Rewind additional parser state when reinterpreting arrow arguments

src/preparser.h

Index: src/preparser.h
diff --git a/src/preparser.h b/src/preparser.h
index 517403d34733eba81ea86c7bedd38162feded389..b00cbbbd477cf2ea175fc4e62f3f180e23de5122 100644
--- a/src/preparser.h
+++ b/src/preparser.h
@@ -114,6 +114,8 @@ class ParserBase : public Traits {
   }
 
  protected:
+  friend class Traits::Type::Checkpoint;
+
   enum AllowEvalOrArgumentsAsIdentifier {
     kAllowEvalOrArguments,
     kDontAllowEvalOrArguments
@@ -124,6 +126,8 @@ class ParserBase : public Traits {
     PARSE_EAGERLY
   };
 
+  class ParserCheckpoint;
+
   // ---------------------------------------------------------------------------
   // FunctionState and BlockState together implement the parser's scope stack.
   // The parser's current scope is in scope_. BlockState and FunctionState
@@ -219,6 +223,38 @@ class ParserBase : public Traits {
     typename Traits::Type::Factory factory_;
 
     friend class ParserTraits;
+    friend class ParserCheckpoint;
+  };
+
+  // Annoyingly, arrow functions first parse as comma expressions, then when we
+  // see the => we have to go back and reinterpret the arguments as being formal
+  // parameters.  To do so we need to reset some of the parser state back to
+  // what it was before the arguments were first seen.
+  class ParserCheckpoint : public Traits::Type::Checkpoint {
+   public:
+    template <typename Parser>
+    explicit ParserCheckpoint(Parser* parser)
+        : Traits::Type::Checkpoint(parser) {
+      function_state_ = parser->function_state_;
+      next_materialized_literal_index_ =
+          function_state_->next_materialized_literal_index_;
+      next_handler_index_ = function_state_->next_handler_index_;
+      expected_property_count_ = function_state_->expected_property_count_;
+    }
+
+    void Restore() {
+      Traits::Type::Checkpoint::Restore();
+      function_state_->next_materialized_literal_index_ =
+          next_materialized_literal_index_;
+      function_state_->next_handler_index_ = next_handler_index_;
+      function_state_->expected_property_count_ = expected_property_count_;
+    }
+
+   private:
+    FunctionState* function_state_;
+    int next_materialized_literal_index_;
+    int next_handler_index_;
+    int expected_property_count_;
   };
 
   class ParsingModeScope BASE_EMBEDDED {
@@ -1034,6 +1070,13 @@ class PreParserTraits {
     typedef PreParserScope Scope;
     typedef PreParserScope ScopePtr;
 
+    class Checkpoint BASE_EMBEDDED {
+     public:
+      template <typename Parser>
+      explicit Checkpoint(Parser* parser) {}
+      void Restore() {}
+    };
+
     // PreParser doesn't need to store generator variables.
     typedef void GeneratorVariable;
     // No interaction with Zones.
@@ -2006,10 +2049,12 @@ ParserBase<Traits>::ParseAssignmentExpression(bool accept_IN, bool* ok) {
   }
 
   if (fni_ != NULL) fni_->Enter();
+  ParserCheckpoint checkpoint(this);
   ExpressionT expression =
       this->ParseConditionalExpression(accept_IN, CHECK_OK);
 
   if (allow_arrow_functions() && peek() == Token::ARROW) {
+    checkpoint.Restore();
     expression = this->ParseArrowFunctionLiteral(lhs_location.beg_pos,
                                                  expression, CHECK_OK);
     return expression;