Media: Introduce the `has_post_thumbnail` filter.

Props rzen, desrosj.
Fixes #44859.
Built from https://develop.svn.wordpress.org/trunk@44542


git-svn-id: http://core.svn.wordpress.org/trunk@44373 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/post-thumbnail-template.php

@@ -23,7 +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 );
 }
 
 /**

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44541';
+$wp_version = '5.1-alpha-44542';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.