Undefined is not a literal.

src/ast.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 53 matching lines @@
 bool Expression::IsStringLiteral() {
   return AsLiteral() != NULL && AsLiteral()->value()->IsString();
 }
 
 
 bool Expression::IsNullLiteral() {
   return AsLiteral() != NULL && AsLiteral()->value()->IsNull();
 }
 
 
-bool Expression::IsUndefinedLiteral() {
-  return AsLiteral() != NULL && AsLiteral()->value()->IsUndefined();
+bool Expression::IsUndefinedLiteral(Isolate* isolate) {
+  VariableProxy* var_proxy = AsVariableProxy();
+  if (var_proxy == NULL) return false;
+  Variable* var = var_proxy->var();
+  // The global identifier "undefined" is immutable. Everything
+  // else could be reassigned.
+  return var != NULL && var->location() == Variable::UNALLOCATED &&
+         var_proxy->name()->Equals(isolate->heap()->undefined_string());
 }
 
 
 VariableProxy::VariableProxy(Isolate* isolate, Variable* var)
     : Expression(isolate),
       name_(var->name()),
       var_(NULL),  // Will be set by the call to BindTo.
       is_this_(var->is_this()),
       is_trivial_(false),
       is_lvalue_(false),
@@ skipping 292 matching lines @@
       maybe_unary->op() == Token::VOID &&
       maybe_unary->expression()->AsLiteral() != NULL;
 }
 
 
 // Check for the pattern: void <literal> equals <expression> or
 // undefined equals <expression>
 static bool MatchLiteralCompareUndefined(Expression* left,
                                          Token::Value op,
                                          Expression* right,
-                                         Expression** expr) {
+                                         Expression** expr,
+                                         Isolate* isolate) {
   if (IsVoidOfLiteral(left) && Token::IsEqualityOp(op)) {
     *expr = right;
     return true;
   }
-  if (left->IsUndefinedLiteral() && Token::IsEqualityOp(op)) {
+  if (left->IsUndefinedLiteral(isolate) && Token::IsEqualityOp(op)) {
     *expr = right;
     return true;
   }
   return false;
 }
 
 
-bool CompareOperation::IsLiteralCompareUndefined(Expression** expr) {
-  return MatchLiteralCompareUndefined(left_, op_, right_, expr) ||
-      MatchLiteralCompareUndefined(right_, op_, left_, expr);
+bool CompareOperation::IsLiteralCompareUndefined(
+    Expression** expr, Isolate* isolate) {
+  return MatchLiteralCompareUndefined(left_, op_, right_, expr, isolate) ||
+      MatchLiteralCompareUndefined(right_, op_, left_, expr, isolate);
 }
 
 
 // Check for the pattern: null equals <expression>
 static bool MatchLiteralCompareNull(Expression* left,
                                     Token::Value op,
                                     Expression* right,
                                     Expression** expr) {
   if (left->IsNullLiteral() && Token::IsEqualityOp(op)) {
     *expr = right;
@@ skipping 773 matching lines @@
     OS::SNPrintF(buffer, "%d", Smi::cast(*value_)->value());
     str = arr;
   } else {
     str = DoubleToCString(value_->Number(), buffer);
   }
   return factory->NewStringFromAscii(CStrVector(str));
 }
 
 
 } }  // namespace v8::internal