fix corner case in unused (#4405)

fixes #4404

lib/compress.js

@@ -5860,7 +5860,15 @@ merge(Compressor.prototype, {
                 if (def.scope === self) assignments.add(def.id, node);
             }
             var node_def, props = [], sym = assign_as_unused(node, props);
-            if (sym && self.variables.get(sym.name) === (node_def = sym.definition())) {
+            if (sym && self.variables.get(sym.name) === (node_def = sym.definition())
+                && !(is_arguments(node_def) && !all(self.argnames, function(argname) {
+                    return !argname.match_symbol(function(node) {
+                        if (node instanceof AST_SymbolFunarg) {
+                            var def = node.definition();
+                            return def.references.length > def.replaced;
+                        }
+                    }, true);
+                }))) {
                 props.forEach(function(prop) {
                     prop.walk(tw);
                 });

test/compress/drop-unused.js

@@ -3112,3 +3112,25 @@ issue_4235: {
     }
     expect_stdout: "undefined"
 }
+
+issue_4404: {
+    options = {
+        pure_getters: "strict",
+        unused: true,
+    }
+    input: {
+        function f(a) {
+            arguments[0] = "PASS";
+            console.log(a);
+        }
+        f("FAIL");
+    }
+    expect: {
+        function f(a) {
+            arguments[0] = "PASS";
+            console.log(a);
+        }
+        f("FAIL");
+    }
+    expect_stdout: "PASS"
+}