Report unexpected lexical decl also without destructuring

src/parsing/parser-base.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_PARSING_PARSER_BASE_H
 #define V8_PARSING_PARSER_BASE_H
 
 #include "src/ast/ast.h"
 #include "src/ast/scopes.h"
 #include "src/bailout-reason.h"
@@ skipping 4914 matching lines @@
   int pos = peek_position();
 
   switch (peek()) {
     case Token::FUNCTION:
     case Token::LBRACE:
       UNREACHABLE();  // Always handled by the callers.
     case Token::CLASS:
       ReportUnexpectedToken(Next());
       *ok = false;
       return impl()->NullStatement();
-    case Token::LET:
-      if (PeekAhead() != Token::LBRACK) break;
+    case Token::LET: {
+      Token::Value next_next = PeekAhead();
+      // "let" followed by either "[" or an identifier means a lexical
+      // declaration, which should not appear here.
+      if (next_next != Token::LBRACK && next_next != Token::IDENTIFIER) break;

[Dan Ehrenberg, 2017/02/17 08:37:14]

Another case you can include here is "let {", which will not be syntactically
valid here and may come up as a destructuring case (by contrast, `let:` and `let
+ let` and `let()` are valid).

[vabr (Chromium), 2017/02/17 09:52:07]

Good idea! Done.

       impl()->ReportMessageAt(scanner()->peek_location(),
                               MessageTemplate::kUnexpectedLexicalDeclaration);
       *ok = false;
       return impl()->NullStatement();
+    }
     default:
       break;
   }
 
   bool starts_with_identifier = peek_any_identifier();
   ExpressionT expr = ParseExpression(true, CHECK_OK);
   if (peek() == Token::COLON && starts_with_identifier &&
       impl()->IsIdentifier(expr)) {
     // The whole expression was a single identifier, and not, e.g.,
     // something starting with an identifier or a parenthesized identifier.
@@ skipping 896 matching lines @@
 }
 
 #undef CHECK_OK
 #undef CHECK_OK_CUSTOM
 #undef CHECK_OK_VOID
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_PARSING_PARSER_BASE_H