1 files changed, 6 insertions, 1 deletions
diff --git a/src/ir/properties.h b/src/ir/properties.h
index 2214eecaf..cddcbc5f2 100644
--- a/src/ir/properties.h
+++ b/src/ir/properties.h
@@ -311,7 +311,12 @@ inline Expression* getImmediateFallthrough(
   } else if (auto* as = curr->dynCast<RefCast>()) {
     return as->ref;
   } else if (auto* as = curr->dynCast<RefAs>()) {
-    return as->value;
+    // Extern conversions are not casts and actually produce new values.
+    // Treating them as fallthroughs would lead to misoptimizations of
+    // subsequent casts.
+    if (as->op != ExternInternalize && as->op != ExternExternalize) {
+      return as->value;
+    }
   } else if (auto* br = curr->dynCast<BrOn>()) {
     return br->ref;
   }