Added internal type assertion

Author: peldax

src/Typesystem/EnumType.php

@@ -45,6 +45,8 @@ final public static function fromEnum(string $enumClass) : EnumItemSet
         }
 
         foreach ($ref->getCases() as $case) {
+            \assert($case instanceof \ReflectionEnumBackedCase);
+            
             $values[] = new EnumItem($case->getBackingValue(), self::getItemDescription($case));
         }