[wp-trac] [WordPress Trac] #42771: WP_Term::get_instance() regression for non-category terms queried with 'category' taxonomy

[WordPress Trac]

#42771: WP_Term::get_instance() regression for non-category terms queried with
'category' taxonomy
--------------------------+-----------------------------
 Reporter:  markjaquith   |      Owner:
     Type:  defect (bug)  |     Status:  new
 Priority:  high          |  Milestone:  Awaiting Review
Component:  Taxonomy      |    Version:  4.9
 Severity:  major         |   Keywords:
  Focuses:                |
--------------------------+-----------------------------
 `WP_Term::get_instance()` changed in 4.9 which caused a lot of other
 issues, probably including:

 #42717
 #42691
 #42605
 #42732

 Other reports here:
 https://wordpress.org/support/topic/category-description-broken-
 after-4-9-update/
 https://wordpress.org/support/topic/4-9-get_category_link-not-working/

 What seems to be happening is this:

 Some old category function gets called on a `term_id` that does not exist
 as a category (let's say `123`).

 e.g. `get_category_link( 123 );`.

 This ends up calling `WP_Term::get_instance( 123, 'category' );`

 This worked before 4.9, because when the taxonomy failed to match...


 {{{
 if ( ! $_term || ( $taxonomy && $taxonomy !== $_term->taxonomy ) ) {
 }}}

 ...`$_term` would be left alone.

 But in 4.9, this happens:

 {{{
 // If there isn't a cached version, hit the database.
 if ( ! $_term || ( $taxonomy && $taxonomy !== $_term->taxonomy ) ) {
     // Any term found in the cache is not a match, so don't use it.
     $_term = false;
 }}}

 The query runs, but it won't find a taxonomy match when iterating the
 results, and thus will return false.

 The old behavior seems... undesirable. You're querying a `category` term
 and potentially getting back a term with another taxonomy.

 We could fix most of the cases in the wild by exempting `category` from
 the `$_term = false;` bailout. But then we're just perpetuating `category`
 being a special case.

--
Ticket URL: <https://core.trac.wordpress.org/ticket/42771>
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