diff --git a/core/lib/Drupal/Core/Form/FormState.php b/core/lib/Drupal/Core/Form/FormState.php
index 6655494..3195202 100644
--- a/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
@@ -250,7 +250,7 @@ class FormState implements FormStateInterface {
    *
    * @var array
    */
-  protected $input;
+  protected $input = [];
 
   /**
    * If TRUE and the method is GET, a form_id is not necessary.
diff --git a/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php b/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
index cf8b2cb..c063666 100644
--- a/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
+++ b/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Core\Form\FormStateDecorator.
- */
-
 namespace Drupal\Core\Form;
 
 use Drupal\Core\Url;
diff --git a/core/lib/Drupal/Core/Form/FormStateValuesTrait.php b/core/lib/Drupal/Core/Form/FormStateValuesTrait.php
index 6626beb..87f7d46 100644
--- a/core/lib/Drupal/Core/Form/FormStateValuesTrait.php
+++ b/core/lib/Drupal/Core/Form/FormStateValuesTrait.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Core\Form\FormStateValuesTrait.
- */
-
 namespace Drupal\Core\Form;
 
 use Drupal\Component\Utility\NestedArray;
@@ -19,12 +14,12 @@
 trait FormStateValuesTrait {
 
   /**
-   * @see \Drupal\Core\Form\FormStateInterface::getValues()
+   * Implements \Drupal\Core\Form\FormStateInterface::getValues()
    */
   abstract public function &getValues();
 
   /**
-   * @see \Drupal\Core\Form\FormStateInterface::getValue()
+   * Implements \Drupal\Core\Form\FormStateInterface::getValue()
    */
   public function &getValue($key, $default = NULL) {
     $exists = NULL;
@@ -36,7 +31,7 @@ public function &getValue($key, $default = NULL) {
   }
 
   /**
-   * @see \Drupal\Core\Form\FormStateInterface::setValues()
+   * Implements \Drupal\Core\Form\FormStateInterface::setValues()
    */
   public function setValues(array $values) {
     $existing_values = &$this->getValues();
@@ -45,7 +40,7 @@ public function setValues(array $values) {
   }
 
   /**
-   * @see \Drupal\Core\Form\FormStateInterface::setValue()
+   * Implements \Drupal\Core\Form\FormStateInterface::setValue()
    */
   public function setValue($key, $value) {
     NestedArray::setValue($this->getValues(), (array) $key, $value, TRUE);
@@ -53,7 +48,7 @@ public function setValue($key, $value) {
   }
 
   /**
-   * @see \Drupal\Core\Form\FormStateInterface::unsetValue()
+   * Implements \Drupal\Core\Form\FormStateInterface::unsetValue()
    */
   public function unsetValue($key) {
     NestedArray::unsetValue($this->getValues(), (array) $key);
@@ -61,7 +56,7 @@ public function unsetValue($key) {
   }
 
   /**
-   * @see \Drupal\Core\Form\FormStateInterface::hasValue()
+   * Implements \Drupal\Core\Form\FormStateInterface::hasValue()
    */
   public function hasValue($key) {
     $exists = NULL;
@@ -70,7 +65,7 @@ public function hasValue($key) {
   }
 
   /**
-   * @see \Drupal\Core\Form\FormStateInterface::isValueEmpty()
+   * Implements \Drupal\Core\Form\FormStateInterface::isValueEmpty()
    */
   public function isValueEmpty($key) {
     $exists = NULL;
@@ -79,7 +74,7 @@ public function isValueEmpty($key) {
   }
 
   /**
-   * @see \Drupal\Core\Form\FormStateInterface::setValueForElement()
+   * Implements \Drupal\Core\Form\FormStateInterface::setValueForElement()
    */
   public function setValueForElement(array $element, $value) {
     return $this->setValue($element['#parents'], $value);
diff --git a/core/lib/Drupal/Core/Form/SubformState.php b/core/lib/Drupal/Core/Form/SubformState.php
index f6ed8c0..543b199 100644
--- a/core/lib/Drupal/Core/Form/SubformState.php
+++ b/core/lib/Drupal/Core/Form/SubformState.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Core\Form\SubformState.
- */
-
 namespace Drupal\Core\Form;
 
 use Drupal\Component\Utility\NestedArray;
@@ -157,4 +152,29 @@ public function setErrorByName($name, $message = '') {
     return $this;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function &getUserInput() {
+    $exists = NULL;
+    $input = &NestedArray::getValue(parent::getUserInput(), $this->getParents('#parents'), $exists);
+
+    // If no input already exists, create it, so it can be referenced.
+    if (!$exists) {
+      $this->setUserInput([]);
+      $input = &NestedArray::getValue(parent::getUserInput(), $this->getParents('#parents'), $exists);
+    }
+
+    return $input;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUserInput(array $user_input) {
+    NestedArray::setValue(parent::getUserInput(), $this->getParents('#parents'), $user_input, TRUE);
+
+    return $this;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Form/SubformStateInterface.php b/core/lib/Drupal/Core/Form/SubformStateInterface.php
index 7329c41..4d04d58 100644
--- a/core/lib/Drupal/Core/Form/SubformStateInterface.php
+++ b/core/lib/Drupal/Core/Form/SubformStateInterface.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Core\Form\SubformStateInterface.
- */
-
 namespace Drupal\Core\Form;
 
