From ba74b0af1a445edc57d0e6c312db54d267e5754d Mon Sep 17 00:00:00 2001
From: Thomas Lively <tlively@google.com>
Date: Mon, 29 Sep 2025 19:32:03 -0700
Subject: [PATCH] [Custom Descriptors] Fix Unsubtyping on soon-to-be-invalid
 types

We've updated Unsubtyping so that it will be correct once we update the
descriptor type validation rules, but in doing so we introduced an
assertion failure when it encounters types that will no longer be valid
in the future but are still valid now. Add a workaround and test.
---
 src/passes/Unsubtyping.cpp            |  5 ++++-
 test/lit/passes/unsubtyping-desc.wast | 19 +++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/passes/Unsubtyping.cpp b/src/passes/Unsubtyping.cpp
index 248ed995fe4..57a9e41d607 100644
--- a/src/passes/Unsubtyping.cpp
+++ b/src/passes/Unsubtyping.cpp
@@ -582,7 +582,10 @@ struct Unsubtyping : Pass {
     }
     if (auto desc = sub.getDescribedType()) {
       if (auto superDesc = super.getDescribedType()) {
-        noteSubtype(*desc, *superDesc);
+        // We will be able to assume this once we fix the validation rules.
+        if (HeapType::isSubType(*desc, *superDesc)) {
+          noteSubtype(*desc, *superDesc);
+        }
       }
     }
   }
diff --git a/test/lit/passes/unsubtyping-desc.wast b/test/lit/passes/unsubtyping-desc.wast
index adfdd1a0c85..50a24a67d41 100644
--- a/test/lit/passes/unsubtyping-desc.wast
+++ b/test/lit/passes/unsubtyping-desc.wast
@@ -206,3 +206,22 @@
   ;; CHECK-NEXT: ))
   (global $bot-mid (ref null $mid) (struct.new $bot (ref.null none)))
 )
+
+;; This will be invalid soon, but in the meantime we should not be confused when
+;; the types described by two related descriptors are unrelated.
+(module
+  (rec
+    ;; CHECK:      (rec
+    ;; CHECK-NEXT:  (type $A (descriptor $super (struct)))
+    (type $A (descriptor $super (struct)))
+    ;; CHECK:       (type $B (descriptor $sub (struct)))
+    (type $B (descriptor $sub (struct)))
+    ;; CHECK:       (type $super (sub (describes $A (struct))))
+    (type $super (sub (describes $A (struct))))
+    ;; CHECK:       (type $sub (sub $super (describes $B (struct))))
+    (type $sub (sub $super (describes $B (struct))))
+  )
+  ;; CHECK:      (global $public (ref null $B) (ref.null none))
+  (global $public (export "public") (ref null $B) (ref.null none))
+)
+;; CHECK:      (export "public" (global $public))