1 files changed, 17 insertions, 1 deletions

diff --git a/src/ir/type-updating.h b/src/ir/type-updating.h
index d224f93dc..160934da1 100644
--- a/src/ir/type-updating.h
+++ b/src/ir/type-updating.h
@@ -248,14 +248,22 @@ struct TypeUpdater
           return; // did not turn
         }
       } else if (auto* iff = curr->dynCast<If>()) {
-        // may not be unreachable if just one side is
+        // We only want to change a concrete type to unreachable here, so undo
+        // anything else. Other changes can be a problem, like refining the type
+        // of an if for GC-using code, as the code all around us only assumes we
+        // are propagating unreachability and not doing a full refinalize.
+        auto old = iff->type;
         iff->finalize();
         if (curr->type != Type::unreachable) {
+          iff->type = old;
           return; // did not turn
         }
       } else if (auto* tryy = curr->dynCast<Try>()) {
+        // See comment on If, above.
+        auto old = tryy->type;
         tryy->finalize();
         if (curr->type != Type::unreachable) {
+          tryy->type = old;
           return; // did not turn
         }
       } else {
@@ -303,9 +311,13 @@ struct TypeUpdater
     if (!curr->type.isConcrete()) {
       return; // nothing concrete to change to unreachable
     }
+    // See comment in propagateTypesUp() for If regarding restoring the type.
+    auto old = curr->type;
     curr->finalize();
     if (curr->type == Type::unreachable) {
       propagateTypesUp(curr);
+    } else {
+      curr->type = old;
     }
   }
 
@@ -313,9 +325,13 @@ struct TypeUpdater
     if (!curr->type.isConcrete()) {
       return; // nothing concrete to change to unreachable
     }
+    // See comment in propagateTypesUp() for Try regarding restoring the type.
+    auto old = curr->type;
     curr->finalize();
     if (curr->type == Type::unreachable) {
       propagateTypesUp(curr);
+    } else {
+      curr->type = old;
     }
   }