[es6] implement default parameters via desugaring

src/preparser.h

Index: src/preparser.h
diff --git a/src/preparser.h b/src/preparser.h
index d38aaaaa26e7f1fa3876c041b708d123c054660a..3b9c58ce9e687e68a669b2b9bece4d89b22e8514 100644
--- a/src/preparser.h
+++ b/src/preparser.h
@@ -66,6 +66,7 @@ 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;
@@ -97,6 +98,7 @@ class ParserBase : public Traits {
         allow_harmony_computed_property_names_(false),
         allow_harmony_rest_params_(false),
         allow_harmony_spreadcalls_(false),
+        allow_harmony_default_parameters_(false),
         allow_strong_mode_(false) {}
 
   // Getters that indicate whether certain syntactical constructs are
@@ -126,6 +128,9 @@ class ParserBase : public Traits {
   bool allow_harmony_spread_arrays() const {
     return allow_harmony_spread_arrays_;
   }
+  bool allow_harmony_default_parameters() const {
+    return allow_harmony_default_parameters_;
+  }
 
   bool allow_strong_mode() const { return allow_strong_mode_; }
 
@@ -167,6 +172,10 @@ class ParserBase : public Traits {
   void set_allow_harmony_spread_arrays(bool allow) {
     allow_harmony_spread_arrays_ = allow;
   }
+  void set_allow_harmony_default_parameters(bool allow) {
+    allow_harmony_default_parameters_ = allow;
+  }
+
 
  protected:
   enum AllowRestrictedIdentifiers {
@@ -914,7 +923,9 @@ class ParserBase : public Traits {
 
   void ParseFormalParameter(FormalParameterScopeT* scope, bool is_rest,
                             ExpressionClassifier* classifier, bool* ok);
-  int ParseFormalParameterList(FormalParameterScopeT* scope, bool* has_rest,
+  int ParseFormalParameterList(FormalParameterScopeT* scope,
+                               ExpressionListT initializers,
+                               bool* hasParameterExpressions, bool* has_rest,
                                ExpressionClassifier* classifier, bool* ok);
   void CheckArityRestrictions(
       int param_count, FunctionLiteral::ArityRestriction arity_restriction,
@@ -1017,6 +1028,7 @@ class ParserBase : public Traits {
   bool allow_harmony_spreadcalls_;
   bool allow_harmony_destructuring_;
   bool allow_harmony_spread_arrays_;
+  bool allow_harmony_default_parameters_;
   bool allow_strong_mode_;
 };
 
@@ -1793,8 +1805,8 @@ class PreParserTraits {
   }
 
   V8_INLINE bool DeclareFormalParameter(DuplicateFinder* scope,
-                                        PreParserIdentifier param,
-                                        bool is_rest);
+                                        PreParserIdentifier param, bool is_rest,
+                                        int pos);
 
   void CheckConflictingVarDeclarations(Scope* scope, bool* ok) {}
 
@@ -1990,7 +2002,7 @@ PreParserExpression PreParserTraits::SpreadCallNew(PreParserExpression function,
 
 bool PreParserTraits::DeclareFormalParameter(
     DuplicateFinder* duplicate_finder, PreParserIdentifier current_identifier,
-    bool is_rest) {
+    bool is_rest, int pos) {
   return pre_parser_->scanner()->FindSymbol(duplicate_finder, 1) != 0;
 }
 
@@ -3661,10 +3673,11 @@ void ParserBase<Traits>::ParseFormalParameter(FormalParameterScopeT* scope,
                                               bool* ok) {
   // FormalParameter[Yield,GeneratorParameter] :
   //   BindingElement[?Yield, ?GeneratorParameter]
+  int pos = peek_position();
   IdentifierT name = ParseAndClassifyIdentifier(classifier, ok);
   if (!*ok) return;
 
-  bool was_declared = Traits::DeclareFormalParameter(scope, name, is_rest);
+  bool was_declared = Traits::DeclareFormalParameter(scope, name, is_rest, pos);
   if (was_declared) {
     classifier->RecordDuplicateFormalParameterError(scanner()->location());
   }
@@ -3673,7 +3686,8 @@ void ParserBase<Traits>::ParseFormalParameter(FormalParameterScopeT* scope,
 
 template <class Traits>
 int ParserBase<Traits>::ParseFormalParameterList(
-    FormalParameterScopeT* scope, bool* is_rest,
+    FormalParameterScopeT* scope, ExpressionListT initializers,
+    bool* hasParameterExpressions, bool* is_rest,
     ExpressionClassifier* classifier, bool* ok) {
   // FormalParameters[Yield,GeneratorParameter] :
   //   [empty]
@@ -3693,14 +3707,43 @@ int ParserBase<Traits>::ParseFormalParameterList(
 
   if (peek() != Token::RPAREN) {
     do {
+      Scope* param_scope = NewScope(scope_, BLOCK_SCOPE);
+      BlockState param_state(&scope_, param_scope);
+      param_scope->set_start_position(peek_position());
       if (++parameter_count > Code::kMaxArguments) {
         ReportMessage(MessageTemplate::kTooManyParameters);
         *ok = false;
         return -1;
       }
+
+      int start_pos = peek_position();
       *is_rest = allow_harmony_rest_params() && Check(Token::ELLIPSIS);
       ParseFormalParameter(scope, *is_rest, classifier, ok);
       if (!*ok) return -1;
+
+      // TODO(caitp, dslomov): set *hasParameterExpressions to true if
+      // formal parameter is an ObjectBindingPattern containing computed
+      // property keys
+
+      ExpressionT initializer = this->EmptyExpression();
+      if (allow_harmony_default_parameters() && Check(Token::ASSIGN)) {
+        // Default parameter initializer
+        static const bool accept_IN = true;
+        ExpressionClassifier classifier;
+        initializer = ParseAssignmentExpression(accept_IN, &classifier, ok);
+        if (!*ok) return -1;
+        *hasParameterExpressions = true;
+
+        // A rest parameter cannot be initialized.
+        if (*is_rest) {
+          Scanner::Location loc(start_pos, scanner()->location().end_pos);
+          ReportMessageAt(loc, MessageTemplate::kBadRestParameterInitializer);
+          *ok = false;
+          return -1;
+        }
+      }
+      param_scope->set_end_position(scanner()->location().end_pos);
+      initializers->Add(initializer, zone());
     } while (!*is_rest && Check(Token::COMMA));
 
     if (*is_rest && peek() == Token::COMMA) {