fix corner case in `merge_vars` (#4136)

fixes #4135

1 files changed, 5 insertions, 2 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 2df76f5f..82bbbf1b 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -4485,9 +4485,12 @@ merge(Compressor.prototype, {
             var head = first.pop();
             var def = head.definition;
             if (!(def.id in prev)) continue;
-            var head_refs = references[def.id];
-            if (!head_refs) continue;
+            if (!references[def.id]) continue;
+            var head_refs = {
+                start: references[def.id].start,
+            };
             while (def.id in merged) def = merged[def.id];
+            head_refs.end = references[def.id].end;
             var skipped = [];
             do {
                 var tail = last.pop();