Issue #3408166 by FatherShawn: fix Set inheritance bug

src/DataStructure/Set.php

@@ -89,7 +89,7 @@ class Set implements SetInterface {
    * {@inheritdoc}
    */
   public function remove(mixed ...$values): void {
-    $filterSet = new Set(NULL);
+    $filterSet = new static(NULL);
     $filterSet->add(...$values);
     // $filter finds values not in the filter set.
     $filter = fn($value) => !$filterSet->has($value);
@@ -118,7 +118,7 @@ class Set implements SetInterface {
     $union = new \AppendIterator();
     $union->append(new \IteratorIterator($this));
     $union->append(new \IteratorIterator($set));
-    return new Set($union);
+    return new static($union);
   }
 
   /**
@@ -127,7 +127,7 @@ class Set implements SetInterface {
   public function intersect(SetInterface $set): Set {
     $filter = fn($value) => $set->has($value);
     $intersection = array_filter($this->members, $filter);
-    return new Set($intersection);
+    return new static($intersection);
   }
 
   /**
@@ -136,7 +136,7 @@ class Set implements SetInterface {
   public function difference(SetInterface $set): Set {
     $filter = fn($value) => !$set->has($value);
     $intersection = array_filter($this->members, $filter);
-    return new Set($intersection);
+    return new static($intersection);
   }
 
   /**