[es6] implement default parameters via desugaring

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index d4607d1007ad845d15815f68b3ce5646b7a849d9..758dd98963e2c61569883c9b7e796db04cae01d4 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -878,6 +878,7 @@ Parser::Parser(ParseInfo* info)
   set_allow_harmony_rest_params(FLAG_harmony_rest_parameters);
   set_allow_harmony_spreadcalls(FLAG_harmony_spreadcalls);
   set_allow_harmony_destructuring(FLAG_harmony_destructuring);
+  set_allow_harmony_default_parameters(FLAG_harmony_default_parameters);
   set_allow_strong_mode(FLAG_strong_mode);
   for (int feature = 0; feature < v8::Isolate::kUseCounterFeatureCount;
        ++feature) {
@@ -1141,9 +1142,13 @@ FunctionLiteral* Parser::ParseLazy(Isolate* isolate, ParseInfo* info,
       scope->set_start_position(shared_info->start_position());
       ExpressionClassifier formals_classifier;
       bool has_rest = false;
+      bool hasParameterExpressions = false;
+      ZoneList<Expression*>* initializers =
+          new (zone()) ZoneList<Expression*>(0, zone());
       if (Check(Token::LPAREN)) {
         // '(' StrictFormalParameters ')'
-        ParseFormalParameterList(scope, &has_rest, &formals_classifier, &ok);
+        ParseFormalParameterList(scope, initializers, &hasParameterExpressions,
+                                 &has_rest, &formals_classifier, &ok);
         if (ok) ok = Check(Token::RPAREN);
       } else {
         // BindingIdentifier
@@ -3396,6 +3401,113 @@ Statement* Parser::DesugarLexicalBindingsInForStatement(
 }
 
 
+ZoneList<Statement*>* Parser::DesugarInitializeParameters(
+    Scope* scope, bool hasParameterExpressions,
+    ZoneList<Expression*>* initializers) {
+  DCHECK(scope->is_function_scope());
+
+  if (hasParameterExpressions) {
+    // If hasParameterExpressions for the function is true, each parameter is
+    // desugared as follows:
+    //
+    // SingleNameBinding :
+    //   let <name> = %_Arguments(<index>);
+    // SingleNameBinding Initializer
+    //   let <name> = IS_UNDEFINED(%_Arguments(<index>)) ? <initializer>
+    //                                                   : %_Arguments(<index>);
+    //
+    // TODO(caitp, dslomov): support BindingPatterns & rest parameters
+    //
+
+    ZoneList<Statement*>* body = new (zone()) ZoneList<Statement*>(0, zone());
+    for (int i = 0; i < initializers->length(); ++i) {
+      Expression* initializer = initializers->at(i);
+
+      // Position of parameter VariableProxy, for hole-checking
+      int pos = scope->parameter_position(i);
+
+      static const int capacity = 1;
+      static const bool is_initializer_block = true;  // ?
+      Block* param_block =
+          factory()->NewBlock(nullptr, capacity, is_initializer_block, pos);
+
+      Variable* param_var = scope->parameter(i);
+      param_var->set_mode(LET);
+
+      // Lexically declare the initialized variable
+      static const VariableMode mode = LET;
+      VariableProxy* proxy = NewUnresolved(param_var->raw_name(), mode);
+      VariableDeclaration* declaration =
+          factory()->NewVariableDeclaration(proxy, mode, scope, pos);
+      scope->AddDeclaration(declaration);
+      proxy = factory()->NewVariableProxy(param_var);
+
+      const AstRawString* fn_name = ast_value_factory()->empty_string();
+      const Runtime::Function* arguments =
+          Runtime::FunctionForId(Runtime::kInlineArguments);
+      ZoneList<Expression*>* arguments_i0 =
+          new (zone()) ZoneList<Expression*>(0, zone());
+      arguments_i0->Add(factory()->NewSmiLiteral(i, RelocInfo::kNoPosition),
+                        zone());
+
+      if (initializer) {
+        // IS_UNDEFINED(%_Arguments(i)) ? <initializer> : %_Arguments(i);
+        ZoneList<Expression*>* arguments_i1 =
+            new (zone()) ZoneList<Expression*>(0, zone());
+        arguments_i1->Add(factory()->NewSmiLiteral(i, RelocInfo::kNoPosition),
+                          zone());
+
+        Expression* arg_or_default = factory()->NewConditional(
+            // condition:
+            factory()->NewCompareOperation(
+                Token::EQ_STRICT,
+                factory()->NewCallRuntime(fn_name, arguments, arguments_i0,
+                                          RelocInfo::kNoPosition),
+                factory()->NewUndefinedLiteral(RelocInfo::kNoPosition),
+                RelocInfo::kNoPosition),
+            // if true:
+            initializer,
+            // if false:
+            factory()->NewCallRuntime(fn_name, arguments, arguments_i1,
+                                      RelocInfo::kNoPosition),
+            RelocInfo::kNoPosition);
+
+        Expression* assign = factory()->NewAssignment(
+            Token::INIT_LET, proxy, arg_or_default, RelocInfo::kNoPosition);
+
+        param_block->AddStatement(
+            factory()->NewExpressionStatement(assign, RelocInfo::kNoPosition),
+            zone());
+        proxy->var()->set_initializer_position(initializer->position());
+      } else {
+        // let <name> = %_Arguments(i)
+        Expression* assign = factory()->NewAssignment(
+            Token::INIT_LET, proxy,
+            factory()->NewCallRuntime(fn_name, arguments, arguments_i0,
+                                      RelocInfo::kNoPosition),
+            RelocInfo::kNoPosition);
+        param_block->AddStatement(
+            factory()->NewExpressionStatement(assign, RelocInfo::kNoPosition),
+            zone());
+        proxy->var()->set_initializer_position(pos);
+      }
+      body->Add(param_block, zone());
+    }
+    return body;
+  } else {
+    // If hasParameterExpressions is false, remove the unnecessary parameter
+    // block scopes.
+    ZoneList<Scope*>* scopes = scope->inner_scopes();
+    for (int i = 0; i < scopes->length(); ++i) {
+      Scope* scope = scopes->at(i);
+      DCHECK(scope->is_block_scope());
+      scope->FinalizeBlockScope();
+    }
+    return nullptr;
+  }
+}
+
+
 Statement* Parser::ParseForStatement(ZoneList<const AstRawString*>* labels,
                                      bool* ok) {
   // ForStatement ::
@@ -3789,7 +3901,8 @@ void ParserTraits::DeclareArrowFunctionParameters(
   parser_->scope_->RemoveUnresolved(expr->AsVariableProxy());
 
