foo.diff

diff --git a/src/Phan/Language/UnionTypeBuilder.php b/src/Phan/Language/UnionTypeBuilder.php
index 98c3587a..f0d0c13d 100644
--- a/src/Phan/Language/UnionTypeBuilder.php
+++ b/src/Phan/Language/UnionTypeBuilder.php
@@ -2,6 +2,8 @@
 
 namespace Phan\Language;
 
+use Phan\Library\Set;
+
 /**
  * Utilities to build a union type.
  * Mostly used internally when the number of types in the resulting union type may be large.
@@ -13,45 +15,36 @@ namespace Phan\Language;
  */
 final class UnionTypeBuilder
 {
-    /** @var list<Type> the list of unique types in this builder instance. */
+    /** @var Set the list of unique types in this builder instance. */
     private $type_set;
 
     /** @param list<Type> $type_set (must be unique) */
     public function __construct(array $type_set = [])
     {
-        $this->type_set = $type_set;
+        $this->type_set = new Set($type_set);
     }
 
     public function addType(Type $type) : void
     {
-        if (\in_array($type, $this->type_set, true)) {
+        if ($this->type_set->contains($type)) {
             return;
         }
-        $this->type_set[] = $type;
+        $this->type_set->attach($type);
     }
 
     public function addUnionType(UnionType $union_type) : void
     {
         $old_type_set = $this->type_set;
         foreach ($union_type->getTypeSet() as $type) {
-            if (!\in_array($type, $old_type_set, true)) {
-                $this->type_set[] = $type;
+            if (!$old_type_set->contains($type)) {
+                $this->type_set->attach($type);
             }
         }
     }
 
     public function removeType(Type $type) : void
     {
-        $i = \array_search($type, $this->type_set, true);
-        if ($i !== false) {
-            // equivalent to unset($new_type_set[$i]) but fills in the gap in array keys.
-            // TODO: How do other ways of unsetting the type affect performance on large projects?
-            $replacement_type = \array_pop($this->type_set);
-            if ($replacement_type !== $type) {
-                // @phan-suppress-next-line PhanPartialTypeMismatchProperty $replacement_type is guaranteed to not be false
-                $this->type_set[$i] = $replacement_type;
-            }
-        }
+        $this->type_set->detach($type);
     }
 
     /**
@@ -67,12 +60,12 @@ final class UnionTypeBuilder
      */
     public function getTypeSet() : array
     {
-        return $this->type_set;
+        return $this->type_set->toArray();
     }
 
     public function clearTypeSet() : void
     {
-        $this->type_set = [];
+        $this->type_set = new Set();;
     }
 
     /**
@@ -80,7 +73,7 @@ final class UnionTypeBuilder
      */
     public function getPHPDocUnionType() : UnionType
     {
-        return UnionType::of($this->type_set, []);
+        return UnionType::of($this->type_set->toArray(), []);
     }
 
     /**