diff --git a/phpstan.dist.neon b/phpstan.dist.neon
index c063526..7a9d50f 100644
--- a/phpstan.dist.neon
+++ b/phpstan.dist.neon
@@ -7,7 +7,7 @@ includes:
     - vendor/phpstan/phpstan-strict-rules/rules.neon
 
 parameters:
-    level: 2
+    level: 9
     strictRules:
         noVariableVariables: false
     paths:
diff --git a/src/DataStructures/StrictList.php b/src/DataStructures/StrictList.php
index d717046..a93a4d4 100644
--- a/src/DataStructures/StrictList.php
+++ b/src/DataStructures/StrictList.php
@@ -31,6 +31,9 @@ use OCC\Basics\Traits\Getter;
  *
  * @author Sebastian Meyer <sebastian.meyer@opencultureconsulting.com>
  * @package opencultureconsulting/basics
+ *
+ * @template AllowedTypes
+ * @extends SplDoublyLinkedList<AllowedTypes>
  */
 class StrictList extends SplDoublyLinkedList
 {
@@ -56,6 +59,8 @@ class StrictList extends SplDoublyLinkedList
      *
      * Fully qualified class names (FQCN) can be specified instead of the
      * generic type "object".
+     *
+     * @var string[]
      */
     protected array $allowedTypes = [];
 
@@ -64,7 +69,7 @@ class StrictList extends SplDoublyLinkedList
      * @see SplDoublyLinkedList::add()
      *
      * @param int $index The index where the new item is to be inserted
-     * @param mixed $item The new item for the index
+     * @param AllowedTypes $item The new item for the index
      *
      * @return void
      *
@@ -86,7 +91,7 @@ class StrictList extends SplDoublyLinkedList
     /**
      * Append items at the end of the list.
      *
-     * @param mixed ...$items One or more items to append
+     * @param AllowedTypes ...$items One or more items to append
      *
      * @return void
      *
@@ -99,7 +104,7 @@ class StrictList extends SplDoublyLinkedList
                 throw new InvalidArgumentException(
                     sprintf(
                         'Parameter %d must be an allowed type, %s given.',
-                        $count + 1,
+                        (int) $count + 1,
                         get_debug_type($item)
                     )
                 );
@@ -113,7 +118,7 @@ class StrictList extends SplDoublyLinkedList
     /**
      * Get the allowed item types.
      *
-     * @return array The list of allowed item types
+     * @return string[] The list of allowed item types
      */
     public function getAllowedTypes(): array
     {
@@ -123,7 +128,7 @@ class StrictList extends SplDoublyLinkedList
     /**
      * Check if item is an allowed type.
      *
-     * @param mixed $item The item to check
+     * @param AllowedTypes $item The item to check
      *
      * @return bool Whether the item is an allowed type
      */
@@ -149,7 +154,7 @@ class StrictList extends SplDoublyLinkedList
      * Magic getter method for $this->allowedTypes.
      * @see Getter
      *
-     * @return array The list of allowed item types
+     * @return string[] The list of allowed item types
      */
     protected function magicGetAllowedTypes(): array
     {
@@ -161,7 +166,7 @@ class StrictList extends SplDoublyLinkedList
      * @see \ArrayAccess::offsetSet()
      *
      * @param ?int $index The index being set or NULL to append
-     * @param mixed $item The new item for the index
+     * @param AllowedTypes $item The new item for the index
      *
      * @return void
      *
@@ -183,7 +188,7 @@ class StrictList extends SplDoublyLinkedList
     /**
      * Prepend items at the start of the list.
      *
-     * @param mixed ...$items One or more items to prepend
+     * @param AllowedTypes ...$items One or more items to prepend
      *
      * @return void
      *
@@ -196,7 +201,7 @@ class StrictList extends SplDoublyLinkedList
                 throw new InvalidArgumentException(
                     sprintf(
                         'Parameter %d must be an allowed type, %s given.',
-                        $count + 1,
+                        (int) $count + 1,
                         get_debug_type($item)
                     )
                 );
@@ -211,7 +216,7 @@ class StrictList extends SplDoublyLinkedList
      * Push an item at the end of the list.
      * @see SplDoublyLinkedList::push()
      *
-     * @param mixed $item The item to push
+     * @param AllowedTypes $item The item to push
      *
      * @return void
      *
@@ -251,14 +256,16 @@ class StrictList extends SplDoublyLinkedList
      */
     public function unserialize($data): void
     {
-        $this->__unserialize(unserialize($data));
+        /** @var mixed[] $dataArray */
+        $dataArray = unserialize($data);
+        $this->__unserialize($dataArray);
     }
 