   bool is_rest = false;
-  bool is_duplicate = DeclareFormalParameter(scope, raw_name, is_rest);
+  int pos = expr->position();
+  bool is_duplicate = DeclareFormalParameter(scope, raw_name, is_rest, pos);
 
   if (is_duplicate && !duplicate_loc->IsValid()) {
     *duplicate_loc = param_location;
@@ -3903,8 +4016,12 @@ FunctionLiteral* Parser::ParseFunctionLiteral(
     Expect(Token::LPAREN, CHECK_OK);
     int start_position = scanner()->location().beg_pos;
     scope_->set_start_position(start_position);
-    num_parameters = ParseFormalParameterList(scope, &has_rest,
-                                              &formals_classifier, CHECK_OK);
+    ZoneList<Expression*>* initializers =
+        new (zone()) ZoneList<Expression*>(0, zone());
+    bool hasParameterExpressions = false;
+    num_parameters =
+        ParseFormalParameterList(scope, initializers, &hasParameterExpressions,
+                                 &has_rest, &formals_classifier, CHECK_OK);
     Expect(Token::RPAREN, CHECK_OK);
     int formals_end_position = scanner()->location().end_pos;
 
@@ -4005,8 +4122,31 @@ FunctionLiteral* Parser::ParseFunctionLiteral(
       }
     }
     if (!is_lazily_parsed) {
-      body = ParseEagerFunctionBody(function_name, pos, fvar, fvar_init_op,
-                                    kind, CHECK_OK);
+      body = DesugarInitializeParameters(scope, hasParameterExpressions,
+                                         initializers);
+      if (hasParameterExpressions) {
+        // TODO(caitp): Function body scope must be a declaration scope
+        Scope* function_body_scope = NewScope(scope, BLOCK_SCOPE);
+        function_body_scope->set_start_position(scope->start_position());
+        function_body_scope->SetScopeName(function_name);
+        BlockState function_body_state(&scope_, function_body_scope);
+        ZoneList<Statement*>* inner_body = ParseEagerFunctionBody(
+            function_name, pos, fvar, fvar_init_op, kind, CHECK_OK);
+
+        // Declare Block node
+        Block* block =
+            factory()->NewBlock(nullptr, inner_body->length(), false, pos);
+        block->set_scope(function_body_scope);
+        for (int i = 0; i < inner_body->length(); ++i) {
+          block->AddStatement(inner_body->at(i), zone());
+        }
+
+        scope->set_end_position(function_body_scope->end_position());
+        body->Add(block, zone());
+      } else {
+        body = ParseEagerFunctionBody(function_name, pos, fvar, fvar_init_op,
+                                      kind, CHECK_OK);
+      }
       materialized_literal_count = function_state.materialized_literal_count();
       expected_property_count = function_state.expected_property_count();
       handler_count = function_state.handler_count();
@@ -4288,6 +4428,8 @@ PreParser::PreParseResult Parser::ParseLazyFunctionBodyWithPreParser(
         allow_harmony_spreadcalls());
     reusable_preparser_->set_allow_harmony_destructuring(
         allow_harmony_destructuring());
+    reusable_preparser_->set_allow_harmony_default_parameters(
+        allow_harmony_default_parameters());
     reusable_preparser_->set_allow_strong_mode(allow_strong_mode());
   }
   PreParser::PreParseResult result = reusable_preparser_->PreParseLazyFunction(