diff --git a/composer.json b/composer.json
index cf7b249ae57..56e544310d8 100644
--- a/composer.json
+++ b/composer.json
@@ -24,7 +24,7 @@
         "composer-runtime-api": "^2",
         "ext-ctype": "*",
         "doctrine/cache": "^1.12.1 || ^2.1.1",
-        "doctrine/collections": "^1.5",
+        "doctrine/collections": "^1.5 || 2.0.x@dev",
         "doctrine/common": "^3.0.3",
         "doctrine/dbal": "^2.13.1 || ^3.2",
         "doctrine/deprecations": "^0.5.3 || ^1",
diff --git a/lib/Doctrine/ORM/EntityRepository.php b/lib/Doctrine/ORM/EntityRepository.php
index 9e633050b40..b55f7799d53 100644
--- a/lib/Doctrine/ORM/EntityRepository.php
+++ b/lib/Doctrine/ORM/EntityRepository.php
@@ -7,6 +7,7 @@
 use BadMethodCallException;
 use Doctrine\Common\Collections\AbstractLazyCollection;
 use Doctrine\Common\Collections\Criteria;
+use Doctrine\Common\Collections\ReadableCollection;
 use Doctrine\Common\Collections\Selectable;
 use Doctrine\Common\Persistence\PersistentObject;
 use Doctrine\DBAL\LockMode;
@@ -326,7 +327,7 @@ protected function getClassMetadata()
      * @return AbstractLazyCollection
      * @psalm-return AbstractLazyCollection<int, T>&Selectable<int, T>
      */
-    public function matching(Criteria $criteria)
+    public function matching(Criteria $criteria): ReadableCollection
     {
         $persister = $this->_em->getUnitOfWork()->getEntityPersister($this->_entityName);
 
diff --git a/lib/Doctrine/ORM/LazyCriteriaCollection.php b/lib/Doctrine/ORM/LazyCriteriaCollection.php
index 21261308401..61c67a0c856 100644
--- a/lib/Doctrine/ORM/LazyCriteriaCollection.php
+++ b/lib/Doctrine/ORM/LazyCriteriaCollection.php
@@ -7,6 +7,7 @@
 use Doctrine\Common\Collections\AbstractLazyCollection;
 use Doctrine\Common\Collections\ArrayCollection;
 use Doctrine\Common\Collections\Criteria;
+use Doctrine\Common\Collections\ReadableCollection;
 use Doctrine\Common\Collections\Selectable;
 use Doctrine\ORM\Persisters\Entity\EntityPersister;
 use ReturnTypeWillChange;
@@ -43,11 +44,8 @@ public function __construct(EntityPersister $entityPersister, Criteria $criteria
 
     /**
      * Do an efficient count on the collection
-     *
-     * @return int
      */
-    #[ReturnTypeWillChange]
-    public function count()
+    public function count(): int
     {
         if ($this->isInitialized()) {
             return $this->collection->count();
@@ -66,7 +64,7 @@ public function count()
      *
      * @return bool TRUE if the collection is empty, FALSE otherwise.
      */
-    public function isEmpty()
+    public function isEmpty(): bool
     {
         if ($this->isInitialized()) {
             return $this->collection->isEmpty();
@@ -80,10 +78,8 @@ public function isEmpty()
      *
      * @param object $element
      * @psalm-param TValue $element
-     *
-     * @return bool
      */
-    public function contains($element)
+    public function contains($element): bool
     {
         if ($this->isInitialized()) {
             return $this->collection->contains($element);
@@ -95,7 +91,7 @@ public function contains($element)
     /**
      * {@inheritDoc}
      */
-    public function matching(Criteria $criteria)
+    public function matching(Criteria $criteria): ReadableCollection
     {
         $this->initialize();
         assert($this->collection instanceof Selectable);
@@ -106,7 +102,7 @@ public function matching(Criteria $criteria)
     /**
      * {@inheritDoc}
      */
-    protected function doInitialize()
+    protected function doInitialize(): void
     {
         $elements         = $this->entityPersister->loadCriteria($this->criteria);
         $this->collection = new ArrayCollection($elements);
diff --git a/lib/Doctrine/ORM/PersistentCollection.php b/lib/Doctrine/ORM/PersistentCollection.php
index 8dac7210fbc..0f97515c40f 100644
--- a/lib/Doctrine/ORM/PersistentCollection.php
+++ b/lib/Doctrine/ORM/PersistentCollection.php
@@ -323,7 +323,7 @@ public function setInitialized($bool): void
     /**
      * {@inheritdoc}
      */
-    public function remove($key)
+    public function remove($key): mixed
     {
         // TODO: If the keys are persistent as well (not yet implemented)
         //       and the collection is not initialized and orphanRemoval is
@@ -408,7 +408,7 @@ public function contains($element): bool
     /**
      * {@inheritdoc}
      */
-    public function get($key)
+    public function get($key): mixed
     {
         if (
             ! $this->initialized
@@ -479,8 +479,7 @@ public function offsetExists($offset): bool
     /**
      * {@inheritdoc}
      */
-    #[ReturnTypeWillChange]
-    public function offsetGet($offset)
+    public function offsetGet($offset): mixed
     {
         return $this->get($offset);
     }
@@ -501,13 +500,10 @@ public function offsetSet($offset, $value): void
 
