Ticket #23983: 23983.patch

wp-includes/post-thumbnail-template.php

@@ -31 +31 @@
  */
 function get_post_thumbnail_id( $post_id = null ) {
         $post_id = ( null === $post_id ) ? get_the_ID() : $post_id;
-        return get_post_meta( $post_id, '_thumbnail_id', true );
+        return apply_filters( 'post_thumbnail_id', get_post_meta( $post_id, '_thumbnail_id', true ), $post_id );
 }
 
 /**