 /**
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestSettingsValidationBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestSettingsValidationBlock.php
index 6e2951a..d1f16d7 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestSettingsValidationBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestSettingsValidationBlock.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains Drupal\block_test\Plugin\Block\TestSettingsValidationBlock.
- */
-
 namespace Drupal\block_test\Plugin\Block;
 
 use Drupal\Core\Block\BlockBase;
diff --git a/core/tests/Drupal/Tests/Core/Form/FormStateDecoratorBaseTest.php b/core/tests/Drupal/Tests/Core/Form/FormStateDecoratorBaseTest.php
index 056a264..e08004a 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormStateDecoratorBaseTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormStateDecoratorBaseTest.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Tests\Core\Form\FormStateDecoratorBaseTest.
- */
-
 namespace Drupal\Tests\Core\Form;
 
 use Drupal\Core\Form\FormInterface;
diff --git a/core/tests/Drupal/Tests/Core/Form/FormStateValuesTraitTest.php b/core/tests/Drupal/Tests/Core/Form/FormStateValuesTraitTest.php
index 543ccf7..2894ac8 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormStateValuesTraitTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormStateValuesTraitTest.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Tests\Core\Form\FormStateValuesTraitTest.
- */
-
 namespace Drupal\Tests\Core\Form;
 
 use Drupal\Core\Form\FormStateValuesTrait;
diff --git a/core/tests/Drupal/Tests/Core/Form/SubformStateTest.php b/core/tests/Drupal/Tests/Core/Form/SubformStateTest.php
index f56ad6b..1455caa 100644
--- a/core/tests/Drupal/Tests/Core/Form/SubformStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/SubformStateTest.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Tests\Core\Form\SubformStateTest.
- */
-
 namespace Drupal\Tests\Core\Form;
 
 use Drupal\Component\Utility\NestedArray;
@@ -22,7 +17,20 @@
 class SubformStateTest extends UnitTestCase {
 
   /**
-   * Test fixture.
+   * The form state's user input test fixture.
+   *
+   * @var mixed[]
+   */
+  protected $formStateUserInput = [
+    'foo' => 'bar',
+    'dog' => [
+      'breed' => 'Pit bull',
+      'name' => 'Dodger',
+    ],
+  ];
+
+  /**
+   * The form state's values test fixture.
    *
    * @var mixed[]
    */
@@ -74,8 +82,13 @@ public function testGetValues(array $parents, $expected) {
 
     $subform = NestedArray::getValue($this->parentForm, $parents);
     $subform_state = SubformState::createForSubform($subform, $this->parentForm, $parent_form_state);
-    $sub_values = $subform_state->getValues();
-    $this->assertSame($expected, $sub_values);
+    $subform_state_values = &$subform_state->getValues();
+    $this->assertSame($expected, $subform_state_values);
+
+    // Modify the retrieved values and confirm they are modified by reference in
+    // the parent form state.
+    $subform_state_values['fish'] = 'Jim';
+    $this->assertSame($subform_state_values, $subform_state->getValues());
   }
 
   /**
@@ -134,8 +147,13 @@ public function testGetValue($parents, $key, $expected, $default = NULL) {
 
     $subform = NestedArray::getValue($this->parentForm, $parents);
     $subform_state = SubformState::createForSubform($subform, $this->parentForm, $parent_form_state);
-    $sub_values = $subform_state->getValue($key, $default);
-    $this->assertSame($expected, $sub_values);
+    $subform_state_value = &$subform_state->getValue($key, $default);
+    $this->assertSame($expected, $subform_state_value);
+
+    // Modify the retrieved values and confirm they are modified by reference in
+    // the parent form state.
+    $subform_state_value = 'Jim';
+    $this->assertSame($subform_state_value, $subform_state->getValue($key));
   }
 
   /**
@@ -304,4 +322,56 @@ public function testSetErrorByName() {
     $this->assertSame($subform_state, $subform_state->setErrorByName($subform_error_name, $message));
   }
 
+  /**
+   * @covers ::getUserInput
+   *
+   * @dataProvider providerGetUserInput
+   */
+  public function testGetUserInput($parent_form_state_user_input, $expected_subform_state_user_input) {
+    $parent_form_state = new FormState();
+    $parent_form_state->setUserInput($parent_form_state_user_input);
+    $parents = ['dog'];
+
+    $subform = NestedArray::getValue($this->parentForm, $parents);
+    $subform_state = SubformState::createForSubform($subform, $this->parentForm, $parent_form_state);
+    $subform_state_user_input = &$subform_state->getUserInput();
+    $this->assertSame($expected_subform_state_user_input, $subform_state_user_input);
+
+    // Modify the retrieved user input and confirm it is modified by reference
+    // in the parent form state.
+    $subform_state_user_input['fish'] = 'Jim';
+    $this->assertSame($subform_state_user_input, $subform_state->getUserInput());
+  }
+
+  /**
+   * Provides data to self::testGetUserInput().
+   */
+  public function providerGetUserInput() {
+    $data = [];
+
+    $data['existing-data'] = [$this->formStateUserInput, $this->formStateUserInput['dog']];
+    $data['non-existing-data'] = [[], []];
+
+    return $data;
+  }
+
+  /**
+   * @covers ::setUserInput
+   */
+  public function testSetUserInput() {
+    $parents = ['dog'];
+
+    $subform_state_user_input = NestedArray::getValue($this->formStateUserInput, $parents);
+
+    $parent_form_state = new FormState();
+    $parent_form_state_user_input = [];
+    NestedArray::setValue($parent_form_state_user_input, $parents, $subform_state_user_input);
+
+    $subform = NestedArray::getValue($this->parentForm, $parents);
+    $subform_state = SubformState::createForSubform($subform, $this->parentForm, $parent_form_state);
+    $this->assertSame($subform_state, $subform_state->setUserInput($subform_state_user_input));
+
+    $this->assertSame($parent_form_state_user_input, $parent_form_state->getUserInput());
+  }
+
 }