fix corner case in reduce_vars (#4950)

fixes #4949

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);
             });

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"
+}