From 18dbceb36f02db6100c1588a77c18c6ca73f1baa Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Fri, 15 Jan 2021 04:33:17 +0000
Subject: [PATCH] fix corner case in `unused` (#4557)

fixes #4556
---
 lib/compress.js          |  3 +++
 test/compress/spreads.js | 20 ++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/lib/compress.js b/lib/compress.js
index fadd7415..01ec7e60 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -5766,6 +5766,9 @@ merge(Compressor.prototype, {
                             var write_only = def.value.write_only;
                             var value = def.value.drop_side_effect_free(compressor);
                             if (def.value !== value) {
+                                sym.references.forEach(function(node) {
+                                    if (node.fixed === sym.fixed) node.fixed = def.value;
+                                });
                                 def.value = null;
                                 if (value) {
                                     AST_Node.warn("Side effects in last use of variable {name} [{file}:{line},{col}]", template(def.name));
diff --git a/test/compress/spreads.js b/test/compress/spreads.js
index 1e4dcbf6..7c98fdcd 100644
--- a/test/compress/spreads.js
+++ b/test/compress/spreads.js
@@ -826,3 +826,23 @@ issue_4363: {
     expect_stdout: "PASS"
     node_version: ">=8"
 }
+
+issue_4556: {
+    options = {
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        console.log(function() {
+            var a = "" + [ a++ ];
+            var b = [ ...a ];
+        }());
+    }
+    expect: {
+        console.log(function() {
+            var a;
+        }());
+    }
+    expect_stdout: "undefined"
+    node_version: ">=6"
+}
-- 
2.34.1