Statically check for assignments to const in harmony mode.

src/preparser.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 181 matching lines @@
   // labels can be simply ignored in all other cases; except for
   // trivial labeled break statements 'label: break label' which is
   // parsed into an empty statement.
 
   // Keep the source position of the statement
   switch (peek()) {
     case i::Token::LBRACE:
       return ParseBlock(ok);
 
     case i::Token::CONST:
+    case i::Token::LET:
     case i::Token::VAR:
       return ParseVariableStatement(kStatement, ok);
 
     case i::Token::SEMICOLON:
       Next();
       return Statement::Default();
 
     case i::Token::IF:
       return ParseIfStatement(ok);
 
@@ skipping 130 matching lines @@
   // ConstBinding ::
   //   Identifier '=' AssignmentExpression
   //
   // TODO(ES6):
   // ConstBinding ::
   //   BindingPattern '=' AssignmentExpression
   bool require_initializer = false;
   if (peek() == i::Token::VAR) {
     Consume(i::Token::VAR);
   } else if (peek() == i::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(i::Token::CONST);
     switch (language_mode()) {
       case i::CLASSIC_MODE:
         break;
       case i::STRICT_MODE: {
         i::Scanner::Location location = scanner_->peek_location();
         ReportMessageAt(location, "strict_const", NULL);
         *ok = false;
         return Statement::Default();
       }
       case i::EXTENDED_MODE:
         if (var_context != kSourceElement &&
             var_context != kForStatement) {
           i::Scanner::Location location = scanner_->peek_location();
           ReportMessageAt(location.beg_pos, location.end_pos,
                           "unprotected_const", NULL);
           *ok = false;
           return Statement::Default();
         }
         require_initializer = true;
         break;
     }
-    Consume(i::Token::CONST);
   } else if (peek() == i::Token::LET) {
-    ASSERT(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()) {
+      i::Scanner::Location location = scanner_->peek_location();
+      ReportMessageAt(location.beg_pos, location.end_pos,
+                      "illegal_let", NULL);
+      *ok = false;
+      return Statement::Default();
+    }
+    Consume(i::Token::LET);
     if (var_context != kSourceElement &&
         var_context != kForStatement) {
       i::Scanner::Location location = scanner_->peek_location();
       ReportMessageAt(location.beg_pos, location.end_pos,
                       "unprotected_let", NULL);
       *ok = false;
       return Statement::Default();
     }
-    Consume(i::Token::LET);
   } else {
     *ok = false;
     return Statement::Default();
   }
 
   // The scope of a var/const declared variable anywhere inside a function
   // is the entire function (ECMA-262, 3rd, 10.1.3, and 12.2). The scope
   // of a let declared variable is the scope of the immediately enclosing
   // block.
   int nvars = 0;  // the number of variables declared
@@ skipping 1329 matching lines @@
       backing_store_.Add(static_cast<byte>((ascii_length >> 14) | 0x80u));
     }
     backing_store_.Add(static_cast<byte>((ascii_length >> 7) | 0x80u));
   }
   backing_store_.Add(static_cast<byte>(ascii_length & 0x7f));
 
   backing_store_.AddBlock(bytes);
   return backing_store_.EndSequence().start();
 }
 } }  // v8::preparser