Paginator with output walker returns count 0 when the query has previously been executed

src/Tools/Pagination/Paginator.php

@@ -228,7 +228,22 @@ private function appendTreeWalker(Query $query, string $walkerClass): void
      */
     private function getCountQuery(): Query
     {
-        $countQuery = $this->cloneQuery($this->query);
+        /*
+            As opposed to using self::cloneQuery, the following code does not transfer
+            a potentially existing result set mapping (either set directly by the user,
+            or taken from the parser result from a previous invocation of Query::parse())
+            to the new query object. This is fine, since we are going to completely change the
+            select clause anyway, so a previously existing result set mapping (RSM) is probably wrong anyway.
+            In the case of using output walkers, we are even creating a new RSM down below.
+            In the case of using a tree walker, we want to have a new RSM created by the parser.
+        */
+        $countQuery = new Query($this->query->getEntityManager());
+        $countQuery->setDQL($this->query->getDQL());
+        $countQuery->setParameters(clone $this->query->getParameters());
+        $countQuery->setCacheable(false);
+        foreach ($this->query->getHints() as $name => $value) {
+            $countQuery->setHint($name, $value);
+        }
 
         if (! $countQuery->hasHint(CountWalker::HINT_DISTINCT)) {
             $countQuery->setHint(CountWalker::HINT_DISTINCT, true);

tests/Tests/ORM/Functional/Ticket/GH12183Test.php

@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\Tests\ORM\Functional\Ticket;
+
+use Doctrine\ORM\Tools\Pagination\Paginator;
+use Doctrine\Tests\Models\CMS\CmsArticle;
+use Doctrine\Tests\OrmFunctionalTestCase;
+
+final class GH12183Test extends OrmFunctionalTestCase
+{
+    protected function setUp(): void
+    {
+        $this->useModelSet('cms');
+
+        parent::setUp();
+
+        $article = new CmsArticle();
+
+        $article->topic = 'Loomings';
+        $article->text  = 'Call me Ishmael.';
+
+        $this->_em->persist($article);
+        $this->_em->flush();
+        $this->_em->clear();
+    }
+
+    public function testPaginatorCountWithTreeWalkerAfterQueryHasBeenExecuted(): void
+    {
+        $query = $this->_em->createQuery('SELECT a FROM Doctrine\Tests\Models\CMS\CmsArticle a');
+
+        // Paginator::count is right when the query has not yet been executed
+        $paginator = new Paginator($query);
+        $paginator->setUseOutputWalkers(false);
+        self::assertSame(1, $paginator->count());
+
+        // Execute the query
+        $result = $query->getResult();
+        self::assertCount(1, $result);
+
+        $paginator = new Paginator($query);
+        $paginator->setUseOutputWalkers(false);
+        self::assertSame(1, $paginator->count());
+    }
+}