Index: wp-includes/post-thumbnail-template.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- wp-includes/post-thumbnail-template.php	(revision cde61269235ba33355cc96b8eaff2a0164aee3c0)
+++ wp-includes/post-thumbnail-template.php	(revision )
@@ -36,7 +36,16 @@
 	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 );
 }
 
 /**