diff -r c7c34916027a -r 177826044cd9 wp/wp-includes/post-thumbnail-template.php
--- a/wp/wp-includes/post-thumbnail-template.php	Mon Oct 14 18:06:33 2019 +0200
+++ b/wp/wp-includes/post-thumbnail-template.php	Mon Oct 14 18:28:13 2019 +0200
@@ -10,7 +10,11 @@
  */
 
 /**
- * Check if post has an image attached.
+ * Determines whether a post has an image attached.
+ *
+ * For more information on this and similar theme functions, check out
+ * the {@link https://developer.wordpress.org/themes/basics/conditional-tags/
+ * Conditional Tags} article in the Theme Developer Handbook.
  *
  * @since 2.9.0
  * @since 4.4.0 `$post` can be a post ID or WP_Post object.
@@ -19,7 +23,19 @@
  * @return bool Whether the post has an image attached.
  */
 function has_post_thumbnail( $post = null ) {
-	return (bool) get_post_thumbnail_id( $post );
+	$thumbnail_id  = get_post_thumbnail_id( $post );
+	$has_thumbnail = (bool) $thumbnail_id;
+
+	/**
+	 * Filters whether a post has a post thumbnail.
+	 *
+	 * @since 5.1.0
+	 *
+	 * @param bool             $has_thumbnail true if the post has a post thumbnail, otherwise false.
+	 * @param int|WP_Post|null $post          Post ID or WP_Post object. Default is global `$post`.
+	 * @param int|string       $thumbnail_id  Post thumbnail ID or empty string.
+	 */
+	return (bool) apply_filters( 'has_post_thumbnail', $has_thumbnail, $post, $thumbnail_id );
 }
 
 /**
@@ -72,19 +88,22 @@
  * @param WP_Query $wp_query Optional. A WP_Query instance. Defaults to the $wp_query global.
  */
 function update_post_thumbnail_cache( $wp_query = null ) {
-	if ( ! $wp_query )
+	if ( ! $wp_query ) {
 		$wp_query = $GLOBALS['wp_query'];
+	}
 
-	if ( $wp_query->thumbnails_cached )
+	if ( $wp_query->thumbnails_cached ) {
 		return;
+	}
 
 	$thumb_ids = array();
 	foreach ( $wp_query->posts as $post ) {
-		if ( $id = get_post_thumbnail_id( $post->ID ) )
+		if ( $id = get_post_thumbnail_id( $post->ID ) ) {
 			$thumb_ids[] = $id;
+		}
 	}
 
-	if ( ! empty ( $thumb_ids ) ) {
+	if ( ! empty( $thumb_ids ) ) {
 		_prime_post_caches( $thumb_ids, false, true );
 	}
 
@@ -145,8 +164,9 @@
 		 *                                        and height values (in that order). Default 'post-thumbnail'.
 		 */
 		do_action( 'begin_fetch_post_thumbnail_html', $post->ID, $post_thumbnail_id, $size );
-		if ( in_the_loop() )
+		if ( in_the_loop() ) {
 			update_post_thumbnail_cache();
+		}
 		$html = wp_get_attachment_image( $post_thumbnail_id, $size, false, $attr );
 
 		/**