[es6] implement optional parameters via desugaring (with scoping)

src/preparser.h

Index: src/preparser.h
diff --git a/src/preparser.h b/src/preparser.h
index c60318be3e35b02e965becaac68f05ec40007840..58f61058c6a8e7859b79e0c8ec0f11b48637367d 100644
--- a/src/preparser.h
+++ b/src/preparser.h
@@ -84,6 +84,7 @@ class ParserBase : public Traits {
  public:
   // Shorten type names defined by Traits.
   typedef typename Traits::Type::Expression ExpressionT;
+  typedef typename Traits::Type::ExpressionList ExpressionListT;
   typedef typename Traits::Type::Identifier IdentifierT;
   typedef typename Traits::Type::FormalParameter FormalParameterT;
   typedef typename Traits::Type::FormalParameterScope FormalParameterScopeT;
@@ -694,10 +695,16 @@ class ParserBase : public Traits {
                                              bool* ok);
 
   void ParseFormalParameter(FormalParameterScopeT* scope,
-                            FormalParameterErrorLocations* locs, bool is_rest,
+                            Scope* initializer_scope,
+                            FormalParameterErrorLocations* locs,
+                            ExpressionT* initializer,
+                            bool* has_initializer, bool is_rest,
                             bool* ok);
   int ParseFormalParameterList(FormalParameterScopeT* scope,
+                               Scope* initializer_scope,
                                FormalParameterErrorLocations* locs,
+                               ExpressionListT initializers,
+                               bool* has_initializers,
                                bool* has_rest, bool* ok);
   void CheckArityRestrictions(
       int param_count, FunctionLiteral::ArityRestriction arity_restriction,
@@ -3343,8 +3350,9 @@ ParserBase<Traits>::ParseMemberExpressionContinuation(
 
 template <class Traits>
 void ParserBase<Traits>::ParseFormalParameter(
-    FormalParameterScopeT* scope, FormalParameterErrorLocations* locs,
-    bool is_rest, bool* ok) {
+    FormalParameterScopeT* scope, Scope* parameter_scope,
+    FormalParameterErrorLocations* locs,
+    ExpressionT* initializer, bool* has_initializer, bool is_rest, bool* ok) {
   // FormalParameter[Yield,GeneratorParameter] :
   //   BindingElement[?Yield, ?GeneratorParameter]
   bool is_strict_reserved;
@@ -3352,6 +3360,17 @@ void ParserBase<Traits>::ParseFormalParameter(
       ParseIdentifierOrStrictReservedWord(&is_strict_reserved, ok);
   if (!*ok) return;
 
+  if (parameter_scope && Check(Token::ASSIGN)) {
+    // Optional argument initializer
+    // let formalName = IS_UNDEFINED(arguments[i]) ? initializer : arguments[i];
+    BlockState state(&scope_, NewScope(parameter_scope, BLOCK_SCOPE));
+    static const bool accept_IN = true;
+    ExpressionClassifier classifier;
+    *initializer = ParseAssignmentExpression(accept_IN, &classifier, ok);
+    if (!*ok) return;
+    *has_initializer = true;
+  }
+
   // Store locations for possible future error reports.
   if (!locs->eval_or_arguments.IsValid() && this->IsEvalOrArguments(name)) {
     locs->eval_or_arguments = scanner()->location();
@@ -3371,8 +3390,10 @@ void ParserBase<Traits>::ParseFormalParameter(
 
 template <class Traits>
 int ParserBase<Traits>::ParseFormalParameterList(
-    FormalParameterScopeT* scope, FormalParameterErrorLocations* locs,
-    bool* is_rest, bool* ok) {
+    FormalParameterScopeT* scope, Scope* parameter_scope,
+    FormalParameterErrorLocations* locs,
+    ExpressionListT initializers, bool* has_initializers, bool* is_rest,
+    bool* ok) {
   // FormalParameters[Yield,GeneratorParameter] :
   //   [empty]
   //   FormalParameterList[?Yield, ?GeneratorParameter]
@@ -3389,6 +3410,8 @@ int ParserBase<Traits>::ParseFormalParameterList(
 
   int parameter_count = 0;
 
+  BlockState parameter_block(&scope_, parameter_scope);
+
   if (peek() != Token::RPAREN) {
     do {
       if (++parameter_count > Code::kMaxArguments) {
@@ -3396,9 +3419,14 @@ int ParserBase<Traits>::ParseFormalParameterList(
         *ok = false;
         return -1;
       }
+      bool has_initializer = false;
+      ExpressionT initializer = this->EmptyExpression();
       *is_rest = allow_harmony_rest_params() && Check(Token::ELLIPSIS);
-      ParseFormalParameter(scope, locs, *is_rest, ok);
+      ParseFormalParameter(scope, parameter_scope, locs, &initializer,
+                           &has_initializer, *is_rest, ok);
       if (!*ok) return -1;
+      initializers->Add(initializer, zone());
+      if (has_initializer) *has_initializers = true;
     } while (!*is_rest && Check(Token::COMMA));
 
     if (*is_rest && peek() == Token::COMMA) {