SubformStateTest.php
similarity index 73%
rename from core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php
rename to core/tests/Drupal/Tests/Core/Form/SubformStateTest.php
index d516d9a..3ec3619 100644
--- a/core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/SubformStateTest.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Contains \Drupal\Tests\Core\Form\SubFormStateTest.
+ * Contains \Drupal\Tests\Core\Form\SubformStateTest.
  */
 
 namespace Drupal\Tests\Core\Form;
@@ -10,14 +10,16 @@
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Form\FormState;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\SubformState;
+use Drupal\Core\Form\SubformStateInterface;
 use Drupal\Tests\UnitTestCase;
 
 /**
- * @coversDefaultClass \Drupal\Core\Form\SubFormState
+ * @coversDefaultClass \Drupal\Core\Form\SubformState
  *
  * @group Form
  */
-class SubFormStateTest extends UnitTestCase {
+class SubformStateTest extends UnitTestCase {
 
   /**
    * Test fixture.
@@ -51,25 +53,9 @@ class SubFormStateTest extends UnitTestCase {
   ];
 
   /**
-   * @covers ::getSubFormState
-   */
-  public function testGetSubFormState() {
-    $form_state = new FormState();
-    $element = [
-      '#parents' => ['foo'],
-      '#array_parents' => ['foo'],
-      'bar' => [],
-    ];
-
-    $sub_form_state = $form_state->getSubFormState($element);
-    $this->assertNotSame($form_state, $sub_form_state);
-    $this->assertInstanceOf(FormStateInterface::class, $sub_form_state);
-  }
-
-  /**
    * @covers ::getValues
    * @covers ::getParents
-   * @covers ::getSubFormProperty
+   * @covers ::getSubformProperty
    *
    * @dataProvider providerTestGetValues
    *
@@ -81,8 +67,8 @@ public function testGetValues(array $parents, $expected) {
     $form_state->setValues($this->initialValues);
 
     $element = NestedArray::getValue($this->element, $parents);
-    $sub_form_state = $form_state->getSubFormState($element);
-    $sub_values = $sub_form_state->getValues();
+    $subform_state = SubformState::createForSubform($element, $form_state);
+    $sub_values = $subform_state->getValues();
     $this->assertSame($expected, $sub_values);
   }
 
@@ -102,7 +88,7 @@ public function providerTestGetValues() {
   /**
    * @covers ::getValues
    * @covers ::getParents
-   * @covers ::getSubFormProperty
+   * @covers ::getSubformProperty
    *
    * @dataProvider providerTestGetValuesBroken
    *
@@ -142,8 +128,8 @@ public function testGetValue($parents, $key, $expected, $default = NULL) {
     $form_state->setValues($this->initialValues);
 
     $element = NestedArray::getValue($this->element, $parents);
-    $sub_form_state = $form_state->getSubFormState($element);
-    $sub_values = $sub_form_state->getValue($key, $default);
+    $subform_state = SubformState::createForSubform($element, $form_state);
+    $sub_values = $subform_state->getValue($key, $default);
     $this->assertSame($expected, $sub_values);
   }
 
@@ -196,8 +182,8 @@ public function testSetValues($parents, $new_values, $expected) {
     $form_state->setValues($this->initialValues);
 
     $element = NestedArray::getValue($this->element, $parents);
-    $sub_form_state = $form_state->getSubFormState($element);
-    $sub_form_state->setValues($new_values);
+    $subform_state = SubformState::createForSubform($element, $form_state);
+    $subform_state->setValues($new_values);
     $this->assertSame($expected, $form_state->getValues());
   }
 
@@ -244,4 +230,28 @@ public function providerTestSetValuesBroken() {
     return $data;
   }
 
+  /**
+   * @covers ::getCompleteFormState
+   */
+  public function testGetCompleteFormStateWithParentCompleteForm() {
+    $decorated_form_state = $this->getMock(FormStateInterface::class);
+    $subform = [];
+    $subform_state = SubformState::createForSubform($subform, $decorated_form_state);
+    $this->assertSame($decorated_form_state, $subform_state->getCompleteFormState());
+  }
+
+  /**
+   * @covers ::getCompleteFormState
+   */
+  public function testGetCompleteFormStateWithParentSubform() {
+    $form_state = $this->getMock(FormStateInterface::class);
+    $decorated_form_state = $this->getMock(SubformStateInterface::class);
+    $decorated_form_state->expects($this->atLeastOnce())
+      ->method('getCompleteFormState')
+      ->willReturn($form_state);
+    $subform = [];
+    $subform_state = SubformState::createForSubform($subform, $decorated_form_state);
+    $this->assertSame($form_state, $subform_state->getCompleteFormState());
+  }
+
 }