Drupal\views\Plugin\views\query\Sql::buildCondition is checking for a non existing class

          // DBTNG doesn't support to add the same subquery twice to the main
          // query and the count query, so clone the subquery to have two instances
          // of the same object. - http://drupal.org/node/1112854
          if (is_object($clause['value']) && $clause['value'] instanceof SelectQuery) {
            $clause['value'] = clone $clause['value'];
          }

The instance of here can never be true because there is no use statement for SelectQuery. Therefore either this code is not needed or we are missing test coverage.