`yii\db\Query::queryScalar()` fixed to use `static` when creating subquery

framework/CHANGELOG.md

@@ -7,6 +7,7 @@ Yii Framework 2 Change Log
 - Bug #6080: Oracle DB schema did not load column types correctly (wenbin1989)
 - Bug #6404: advanced application template `Alert` widget was generating duplicate IDs in case of multiple flashes (SDKiller)
 - Bug #6557: Link URLs generated by `yii\widgets\Menu` are not encoded (qiangxue)
+- Bug #6621: `yii\db\Query::queryScalar()` fixed to use `static` when creating subquery (klimov-paul)
 - Enh #6247: Logger and error handler are now using slightly less memory (stepanselyuk, samdark)
 - Enh #6434: Added `yii\behaviors\SluggableBehavior::immutable` to support keeping the generated slug unchanged (trntv)
 - Enh #6467: `ActiveForm` will scroll to the nearest visible element when the first error input is hidden (newartix)

framework/db/Query.php

@@ -372,7 +372,7 @@ class Query extends Component implements QueryInterface
         if (empty($this->groupBy) && empty($this->union) && !$this->distinct) {
             return $command->queryScalar();
         } else {
-            return (new Query)->select([$selectExpression])
+            return (new static)->select([$selectExpression])
                 ->from(['c' => $this])
                 ->createCommand($command->db)
                 ->queryScalar();