diff --git a/lib/Doctrine/ORM/Query/SqlWalker.php b/lib/Doctrine/ORM/Query/SqlWalker.php
index 186187025..bc86ec8f6 100644
--- a/lib/Doctrine/ORM/Query/SqlWalker.php
+++ b/lib/Doctrine/ORM/Query/SqlWalker.php
@@ -2276,7 +2276,7 @@ class SqlWalker implements TreeWalker
      * @return string The list in parentheses of valid child discriminators from the given class
      * @throws QueryException
      */
-    private function getChildDiscriminatorsFromClassMetadata(ClassMetadataInfo $rootClass, AST\InstanceOfExpression $instanceOfExpr)
+    private function getChildDiscriminatorsFromClassMetadata(ClassMetadataInfo $rootClass, AST\InstanceOfExpression $instanceOfExpr): string
     {
         $sqlParameterList = [];
         $discriminators = [];
diff --git a/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfAbstractTest.php b/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfAbstractTest.php
index f0ea53fe8..5fa5cdc86 100644
--- a/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfAbstractTest.php
+++ b/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfAbstractTest.php
@@ -6,7 +6,7 @@ use Doctrine\Tests\OrmFunctionalTestCase;
 
 class Ticket4646InstanceOfAbstractTest extends OrmFunctionalTestCase
 {
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
@@ -16,7 +16,7 @@ class Ticket4646InstanceOfAbstractTest extends OrmFunctionalTestCase
         ]);
     }
 
-    public function testInstanceOf()
+    public function testInstanceOf(): void
     {
         $this->_em->persist(new EmployeeTicket4646Abstract());
         $this->_em->flush();
@@ -26,8 +26,8 @@ class Ticket4646InstanceOfAbstractTest extends OrmFunctionalTestCase
         $query = $this->_em->createQuery($dql);
         $result = $query->getResult();
 
-        $this->assertCount(1, $result);
-        $this->assertContainsOnlyInstancesOf(PersonTicket4646Abstract::class, $result);
+        self::assertCount(1, $result);
+        self::assertContainsOnlyInstancesOf(PersonTicket4646Abstract::class, $result);
     }
 }
 
@@ -49,7 +49,7 @@ abstract class PersonTicket4646Abstract
      */
     private $id;
 
-    public function getId()
+    public function getId(): ?int
     {
         return $this->id;
     }
diff --git a/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfMultiLevelTest.php b/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfMultiLevelTest.php
index 6e4160b24..2c111a9e6 100644
--- a/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfMultiLevelTest.php
+++ b/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfMultiLevelTest.php
@@ -6,7 +6,7 @@ use Doctrine\Tests\OrmFunctionalTestCase;
 
 class Ticket4646InstanceOfMultiLevelTest extends OrmFunctionalTestCase
 {
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
@@ -17,7 +17,7 @@ class Ticket4646InstanceOfMultiLevelTest extends OrmFunctionalTestCase
         ]);
     }
 
-    public function testInstanceOf()
+    public function testInstanceOf(): void
     {
         $this->_em->persist(new PersonTicket4646MultiLevel());
         $this->_em->persist(new EmployeeTicket4646MultiLevel());
@@ -29,8 +29,8 @@ class Ticket4646InstanceOfMultiLevelTest extends OrmFunctionalTestCase
         $query = $this->_em->createQuery($dql);
         $result = $query->getResult();
 
-        $this->assertCount(3, $result);
-        $this->assertContainsOnlyInstancesOf(PersonTicket4646MultiLevel::class, $result);
+        self::assertCount(3, $result);
+        self::assertContainsOnlyInstancesOf(PersonTicket4646MultiLevel::class, $result);
     }
 }
 
@@ -54,7 +54,7 @@ class PersonTicket4646MultiLevel
      */
     private $id;
 
-    public function getId()
+    public function getId(): ?int
     {
         return $this->id;
     }
diff --git a/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfParametricTest.php b/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfParametricTest.php
index 302934e8c..bf11c3a85 100644
--- a/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfParametricTest.php
+++ b/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfParametricTest.php
@@ -6,7 +6,7 @@ use Doctrine\Tests\OrmFunctionalTestCase;
 
 class Ticket4646InstanceOfParametricTest extends OrmFunctionalTestCase
 {
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
         $this->_schemaTool->createSchema([
@@ -15,7 +15,7 @@ class Ticket4646InstanceOfParametricTest extends OrmFunctionalTestCase
         ]);
     }
 
-    public function testInstanceOf()
+    public function testInstanceOf(): void
     {
         $this->_em->persist(new PersonTicket4646Parametric());
         $this->_em->persist(new EmployeeTicket4646Parametric());
@@ -28,8 +28,8 @@ class Ticket4646InstanceOfParametricTest extends OrmFunctionalTestCase
             $this->_em->getClassMetadata(PersonTicket4646Parametric::class)
         );
         $result = $query->getResult();
-        $this->assertCount(2, $result);
-        $this->assertContainsOnlyInstancesOf(PersonTicket4646Parametric::class, $result);
+        self::assertCount(2, $result);
+        self::assertContainsOnlyInstancesOf(PersonTicket4646Parametric::class, $result);
     }
 }
 
@@ -52,7 +52,7 @@ class PersonTicket4646Parametric
      */
     private $id;
 
-    public function getId()
+    public function getId(): ?int
     {
         return $this->id;
     }
diff --git a/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfTest.php b/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfTest.php
index 363611d4d..0ed97243c 100644
--- a/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfTest.php
+++ b/tests/Doctrine/Tests/ORM/Functional/Ticket/Ticket4646InstanceOfTest.php
@@ -6,7 +6,7 @@ use Doctrine\Tests\OrmFunctionalTestCase;
 
 class Ticket4646InstanceOfTest extends OrmFunctionalTestCase
 {
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
@@ -16,7 +16,7 @@ class Ticket4646InstanceOfTest extends OrmFunctionalTestCase
         ]);
     }
 
-    public function testInstanceOf()
+    public function testInstanceOf(): void
     {
         $this->_em->persist(new PersonTicket4646());
         $this->_em->persist(new EmployeeTicket4646());
@@ -27,8 +27,8 @@ class Ticket4646InstanceOfTest extends OrmFunctionalTestCase
         $query = $this->_em->createQuery($dql);
         $result = $query->getResult();
 
-        $this->assertCount(2, $result);
-        $this->assertContainsOnlyInstancesOf(PersonTicket4646::class, $result);
+        self::assertCount(2, $result);
+        self::assertContainsOnlyInstancesOf(PersonTicket4646::class, $result);
     }
 }
 
@@ -51,7 +51,7 @@ class PersonTicket4646
      */
     private $id;
 
-    public function getId()
+    public function getId(): ?int
     {
         return $this->id;
     }