2 files changed, 37 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;
   }
diff --git a/test/lit/passes/optimize-instructions-gc.wast b/test/lit/passes/optimize-instructions-gc.wast
index c5100434d..c0ea515d0 100644
--- a/test/lit/passes/optimize-instructions-gc.wast
+++ b/test/lit/passes/optimize-instructions-gc.wast
@@ -3251,4 +3251,35 @@
       )
     )
   )
+
+  ;; CHECK:      (func $cast-internalized-extern (type $externref_=>_none) (param $externref externref)
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (ref.cast $A
+  ;; CHECK-NEXT:    (extern.internalize
+  ;; CHECK-NEXT:     (local.get $externref)
+  ;; CHECK-NEXT:    )
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  ;; NOMNL:      (func $cast-internalized-extern (type $externref_=>_none) (param $externref externref)
+  ;; NOMNL-NEXT:  (drop
+  ;; NOMNL-NEXT:   (ref.cast $A
+  ;; NOMNL-NEXT:    (extern.internalize
+  ;; NOMNL-NEXT:     (local.get $externref)
+  ;; NOMNL-NEXT:    )
+  ;; NOMNL-NEXT:   )
+  ;; NOMNL-NEXT:  )
+  ;; NOMNL-NEXT: )
+  (func $cast-internalized-extern (param $externref externref)
+    ;; We cannot optimize this cast, and in particular we should not treat the
+    ;; externref as falling through to the cast and incorrectly inferring that
+    ;; the cast cannot succeed.
+    (drop
+      (ref.cast $A
+        (extern.internalize
+          (local.get $externref)
+        )
+      )
+    )
+  )
 )