Statically check for assignments to const in harmony mode.

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index 339206b1346726330632ff918ecdfbd5b6148369..46077458376fac3ecb479d3113d3357350186b38 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -1263,6 +1263,7 @@ Statement* Parser::ParseStatement(ZoneStringList* labels, bool* ok) {
       return ParseBlock(labels, ok);
 
     case Token::CONST:  // fall through
+    case Token::LET:
     case Token::VAR:
       stmt = ParseVariableStatement(kStatement, ok);
       break;
@@ -1704,6 +1705,16 @@ Block* Parser::ParseVariableDeclarations(
   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:
@@ -1729,7 +1740,17 @@ Block* Parser::ParseVariableDeclarations(
     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) {
@@ -2668,6 +2689,7 @@ Expression* Parser::ParseAssignmentExpression(bool accept_IN, bool* ok) {
     // Assignment to eval or arguments is disallowed in strict mode.
     CheckStrictModeLValue(expression, "strict_lhs_assignment", CHECK_OK);
   }
+  MarkAsLValue(expression);
 
   Token::Value op = Next();  // Get assignment operator.
   int pos = scanner().location().beg_pos;
@@ -2901,6 +2923,7 @@ Expression* Parser::ParseUnaryExpression(bool* ok) {
       // Prefix expression operand in strict mode may not be eval or arguments.
       CheckStrictModeLValue(expression, "strict_lhs_prefix", CHECK_OK);
     }
+    MarkAsLValue(expression);
 
     int position = scanner().location().beg_pos;
     return new(zone()) CountOperation(isolate(),
@@ -2936,6 +2959,7 @@ Expression* Parser::ParsePostfixExpression(bool* ok) {
       // Postfix expression operand in strict mode may not be eval or arguments.
       CheckStrictModeLValue(expression, "strict_lhs_prefix", CHECK_OK);
     }
+    MarkAsLValue(expression);
 
     Token::Value next = Next();
     int position = scanner().location().beg_pos;
@@ -4294,6 +4318,15 @@ Handle<String> Parser::ParseIdentifierName(bool* ok) {
 }
 
 
+void Parser::MarkAsLValue(Expression* expression) {
+  VariableProxy* proxy = expression != NULL
+      ? expression->AsVariableProxy()
+      : NULL;
+
+  if (proxy != NULL) proxy->MarkAsLValue();
+}
+
+
 // Checks LHS expression for assignment and prefix/postfix increment/decrement
 // in strict mode.
 void Parser::CheckStrictModeLValue(Expression* expression,