After https://gerrit.wikimedia.org/r/c/mediawiki/core/+/939777 phan does not see a DBConnRef in IndexPager::$mDb and reports in Linter:
includes/LintErrorsPager.php:136 PhanUndeclaredMethod Call to undeclared method \Wikimedia\Rdbms\IDatabase::fieldExists includes/LintErrorsPager.php:154 PhanUndeclaredMethod Call to undeclared method \Wikimedia\Rdbms\IDatabase::fieldExists
(Global function wfGetDB returns DBConnRef, which implements IMaintainableDatabase::fieldExists, but that method is not in IDatabase)