Exclude breaks wp_list_categories for non-hierarchical categories

[thomask]

I'm not sure if this is a bug or just some unhappy intent

wp_list_categories( array (

'taxonomy' => 'post_tag',
'hide_empty' => false,
'exclude' => '123'

) );
return always "no category found" message. When "exclude" is not there, it works normaly, the same for any other option. Hierarchical taxonomies are ok, problem is only with non-hierarchical (even the custom ones).

Problem can be solved by adding hierarchical=false parameter, but it is nowhere written, and not very obvious.

[boonebgorges]

Confirmed.

The back story here is kinda complicated. In [8614], wp_link_categories() was modified so that the 'exclude' parameter always got translated into 'exclude_tree'. This makes sense when you use that function for categories (which are hierarchical). But when using it for non-hierarchical taxonomies, it has the side effect of short-circuiting some queries in get_terms(). In particular, get_terms() will run get_terms( 'child_of' => $excluded_item ) for each member of $exclude_tree. But that secondary run of get_terms() automatically sets child_of to 0 for non-hierarchical taxonomies, under what I gather are the auspices of saving an unnecessary query. But in your case, it was causing the secondary call to get_terms() to return *all* terms in the taxonomy, with the result that *all* terms were being excluded from the query. Thus "no category found".

Removing the lines in get_terms() that force 'child_of=0' for non-hierarchical taxonomies fixes the problem. What's more: it appears to actually *save* database queries in at least some cases, since a check for term children will return an empty array in these cases, avoiding later lookups. And since [29815], I think we have decent tests for child_of, and better understanding of it, so I feel comfortable taking it out.

Thanks for the report.

[boonebgorges]

In 30265:

Don't force child_of=0 for non-hierarchical taxonomies in get_terms().

This forcing appears to have been introduced to save unnecessary queries, but
(a) in some cases it appeared to be causing *more* queries, and (b) it caused
incorrect results when the 'exclude_tree' param of get_terms() called
get_terms() on each item in the tree using the 'child_of' param.

Fixes #30275.