From bd379654d353b91753b2acf0a449a8a19fd948df Mon Sep 17 00:00:00 2001
From: Jeongwon Park <sanguineman@naver.com>
Date: Mon, 24 Aug 2026 20:05:26 +0900
Subject: [PATCH] fix(interpreter): reject subtypes with multiple supertypes

---
 interpreter/valid/valid.ml       | 1 +
 test/core/gc/type-subtyping.wast | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/interpreter/valid/valid.ml b/interpreter/valid/valid.ml
index 819a97fbfe..9b3756a54c 100644
--- a/interpreter/valid/valid.ml
+++ b/interpreter/valid/valid.ml
@@ -159,6 +159,7 @@ let check_comptype (c : context) (ct : comptype) at =
 
 let check_subtype (c : context) (sut : subtype) at =
   let SubT (_fin, uts, ct) = sut in
+  require (List.length uts <= 1) at "multiple supertypes";
   List.iter (fun ut -> check_typeuse c ut at) uts;
   check_comptype c ct at
 
diff --git a/test/core/gc/type-subtyping.wast b/test/core/gc/type-subtyping.wast
index b507c5a6bc..e2775ae5cc 100644
--- a/test/core/gc/type-subtyping.wast
+++ b/test/core/gc/type-subtyping.wast
@@ -949,6 +949,15 @@
   "sub type"
 )
 
+;; Testing the number of allowed supertypes
+
+(assert_invalid
+  (module
+    (type $parent1 (sub (struct)))
+    (type $parent2 (sub (struct)))
+    (type $child (sub $parent1 $parent2 (struct))))
+  "multiple supertypes"
+)
 
 ;; Testing exported functions with non-flat types
 (module