[NodeTypeResolver] Apply logic Mixed inside Union check on TypeFactory::uniquateTypes() (#7346)

Author: samsonasik

src/NodeTypeResolver/PHPStan/Type/TypeFactory.php

@@ -64,8 +64,10 @@ public function uniquateTypes(array $types, bool $keepConstant = false): array
         $hasFalse = false;
         $hasTrue = false;
         foreach ($types as $type) {
+            // mixed only allowed to be standalone
             if ($type instanceof MixedType) {
-                $type = new MixedType();
+                /** @var array<TType> */
+                return [new MixedType()];
             }
 
             $type = $this->normalizeObjectType($totalTypes, $type);
@@ -168,12 +170,6 @@ private function createUnionOrSingleType(array $types): Type
             return $types[0];
         }
 
-        foreach ($types as $type) {
-            if ($type instanceof MixedType) {
-                return new MixedType();
-            }
-        }
-
         return new UnionType($types);
     }