DebugInfo: Don't trample in replaceCurrent() (#5915)

Copy the old expression's debug info if the new has none. But if the new has
its own, trust that.

Followup to #5914

1 files changed, 6 insertions, 1 deletions

diff --git a/src/wasm-traversal.h b/src/wasm-traversal.h
index 80378242a..212ded365 100644
--- a/src/wasm-traversal.h
+++ b/src/wasm-traversal.h
@@ -124,7 +124,12 @@ struct Walker : public VisitorType {
     // Copy debug info, if present.
     if (currFunction) {
       auto& debugLocations = currFunction->debugLocations;
-      if (!debugLocations.empty()) {
+      // Early exit if there is no debug info at all. Also, leave if we already
+      // have debug info on the new expression, which we don't want to trample:
+      // if there is no debug info we do want to copy, as a replacement
+      // operation suggests the new code plays the same role (it is an optimized
+      // version of the old), but if the code is already annotated, trust that.
+      if (!debugLocations.empty() && !debugLocations.count(expression)) {
         auto* curr = getCurrent();
         auto iter = debugLocations.find(curr);
         if (iter != debugLocations.end()) {