     /**
      * {@inheritdoc}
-     *
-     * @return object|null
      */
-    #[ReturnTypeWillChange]
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
-        return $this->remove($offset);
+        $this->remove($offset);
     }
 
     public function isEmpty(): bool
diff --git a/lib/Doctrine/ORM/Persisters/SqlExpressionVisitor.php b/lib/Doctrine/ORM/Persisters/SqlExpressionVisitor.php
index 967d4b4f65c..5f793bb070d 100644
--- a/lib/Doctrine/ORM/Persisters/SqlExpressionVisitor.php
+++ b/lib/Doctrine/ORM/Persisters/SqlExpressionVisitor.php
@@ -35,10 +35,8 @@ public function __construct(BasicEntityPersister $persister, ClassMetadata $clas
 
     /**
      * Converts a comparison expression into the target query language output.
-     *
-     * @return mixed
      */
-    public function walkComparison(Comparison $comparison)
+    public function walkComparison(Comparison $comparison): mixed
     {
         $field = $comparison->getField();
         $value = $comparison->getValue()->getValue(); // shortcut for walkValue()
@@ -65,7 +63,7 @@ public function walkComparison(Comparison $comparison)
      *
      * @throws RuntimeException
      */
-    public function walkCompositeExpression(CompositeExpression $expr)
+    public function walkCompositeExpression(CompositeExpression $expr): mixed
     {
         $expressionList = [];
 
@@ -90,7 +88,7 @@ public function walkCompositeExpression(CompositeExpression $expr)
      *
      * @return string
      */
-    public function walkValue(Value $value)
+    public function walkValue(Value $value): mixed
     {
         return '?';
     }
diff --git a/lib/Doctrine/ORM/Persisters/SqlValueVisitor.php b/lib/Doctrine/ORM/Persisters/SqlValueVisitor.php
index a61d0a25f86..795cb7b856e 100644
--- a/lib/Doctrine/ORM/Persisters/SqlValueVisitor.php
+++ b/lib/Doctrine/ORM/Persisters/SqlValueVisitor.php
@@ -25,7 +25,7 @@ class SqlValueVisitor extends ExpressionVisitor
      *
      * {@inheritDoc}
      */
-    public function walkComparison(Comparison $comparison)
+    public function walkComparison(Comparison $comparison): mixed
     {
         $value    = $this->getValueFromComparison($comparison);
         $field    = $comparison->getField();
@@ -48,7 +48,7 @@ public function walkComparison(Comparison $comparison)
      *
      * {@inheritDoc}
      */
-    public function walkCompositeExpression(CompositeExpression $expr)
+    public function walkCompositeExpression(CompositeExpression $expr): mixed
     {
         foreach ($expr->getExpressionList() as $child) {
             $this->dispatch($child);
@@ -62,7 +62,7 @@ public function walkCompositeExpression(CompositeExpression $expr)
      *
      * {@inheritDoc}
      */
-    public function walkValue(Value $value)
+    public function walkValue(Value $value): mixed
     {
         return null;
     }
diff --git a/lib/Doctrine/ORM/Query/QueryExpressionVisitor.php b/lib/Doctrine/ORM/Query/QueryExpressionVisitor.php
index a17323c0002..4de269ce704 100644
--- a/lib/Doctrine/ORM/Query/QueryExpressionVisitor.php
+++ b/lib/Doctrine/ORM/Query/QueryExpressionVisitor.php
@@ -77,10 +77,7 @@ private static function convertComparisonOperator($criteriaOperator)
         return self::$operatorMap[$criteriaOperator] ?? null;
     }
 
-    /**
-     * {@inheritDoc}
-     */
-    public function walkCompositeExpression(CompositeExpression $expr)
+    public function walkCompositeExpression(CompositeExpression $expr): mixed
     {
         $expressionList = [];
 
@@ -103,7 +100,7 @@ public function walkCompositeExpression(CompositeExpression $expr)
     /**
      * {@inheritDoc}
      */
-    public function walkComparison(Comparison $comparison)
+    public function walkComparison(Comparison $comparison): mixed
     {
         if (! isset($this->queryAliases[0])) {
             throw new QueryException('No aliases are set before invoking walkComparison().');
@@ -197,10 +194,7 @@ public function walkComparison(Comparison $comparison)
         }
     }
 
-    /**
-     * {@inheritDoc}
-     */
-    public function walkValue(Value $value)
+    public function walkValue(Value $value): mixed
     {
         return $value->getValue();
     }