From de376c3d3316fc4a33a876793eb4d693422415a0 Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Fri, 21 May 2021 11:49:07 +0100
Subject: [PATCH] fix corner case in `reduce_vars` (#4950)

fixes #4949
---
 lib/compress.js              |  9 ++++++---
 test/compress/reduce_vars.js | 20 ++++++++++++++++++++
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index 8975f47b..4f50f52a 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -580,9 +580,12 @@ merge(Compressor.prototype, {
         }
 
         function safe_to_assign(tw, def, declare) {
-            if (!(declare || all(def.orig, function(sym) {
-                return !(sym instanceof AST_SymbolConst);
-            }))) return false;
+            if (!declare) {
+                if (is_funarg(def) && def.scope.uses_arguments && !tw.has_directive("use strict")) return false;
+                if (!all(def.orig, function(sym) {
+                    return !(sym instanceof AST_SymbolConst);
+                })) return false;
+            }
             if (def.fixed === undefined) return declare || all(def.orig, function(sym) {
                 return !(sym instanceof AST_SymbolLet);
             });
diff --git a/test/compress/reduce_vars.js b/test/compress/reduce_vars.js
index 04d96b51..33d59982 100644
--- a/test/compress/reduce_vars.js
+++ b/test/compress/reduce_vars.js
@@ -7722,3 +7722,23 @@ issue_4943_2: {
         "bar",
     ]
 }
+
+issue_4949: {
+    options = {
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        (function f(a) {
+            a = 0;
+            console.log(a++, arguments[0]);
+        })(0);
+    }
+    expect: {
+        (function(a) {
+            a = 0;
+            console.log(a++, arguments[0]);
+        })(0);
+    }
+    expect_stdout: "0 1"
+}