Does not work with Smart Date dates

Just reporting here that contextual_range_filter does work with Smart Date range field type. Make sure your contextual filters are enabled at /admin/config/content/contextual-range-filter

Configure your smart date range field contextual filter as follows:
When the filter value is NOT available

• Provide default value
• Type: PHP code

if (isset($entity['node'])) {
  // Get the date range
  $start_date = $entity['node']->field_smartdate_range->value;
  $end_date = $entity['node']->field_smartdate_range->end_value;
  $range = $start_date . '--' . $end_date;
  return $range;
}

For lower limit range only:

if (isset($entity['node'])) {
  // Get the date range
  $start_date = $entity['node']->field_smartdate->value;
  $range = $start_date . '--';
  return $range;
}

Hope this helps someone.