class FormTestVerticalTabsAccessForm

class FormTestVerticalTabsAccessForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_vertical_tabs_access_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['vertical_tabs1'] = [
            '#type' => 'vertical_tabs',
        ];
        $form['tab1'] = [
            '#type' => 'fieldset',
            '#title' => t('Tab 1'),
            '#collapsible' => TRUE,
            '#group' => 'vertical_tabs1',
        ];
        $form['tab1']['field1'] = [
            '#title' => t('Field 1'),
            '#type' => 'checkbox',
            '#default_value' => TRUE,
        ];
        $form['tab2'] = [
            '#type' => 'fieldset',
            '#title' => t('Tab 2'),
            '#collapsible' => TRUE,
            '#group' => 'vertical_tabs1',
        ];
        $form['tab2']['field2'] = [
            '#title' => t('Field 2'),
            '#type' => 'textfield',
            '#default_value' => 'field2',
        ];
        $form['fieldset1'] = [
            '#type' => 'fieldset',
            '#title' => t('Fieldset'),
        ];
        $form['fieldset1']['field3'] = [
            '#type' => 'checkbox',
            '#title' => t('Field 3'),
            '#default_value' => TRUE,
        ];
        $form['container'] = [
            '#type' => 'container',
        ];
        $form['container']['field4'] = [
            '#type' => 'checkbox',
            '#title' => t('Field 4'),
            '#default_value' => TRUE,
        ];
        $form['container']['subcontainer'] = [
            '#type' => 'container',
        ];
        $form['container']['subcontainer']['field5'] = [
            '#type' => 'checkbox',
            '#title' => t('Field 5'),
            '#default_value' => TRUE,
        ];
        $form['vertical_tabs2'] = [
            '#type' => 'vertical_tabs',
        ];
        $form['tab3'] = [
            '#type' => 'fieldset',
            '#title' => t('Tab 3'),
            '#collapsible' => TRUE,
            '#group' => 'vertical_tabs2',
        ];
        $form['tab3']['field6'] = [
            '#title' => t('Field 6'),
            '#type' => 'checkbox',
            '#default_value' => TRUE,
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        $values = $form_state->getValues();
        if (empty($values['field1'])) {
            $form_state->setErrorByName('tab1][field1', t('This checkbox inside a vertical tab does not have its default value.'));
        }
        if ($values['field2'] != 'field2') {
            $form_state->setErrorByName('tab2][field2', t('This textfield inside a vertical tab does not have its default value.'));
        }
        if (empty($values['field3'])) {
            $form_state->setErrorByName('fieldset][field3', t('This checkbox inside a fieldset does not have its default value.'));
        }
        if (empty($values['field4'])) {
            $form_state->setErrorByName('container][field4', t('This checkbox inside a container does not have its default value.'));
        }
        if (empty($values['field5'])) {
            $form_state->setErrorByName('container][subcontainer][field5', t('This checkbox inside a nested container does not have its default value.'));
        }
        if (empty($values['field5'])) {
            $form_state->setErrorByName('tab3][field6', t('This checkbox inside a vertical tab whose fieldset access is allowed does not have its default value.'));
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->messenger()
            ->addStatus(t('The form submitted correctly.'));
    }

}