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While tests are having trouble passing now in trunk, it would be nice to add a test for the case when something other than WP_Term is returned.

I thought about tests before submitting this, but wasn't sure what form a test would take. For example, I could generate a test that fails when get_term() returns null - but that test would then always fail. What would success look like in that scenario?

The inverse also seems not very helpful, if we test that we got null back in a way we expected, then it would always pass. Something like:

class Tests_Term_Null_Term_Handling extends WP_UnitTestCase {
    public function test_orphaned_hierarchy_causes_null_term_access() {
        global $wpdb;
        register_taxonomy( 'wptests_tax', 'post', array( 'hierarchical' => true ) );    
        $parent_term = self::factory()->term->create( array( 'taxonomy' => 'wptests_tax' ) );
        $non_existent_child_id = 99999;
        $hierarchy = array( $parent_term => array( $non_existent_child_id ) );
        update_option( 'wptests_tax_children', $hierarchy ); 
        $this->assertNull( get_term( $non_existent_child_id, 'wptests_tax' ), 'Non-existent term should return null' );
    }
}

If you are ok with a test like that, I'm happy to clean it up ( and add comments ) and add that to the patch.

Yeah, I mean whatever scenario caused the error to happen in the first place on your site would be good to add a test for.

I honestly don't know what happened, I noticed it on sites that have been running for more than 15 years and have probably been generating this warning for years.

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 60661
GitHub commit: 9bc4e2f

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.