Always lazy compile arrow functions

src/preparser.h

Index: src/preparser.h
diff --git a/src/preparser.h b/src/preparser.h
index a5aa3709a6d509174fe9c02530b80cd940c907ab..2b07de94c47037d58c1f1a0652f96a75fe2d2677 100644
--- a/src/preparser.h
+++ b/src/preparser.h
@@ -1083,6 +1083,7 @@ class PreParserExpression {
   // More dummy implementations of things PreParser doesn't need to track:
   void set_index(int index) {}  // For YieldExpressions
   void set_should_eager_compile() {}
+  FunctionKind kind() const { return FunctionKind::kNormalFunction; }
 
   int position() const { return RelocInfo::kNoPosition; }
   void set_function_token_position(int position) {}
@@ -3279,8 +3280,12 @@ ParserBase<Traits>::ParseLeftHandSideExpression(
           pos = peek_position();
           // Also the trailing parenthesis are a hint that the function will
           // be called immediately. If we happen to have parsed a preceding
-          // function literal eagerly, we can also compile it eagerly.
-          if (result->IsFunctionLiteral() && mode() == PARSE_EAGERLY) {
+          // function literal eagerly, we can also compile it eagerly (unless
+          // that literal was an arrow function, in which case we always
+          // compile lazily in order to get scoping right for its parameters,
+          // see https://code.google.com/p/v8/issues/detail?id=4395).
+          if (result->IsFunctionLiteral() && mode() == PARSE_EAGERLY &&
+              !IsArrowFunction(result->AsFunctionLiteral()->kind())) {

[caitp (gmail), 2015/09/19 21:34:36]

The only time you _need_ to force re-parsing of arrows is when the parameter
list contains expressions (eg an initializer), so we could probably add an extra
condition to escape lazy parsing here. But, it might not matter that much
anyway, so.

[rossberg, 2015/09/22 15:33:05]

That's a good point. It might make sense to avoid enforcing lazy parsing for
arrow functions with simple parameters. I expect arrow IIFE to become a not
too-uncommon use case.

             result->AsFunctionLiteral()->set_should_eager_compile();
           }
         }