Better error reporting for "return();"

src/preparser.h

Index: src/preparser.h
diff --git a/src/preparser.h b/src/preparser.h
index 862b7fb40dd894b53dfcec80d36aedde5b441e3f..238fb89ac8c9b6a9afece0a03c638fa3ce064e07 100644
--- a/src/preparser.h
+++ b/src/preparser.h
@@ -502,7 +502,9 @@ class ParserBase : public Traits {
   }
 
   void ReportUnexpectedToken(Token::Value token);
-  void ReportUnexpectedTokenAt(Scanner::Location location, Token::Value token);
+  void ReportUnexpectedTokenAt(
+      Scanner::Location location, Token::Value token,
+      MessageTemplate::Template message = MessageTemplate::kUnexpectedToken);
 
 
   void ReportClassifierError(const ExpressionClassifier::Error& error) {
@@ -1814,10 +1816,10 @@ void ParserBase<Traits>::ReportUnexpectedToken(Token::Value token) {
 }
 
 
-template<class Traits>
+template <class Traits>
 void ParserBase<Traits>::ReportUnexpectedTokenAt(
-    Scanner::Location source_location, Token::Value token) {
-
+    Scanner::Location source_location, Token::Value token,
+    MessageTemplate::Template message) {
   // Four of the tokens are treated specially
   switch (token) {
     case Token::EOS:
@@ -1850,8 +1852,7 @@ void ParserBase<Traits>::ReportUnexpectedTokenAt(
     default:
       const char* name = Token::String(token);
       DCHECK(name != NULL);
-      Traits::ReportMessageAt(source_location,
-                              MessageTemplate::kUnexpectedToken, name);
+      Traits::ReportMessageAt(source_location, message, name);
   }
 }
 
@@ -3610,7 +3611,17 @@ ParserBase<Traits>::ParseArrowFunctionLiteral(
     FunctionState function_state(&function_state_, &scope_, scope,
                                  kArrowFunction, &function_factory);
 
-    Expect(Token::ARROW, CHECK_OK);
+    // ParseArrowFunctionLiteral can be called in two ways: after seeing a
+    // formal parameter list that *must* be followed by an arrow, to wit "()" or
+    // "(...x)", or after reading a comma expression that is followed by =>.  In
+    // the former cases we want to give a good error to the user who might have
+    // typed e.g. "return();".
+    if (!Check(Token::ARROW)) {
+      ReportUnexpectedTokenAt(scanner_->peek_location(), peek(),
+                              MessageTemplate::kMissingArrow);
+      *ok = false;
+      return this->EmptyExpression();
+    }
 
     if (peek() == Token::LBRACE) {
       // Multiple statement body