The descendant query in `WP_Comment_Query::fill_descendants()` uses the clauses
of the main `get_comment_ids()` query as a basis, discarding the `parent`,
`parent__in`, and `parent__not_in` clauses. As implemented in WP 4.4 [34546],
the WHERE clause was assembled in such a way that any modifications applied
using the `comments_clauses` filter were not inherited by `fill_descendants()`.
This resulted in descendant queries that did not always properly filter
results, and sometimes contained syntax errors.
The current changeset fixes the problem by using the post-filter WHERE clause
as the basis for the `fill_descendants()` query. This change requires a new
approach for eliminating the unneeded parent-related clauses: instead of
eliminating values in an associative array, we must use regular expressions.
Merges [36277] to the 4.4 branch.
Props boonebgorges, firebird75.
Fixes#35192.
git-svn-id: https://develop.svn.wordpress.org/branches/4.4@36357 602fd350-edb4-49c9-b593-d223f7449a82
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