[parser] report better errors for multiple ForBindings in ForIn/Of loops

src/preparser.cc

 // Copyright 2011 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.
 
 #include <cmath>
 
 #include "src/allocation.h"
 #include "src/base/logging.h"
 #include "src/conversions-inl.h"
 #include "src/conversions.h"
@@ skipping 392 matching lines @@
   return Statement::Default();
 }
 
 
 PreParser::Statement PreParser::ParseVariableStatement(
     VariableDeclarationContext var_context,
     bool* ok) {
   // VariableStatement ::
   //   VariableDeclarations ';'
 
-  Statement result =
-      ParseVariableDeclarations(var_context, nullptr, nullptr, CHECK_OK);
+  Statement result = ParseVariableDeclarations(var_context, nullptr, nullptr,
+                                               nullptr, CHECK_OK);
   ExpectSemicolon(CHECK_OK);
   return result;
 }
 
 
 // If the variable declaration declares exactly one non-const
 // variable, then *var is set to that variable. In all other cases,
 // *var is untouched; in particular, it is the caller's responsibility
 // to initialize it properly. This mechanism is also used for the parsing
 // of 'for-in' loops.
 PreParser::Statement PreParser::ParseVariableDeclarations(
     VariableDeclarationContext var_context, int* num_decl,
-    Scanner::Location* first_initializer_loc, bool* ok) {
+    Scanner::Location* first_initializer_loc, Scanner::Location* bindings_loc,
+    bool* ok) {
   // VariableDeclarations ::
   //   ('var' | 'const') (Identifier ('=' AssignmentExpression)?)+[',']
   //
   // The ES6 Draft Rev3 specifies the following grammar for const declarations
   //
   // ConstDeclaration ::
   //   const ConstBinding (',' ConstBinding)* ';'
   // ConstBinding ::
   //   Identifier '=' AssignmentExpression
   //
@@ skipping 33 matching lines @@
   } 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
+  int bindings_start = peek_position();
   do {
     // Parse variable name.
     if (nvars > 0) Consume(Token::COMMA);
     ParseIdentifier(kDontAllowEvalOrArguments, CHECK_OK);
     Scanner::Location variable_loc = scanner()->location();
     nvars++;
     if (peek() == Token::ASSIGN || require_initializer ||
         // require initializers for multiple consts.
         (is_strict_const && peek() == Token::COMMA)) {
       Expect(Token::ASSIGN, CHECK_OK);
       ParseAssignmentExpression(var_context != kForStatement, CHECK_OK);
 
       variable_loc.end_pos = scanner()->location().end_pos;
       if (first_initializer_loc && !first_initializer_loc->IsValid()) {
         *first_initializer_loc = variable_loc;
       }
     }
   } while (peek() == Token::COMMA);
 
+  if (bindings_loc) {
+    *bindings_loc =
+        Scanner::Location(bindings_start, scanner()->location().end_pos);
+  }
+
   if (num_decl != NULL) *num_decl = nvars;
   return Statement::Default();
 }
 
 
 PreParser::Statement PreParser::ParseExpressionOrLabelledStatement(bool* ok) {
   // ExpressionStatement | LabelledStatement ::
   //   Expression ';'
   //   Identifier ':' Statement
 
@@ skipping 215 matching lines @@
   // ForStatement ::
   //   'for' '(' Expression? ';' Expression? ';' Expression? ')' Statement
 
   Expect(Token::FOR, CHECK_OK);
   Expect(Token::LPAREN, CHECK_OK);
   bool is_let_identifier_expression = false;
   if (peek() != Token::SEMICOLON) {
     ForEachStatement::VisitMode mode;
     if (peek() == Token::VAR || peek() == Token::CONST ||
         (peek() == Token::LET && is_strict(language_mode()))) {
-      bool is_lexical = peek() == Token::LET ||
-                        (peek() == Token::CONST && is_strict(language_mode()));
       int decl_count;
       Scanner::Location first_initializer_loc = Scanner::Location::invalid();
+      Scanner::Location bindings_loc = Scanner::Location::invalid();
       ParseVariableDeclarations(kForStatement, &decl_count,
-                                &first_initializer_loc, CHECK_OK);
-      bool has_initializers = first_initializer_loc.IsValid();
-      bool accept_IN = decl_count == 1 && !(is_lexical && has_initializers);
+                                &first_initializer_loc, &bindings_loc,
+                                CHECK_OK);
+      bool accept_IN = decl_count >= 1;
       bool accept_OF = true;
       if (accept_IN && CheckInOrOf(accept_OF, &mode, ok)) {
         if (!*ok) return Statement::Default();
+        if (decl_count != 1) {
+          const char* loop_type =
+              mode == ForEachStatement::ITERATE ? "for-of" : "for-in";
+          PreParserTraits::ReportMessageAt(
+              bindings_loc, "for_inof_loop_multi_bindings", loop_type);
+          *ok = false;
+          return Statement::Default();
+        }
         if (first_initializer_loc.IsValid() &&
             (is_strict(language_mode()) || mode == ForEachStatement::ITERATE)) {
           if (mode == ForEachStatement::ITERATE) {
             ReportMessageAt(first_initializer_loc, "for_of_loop_initializer");
           } else {
             // TODO(caitp): This should be an error in sloppy mode, too.
             ReportMessageAt(first_initializer_loc, "for_in_loop_initializer");
           }
           *ok = false;
           return Statement::Default();
@@ skipping 301 matching lines @@
   ParseIdentifier(kAllowEvalOrArguments, CHECK_OK);
   ParseArguments(ok);
 
   return Expression::Default();
 }
 
 #undef CHECK_OK
 
 
 } }  // v8::internal