improve sandbox fidelity (#3415)

1 files changed, 9 insertions, 4 deletions

diff --git a/lib/compress.js b/lib/compress.js
index f7730cd5..5d6567f7 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -451,6 +451,7 @@ merge(Compressor.prototype, {
             if (tw.safe_ids[def.id]) {
                 if (def.fixed == null) {
                     if (is_arguments(def)) return false;
+                    if (def.global && def.name == "arguments") return false;
                     def.fixed = make_node(AST_Undefined, def.orig);
                 }
                 return true;
@@ -894,9 +895,13 @@ merge(Compressor.prototype, {
         return orig.length == 1 && orig[0] instanceof AST_SymbolLambda;
     });
 
-    function is_lhs_read_only(lhs) {
+    function is_lhs_read_only(lhs, compressor) {
         if (lhs instanceof AST_This) return true;
-        if (lhs instanceof AST_SymbolRef) return lhs.definition().orig[0] instanceof AST_SymbolLambda;
+        if (lhs instanceof AST_SymbolRef) {
+            var def = lhs.definition();
+            return def.orig[0] instanceof AST_SymbolLambda
+                || compressor.exposed(def) && identifier_atom[def.name];
+        }
         if (lhs instanceof AST_PropAccess) {
             lhs = lhs.expression;
             if (lhs instanceof AST_SymbolRef) {
@@ -905,7 +910,7 @@ merge(Compressor.prototype, {
             }
             if (!lhs) return true;
             if (lhs.is_constant()) return true;
-            return is_lhs_read_only(lhs);
+            return is_lhs_read_only(lhs, compressor);
         }
         return false;
     }
@@ -1220,7 +1225,7 @@ merge(Compressor.prototype, {
                     var stop_if_hit = null;
                     var lhs = get_lhs(candidate);
                     var side_effects = lhs && lhs.has_side_effects(compressor);
-                    var scan_lhs = lhs && !side_effects && !is_lhs_read_only(lhs);
+                    var scan_lhs = lhs && !side_effects && !is_lhs_read_only(lhs, compressor);
                     var scan_rhs = foldable(get_rhs(candidate));
                     if (!scan_lhs && !scan_rhs) continue;
                     // Locate symbols which may execute code outside of scanning range