class FormTestLimitValidationErrorsForm

class FormTestLimitValidationErrorsForm extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'form_test_limit_validation_errors_form';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['title'] = [
      '#type' => 'textfield',
      '#title' => 'Title',
      '#required' => TRUE,
    ];
    $form['test'] = [
      '#title' => 'Test',
      '#type' => 'textfield',
      '#element_validate' => [
        '::elementValidateLimitValidationErrors',
      ],
    ];
    $form['test_numeric_index'] = [
      '#tree' => TRUE,
    ];
    $form['test_numeric_index'][0] = [
      '#title' => 'Test (numeric index)',
      '#type' => 'textfield',
      '#element_validate' => [
        '::elementValidateLimitValidationErrors',
      ],
    ];
    $form['test_substring'] = [
      '#tree' => TRUE,
    ];
    $form['test_substring']['foo'] = [
      '#title' => 'Test (substring) foo',
      '#type' => 'textfield',
      '#element_validate' => [
        '::elementValidateLimitValidationErrors',
      ],
    ];
    $form['test_substring']['foobar'] = [
      '#title' => 'Test (substring) foobar',
      '#type' => 'textfield',
      '#element_validate' => [
        '::elementValidateLimitValidationErrors',
      ],
    ];
    $form['actions']['partial'] = [
      '#type' => 'submit',
      '#limit_validation_errors' => [
        [
          'test',
        ],
      ],
      '#submit' => [
        '::partialSubmitForm',
      ],
      '#value' => $this->t('Partial validate'),
    ];
    $form['actions']['partial_numeric_index'] = [
      '#type' => 'submit',
      '#limit_validation_errors' => [
        [
          'test_numeric_index',
          0,
        ],
      ],
      '#submit' => [
        '::partialSubmitForm',
      ],
      '#value' => $this->t('Partial validate (numeric index)'),
    ];
    $form['actions']['substring'] = [
      '#type' => 'submit',
      '#limit_validation_errors' => [
        [
          'test_substring',
          'foo',
        ],
      ],
      '#submit' => [
        '::partialSubmitForm',
      ],
      '#value' => $this->t('Partial validate (substring)'),
    ];
    $form['actions']['full'] = [
      '#type' => 'submit',
      '#value' => $this->t('Full validate'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function elementValidateLimitValidationErrors($element, FormStateInterface $form_state) {
    if ($element['#value'] == 'invalid') {
      $form_state->setError($element, $this->t('@label element is invalid', [
        '@label' => $element['#title'],
      ]));
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
  }
  
  /**
   * {@inheritdoc}
   */
  public function partialSubmitForm(array &$form, FormStateInterface $form_state) {
    // The title has not been validated, thus its value - in case of the test
    // case an empty string - may not be set.
    if (!$form_state->hasValue('title') && $form_state->hasValue('test')) {
      $this->messenger()
        ->addStatus('Only validated values appear in the form values.');
    }
  }

}