Lack of #title and #title_display in interval_element_process interfere with required validation

Hi,
I had a node form which an interval field was required. If the field value was empty drupal would stop the form from submitting but would not display an error message.

Drupal relies on the #title element to display required form errors
from form.inc:

// Although discouraged, a #title is not mandatory for form elements. In
        // case there is no #title, we cannot set a form error message.
        // Instead of setting no #title, form constructors are encouraged to set
        // #title_display to 'invisible' to improve accessibility.
        if (isset($elements['#title'])) {
          form_error($elements, $t('!name field is required.', array('!name' => $elements['#title'])));
        }
        else {
          form_error($elements);
        }

Adding the title and title_display solved the issue for me

function interval_element_process($element, &$form_state, $form) {
  $value = !empty($element['#default_value'])?$element['#default_value']:array('interval' => NULL, 'period' => NULL);
  $element['interval'] = array(
    '#type' => 'textfield',
    '#default_value' => $value['interval'],
    '#required' => $element['#required'],
    '#title' => $element['#title'],
    '#title_display' => 'invisible',
  );

Patch coming.