Replicate default behaviour for Custom taxonomies while creating/updating a post.

[shashwatmittal]

Generally when creating/updating a post, while inserting the default taxonomies, there is a check to ensure that the taxonomy is associated with the given post type.
But the same doesn't applies with custom taxonomies. In case the data is passed, and the taxonomies is not associated with the passed post type, it still gets added. We should add a check for the same.

if ( ! empty( $postarr['tax_input'] ) ) {
		foreach ( $postarr['tax_input'] as $taxonomy => $tags ) {
			$taxonomy_obj = get_taxonomy( $taxonomy );

			if ( ! $taxonomy_obj ) {
				/* translators: %s: Taxonomy name. */
				_doing_it_wrong( __FUNCTION__, sprintf( __( 'Invalid taxonomy: %s.' ), $taxonomy ), '4.4.0' );
				continue;
			}

			// array = hierarchical, string = non-hierarchical.
			if ( is_array( $tags ) ) {
				$tags = array_filter( $tags );
			}

			if ( current_user_can( $taxonomy_obj->cap->assign_terms ) ) {
				wp_set_post_terms( $post_ID, $tags, $taxonomy );
			}
		}
	}

The above code should check if the taxonomy passed in the tax_input exists for current post type or not.

Add a check before inserting custom taxonomies to post type. Checks if the custom taxonomy is associated with the post or not before inserting. The same things happens for default taxonomies already.

Trac ticket: https://core.trac.wordpress.org/ticket/50894

[helen]

Because this is WordPress, I have a feeling there are probably people assigning terms to post types relying on exactly this lack of checking. I don't really have a strong opinion on whether we should change that, certainly technically it seems more correct. What I do know right this moment is that we are in beta 3 for 5.6 and I bet this will break somebody's custom code if not a distributed plugin. Would love some more opinions here from folks on whether we should make this change given what I understand to be potential implications.