From 7b4fd858ba23e6c1402ad33d8cc4a8a8e7966614 Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Thu, 18 Feb 2021 15:24:55 +0000
Subject: [PATCH] fix corner case in `reduce_vars` (#4663)

fixes #4662
---
 lib/compress.js              |  6 +++++-
 test/compress/drop-unused.js | 23 +++++++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/lib/compress.js b/lib/compress.js
index 09e78d46..a58e2816 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -864,7 +864,11 @@ merge(Compressor.prototype, {
                     push_ref(d, lhs);
                     if (d.fixed) {
                         lhs.fixed = d.fixed;
-                        lhs.fixed.assigns = [ node ];
+                        if (lhs.fixed.assigns) {
+                            lhs.fixed.assigns.push(node);
+                        } else {
+                            lhs.fixed.assigns = [ node ];
+                        }
                     }
                 } else {
                     lhs.walk(tw);
diff --git a/test/compress/drop-unused.js b/test/compress/drop-unused.js
index 914e83ad..28253568 100644
--- a/test/compress/drop-unused.js
+++ b/test/compress/drop-unused.js
@@ -3251,3 +3251,26 @@ issue_4558_2: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4662: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 0;
+        function f(b, c) {
+            console.log(b, c);
+        }
+        f(++a, a = a, a);
+    }
+    expect: {
+        var a = 0;
+        (function(b, c) {
+            console.log(b, c);
+        })(++a, a = 1);
+    }
+    expect_stdout: "1 1"
+}