Source

File: wp-includes/post-thumbnail-template.php

function has_post_thumbnail( $post = null ) {
	$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 );
}