update_post_thumbnail_cache returns notice when get_the_post_thumbnail used with ID after setup_postdata()

[tomcent]

Situation:

• Inside main loop
• foreach over array of (related) post_id's,
• use setup_postdata($post) for each item
• Items call get_template_part(something)
• Template parts uses get_the_post_thumbnail which allow a post_ID
• get_the_post_thumbnail calls update_post_thumbnail_cache (because in_the_loop)
• update_post_thumbnail_cache gives notice on line 101: Trying to get property 'ID' of non-object
• Restore main loop with wp_reset_postdata()

Possible fix:
Change this

<?php
foreach ( $wp_query->posts as $post ) {
        $id = get_post_thumbnail_id( $post->ID );
        if ( $id ) {
                $thumb_ids[] = $id;
        }
}

To this

<?php
foreach ( $wp_query->posts as $post ) {
        if (is_object($post)) {
                $post = $post->ID;
        }
        $id = get_post_thumbnail_id( $post );
        if ( $id ) {
                $thumb_ids[] = $id;
        }
}

in /wp-includes/post-thumbnail-template.php line 101

[tomcent]

Replying to tomcent:

update_post_thumbnail_cache gives notice on line 101: Trying to get property 'ID' of non-object

Change in /wp-includes/post-thumbnail-template.php line 101

This is now line 106 after the WP 5.5 update.
The issue is still present on this version.

This PR addresses an issue where notices such as "Trying to get property 'ID' of non-object" could occur when working with $wp_query->posts in loops. The code has been updated to validate that $post is an instance of WP_Post before attempting to access its properties. If $post is valid, it is converted to its post ID to ensure compatibility with get_post_thumbnail_id(). This change prevents errors in scenarios where $post may not be a proper object, improving the reliability and stability of the loop when handling post data.

Trac ticket: https://core.trac.wordpress.org/ticket/50847