add current_page_item to custom post type if active in WordPress

add_filter( 'nav_menu_css_class', 'thd_menu_classes', 10, 2 );
function thd_menu_classes( $classes , $item ){
  if ( get_post_type() == 'product' ) {
    // remove unwanted if found
    $classes = str_replace( 'current_page_parent', '', $classes );
    // check if url contains name of custom post type
    if ( strpos($item->url, '/product/' )) {
      $classes = str_replace( 'menu-item', 'menu-item current_page_item', $classes );
      remove_filter( 'nav_menu_css_class', 'thd_menu_classes', 10, 2 );
    }
  }
  return $classes;
}

// replace 'product' on line 3 with name of your custom post type
// replace '/product/' on line 7 with part of url you want to filter