Ticket #23983: 23983.2.patch

wp-includes/post-thumbnail-template.php

@@ -36 +36 @@
         if ( ! $post ) {
                 return '';
         }
-        return get_post_meta( $post->ID, '_thumbnail_id', true );
+
+        /**
+         * Filters the post thumbnail ID.
+         *
+         * @since 4.8.0
+         *
+         * @param string  $post_thumbnail_id The post thumbnail ID.
+         * @param WP_Post $post              The post object.
+         */
+        return (int) apply_filters( 'post_thumbnail_id', get_post_meta( $post->ID, '_thumbnail_id', true ), $post );
 }
 
 /**