...那个login本来就不在common\model\User.php里，另外Yii::$app->user里的这个user也不是common\model\User.php，而是yii\web\User，那么login也就找到了;

public function login(IdentityInterface $identity, $duration = 0)
    {
        if ($this->beforeLogin($identity, false, $duration)) {
            $this->switchIdentity($identity, $duration);
            $id = $identity->getId();
            $ip = Yii::$app->getRequest()->getUserIP();
            if ($this->enableSession) {
                $log = "User '$id' logged in from $ip with duration $duration.";
            } else {
                $log = "User '$id' logged in from $ip. Session not enabled.";
            }
            Yii::info($log, __METHOD__);
            $this->afterLogin($identity, false, $duration);
        }

        return !$this->getIsGuest();
    }