     /**
      * Prepend the list with an item.
      * @see SplDoublyLinkedList::unshift()
      *
-     * @param mixed $item The item to unshift
+     * @param AllowedTypes $item The item to unshift
      *
      * @return void
      *
@@ -280,7 +287,7 @@ class StrictList extends SplDoublyLinkedList
     /**
      * Create a type-sensitive, traversable list of items.
      *
-     * @param iterable $items Initial set of items
+     * @param iterable<AllowedTypes> $items Initial set of items
      * @param string[] $allowedTypes Allowed types of items (optional)
      *
      * @throws InvalidArgumentException
@@ -299,7 +306,7 @@ class StrictList extends SplDoublyLinkedList
     /**
      * Get debug information for $this.
      *
-     * @return array Array of debug information
+     * @return mixed[] Array of debug information
      */
     public function __debugInfo(): array
     {
@@ -309,30 +316,33 @@ class StrictList extends SplDoublyLinkedList
     /**
      * Get array representation of $this.
      *
-     * @return array Array representation
+     * @return mixed[] Array representation
      */
     public function __serialize(): array
     {
         return [
             'StrictList::allowedTypes' => $this->allowedTypes,
-            'SplDoublyLinkedList::flags' => $this->getIteratorMode(),
-            'SplDoublyLinkedList::dllist' => iterator_to_array($this)
+            'SplDoublyLinkedList::dllist' => iterator_to_array($this),
+            'SplDoublyLinkedList::flags' => $this->getIteratorMode()
         ];
     }
 
     /**
      * Restore $this from array representation.
      *
-     * @param array $data Array representation
+     * @param mixed[] $data Array representation
      *
      * @return void
      */
     public function __unserialize(array $data): void
     {
-        $this->__construct(
-            $data['SplDoublyLinkedList::dllist'],
-            $data['StrictList::allowedTypes']
-        );
-        $this->setIteratorMode($data['SplDoublyLinkedList::flags']);
+        /** @var string[] $allowedTypes */
+        $allowedTypes = $data['StrictList::allowedTypes'];
+        /** @var iterable<AllowedTypes> $items */
+        $items = $data['SplDoublyLinkedList::dllist'];
+        $this->__construct($items, $allowedTypes);
+        /** @var int $flags */
+        $flags = $data['SplDoublyLinkedList::flags'];
+        $this->setIteratorMode($flags);
     }
 }
diff --git a/src/DataStructures/StrictQueue.php b/src/DataStructures/StrictQueue.php
index cfaf2ca..5b09323 100644
--- a/src/DataStructures/StrictQueue.php
+++ b/src/DataStructures/StrictQueue.php
@@ -29,6 +29,9 @@ use RuntimeException;
  *
  * @author Sebastian Meyer <sebastian.meyer@opencultureconsulting.com>
  * @package opencultureconsulting/basics
+ *
+ * @template AllowedTypes
+ * @extends StrictList<AllowedTypes>
  */
 class StrictQueue extends StrictList
 {
@@ -36,7 +39,7 @@ class StrictQueue extends StrictList
      * Dequeue an item from the queue.
      * @see \SplQueue::dequeue()
      *
-     * @return mixed The dequeued item
+     * @return AllowedTypes The dequeued item
      */
     public function dequeue(): mixed
     {
@@ -47,7 +50,7 @@ class StrictQueue extends StrictList
      * Add an item to the queue.
      * @see \SplQueue::enqueue()
      *
-     * @param mixed $item The item to enqueue
+     * @param AllowedTypes $item The item to enqueue
      *
      * @return void
      *
@@ -84,7 +87,7 @@ class StrictQueue extends StrictList
     /**
      * Create a type-sensitive, traversable queue of items.
      *
-     * @param iterable $items Initial set of items
+     * @param iterable<AllowedTypes> $items Initial set of items
      * @param string[] $allowedTypes Allowed types of items (optional)
      */
     public function __construct(iterable $items = [], array $allowedTypes = [])
diff --git a/src/DataStructures/StrictStack.php b/src/DataStructures/StrictStack.php
index bbb79d8..59699f4 100644
--- a/src/DataStructures/StrictStack.php
+++ b/src/DataStructures/StrictStack.php
@@ -29,13 +29,16 @@ use RuntimeException;
  *
  * @author Sebastian Meyer <sebastian.meyer@opencultureconsulting.com>
  * @package opencultureconsulting/basics
+ *
+ * @template AllowedTypes
+ * @extends StrictList<AllowedTypes>
  */
 class StrictStack extends StrictList
 {
     /**
      * Add an item to the stack.
      *
-     * @param mixed $item The item to stack
+     * @param AllowedTypes $item The item to stack
      *
      * @return void
      *
@@ -49,7 +52,7 @@ class StrictStack extends StrictList
     /**
      * Unstack an item from the stack.
      *
-     * @return mixed The unstacked item
+     * @return AllowedTypes The unstacked item
      */
     public function unstack(): mixed
     {
@@ -82,7 +85,7 @@ class StrictStack extends StrictList
     /**
      * Create a type-sensitive, traversable stack of items.
      *
-     * @param iterable $items Initial set of items
+     * @param iterable<AllowedTypes> $items Initial set of items
      * @param string[] $allowedTypes Allowed types of items (optional)
      */
     public function __construct(iterable $items = [], array $allowedTypes = [])
diff --git a/src/Traits/Singleton.php b/src/Traits/Singleton.php
index c12ac03..41d94b8 100644
--- a/src/Traits/Singleton.php
+++ b/src/Traits/Singleton.php
@@ -41,6 +41,8 @@ trait Singleton
      * Get a singleton instance of this class.
      *
      * @param mixed ...$args Constructor parameters
+     *
+     * @return static
      */
     final public static function getInstance(mixed ...$args): static
     {