Make let/const outside of the extended mode early errors (under harmony flag).

src/parser.cc

 // Copyright 2011 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 1245 matching lines @@
   // parsed into an empty statement.
 
   // Keep the source position of the statement
   int statement_pos = scanner().peek_location().beg_pos;
   Statement* stmt = NULL;
   switch (peek()) {
     case Token::LBRACE:
       return ParseBlock(labels, ok);
 
     case Token::CONST:  // fall through
+    case Token::LET:
     case Token::VAR:
       stmt = ParseVariableStatement(kStatement, ok);
       break;
 
     case Token::SEMICOLON:
       Next();
       return EmptyStatement();
 
     case Token::IF:
       stmt = ParseIfStatement(labels, ok);
@@ skipping 407 matching lines @@
   // True if the binding needs initialization. 'let' and 'const' declared
   // bindings are created uninitialized by their declaration nodes and
   // need initialization. 'var' declared bindings are always initialized
   // immediately by their declaration nodes.
   bool needs_init = false;
   bool is_const = false;
   Token::Value init_op = Token::INIT_VAR;
   if (peek() == Token::VAR) {
     Consume(Token::VAR);
   } else if (peek() == Token::CONST) {
+    // TODO(ES6): The ES6 Draft Rev4 section 12.2.2 reads:
+    //
+    // ConstDeclaration : const ConstBinding (',' ConstBinding)* ';'
+    //
+    // * It is a Syntax Error if the code that matches this production is not
+    //   contained in extended code.
+    //
+    // However disallowing const in classic mode will break compatibility with
+    // existing pages. Therefore we keep allowing const with the old
+    // non-harmony semantics in classic mode.
     Consume(Token::CONST);
     switch (top_scope_->language_mode()) {
       case CLASSIC_MODE:
         mode = CONST;
         init_op = Token::INIT_CONST;
         break;
       case STRICT_MODE:
         ReportMessage("strict_const", Vector<const char*>::empty());
         *ok = false;
         return NULL;
       case EXTENDED_MODE:
         if (var_context != kSourceElement &&
             var_context != kForStatement) {
           // In extended mode 'const' declarations are only allowed in source
           // element positions.
           ReportMessage("unprotected_const", Vector<const char*>::empty());
           *ok = false;
           return NULL;
         }
         mode = CONST_HARMONY;
         init_op = Token::INIT_CONST_HARMONY;
     }
     is_const = true;
     needs_init = true;
   } else if (peek() == Token::LET) {
-    ASSERT(top_scope_->is_extended_mode());
+    // ES6 Draft Rev4 section 12.2.1:
+    //
+    // LetDeclaration : let LetBindingList ;
+    //
+    // * It is a Syntax Error if the code that matches this production is not
+    //   contained in extended code.
+    if (!is_extended_mode()) {
+      ReportMessage("illegal_let", Vector<const char*>::empty());
+      *ok = false;
+      return NULL;
+    }
     Consume(Token::LET);
     if (var_context != kSourceElement &&
         var_context != kForStatement) {
       // Let declarations are only allowed in source element positions.
       ASSERT(var_context == kStatement);
       ReportMessage("unprotected_let", Vector<const char*>::empty());
       *ok = false;
       return NULL;
     }
     mode = LET;
@@ skipping 3710 matching lines @@
       result = parser.ParseProgram(source,
                                    info->is_global(),
                                    info->language_mode());
     }
   }
   info->SetFunction(result);
   return (result != NULL);
 }
 
 } }  // namespace v8::internal