Prohibit let in lexical bindings

src/preparser.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_PREPARSER_H
 #define V8_PREPARSER_H
 
 #include "src/bailout-reason.h"
 #include "src/expression-classifier.h"
 #include "src/func-name-inferrer.h"
@@ skipping 600 matching lines @@
       // parameter list, show the "arrow formal parameters" error if the formals
       // started with a parenthesis, and the binding pattern error otherwise.
       const ExpressionClassifier::Error& error =
           parenthesized_formals ? classifier->arrow_formal_parameters_error()
                                 : classifier->binding_pattern_error();
       ReportClassifierError(error);
       *ok = false;
     }
   }
 
+  void ValidateLetPattern(const ExpressionClassifier* classifier, bool* ok) {
+    if (!classifier->is_valid_let_pattern()) {
+      ReportClassifierError(classifier->let_pattern_error());
+      *ok = false;
+    }
+  }
+
   void ExpressionUnexpectedToken(ExpressionClassifier* classifier) {
     MessageTemplate::Template message = MessageTemplate::kUnexpectedToken;
     const char* arg;
     GetUnexpectedTokenMessage(peek(), &message, &arg);
     classifier->RecordExpressionError(scanner()->peek_location(), message, arg);
   }
 
   void BindingPatternUnexpectedToken(ExpressionClassifier* classifier) {
     MessageTemplate::Template message = MessageTemplate::kUnexpectedToken;
     const char* arg;
@@ skipping 1454 matching lines @@
         scanner()->FindSymbol(classifier->duplicate_finder(), 1) != 0) {
       classifier->RecordDuplicateFormalParameterError(scanner()->location());
     }
     return name;
   } else if (is_sloppy(language_mode()) &&
              (next == Token::FUTURE_STRICT_RESERVED_WORD ||
               next == Token::LET || next == Token::STATIC ||
               (next == Token::YIELD && !is_generator()))) {
     classifier->RecordStrictModeFormalParameterError(
         scanner()->location(), MessageTemplate::kUnexpectedStrictReserved);
+    if (next == Token::LET) {
+      classifier->RecordLetPatternError(scanner()->location(),
+                                        MessageTemplate::kLetInLexicalBinding);
+    }
     return this->GetSymbol(scanner());
   } else {
     this->ReportUnexpectedToken(next);
     *ok = false;
     return Traits::EmptyIdentifier();
   }
 }
 
 
 template <class Traits>
@@ skipping 2072 matching lines @@
       return;
     }
     has_seen_constructor_ = true;
     return;
   }
 }
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_PREPARSER_H