register_taxonomy_for_object_type cannot be used with non-post types

[leewillis77]

register_taxonomy_for_object_type checks that the object type is a post_type object, and will not let you register the taxonomy for the object type if it is not:

​http://core.trac.wordpress.org/browser/trunk/src/wp-includes/taxonomy.php#L475

During discussion with nacin on IRC relating to #11058 he suggesting removing the get_post_type_object check from the new unregister_taxonomy_from_object_type function since it didn't seem to be sensible.

This ticket is open to investigate the restriction in register_taxonomy_for_object_type and see if it can also be removed.

Patch attached in case the collective wisdom decides that it is unnecessary.

[leewillis77]

Proposed patch

[SergeyBiryukov]

Introduced in [12597] along with the function itself.

[ryan]

Which object types do we have in mind? Seems we'd have to make sure that the taxonomy isn't registered for multiple object types that aren't all of the same class ( post | user | whatever ).

[nacin]

On further thought, this function should be limited to post types. I was wrong earlier.

Registering a taxonomy for an object type that isn't a post type is a fairly dangerous affair. If a single taxonomy handles both a post type and a non-post type, you end up with ID conflicts. I'm not sure I can think of a situation where it should be allowed. Removing this check therefore probably isn't prudent — actual assignment of a non-post object type should occur only on registration of the taxonomy.

Wontfix.