[es6] implement default parameters via desugaring

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index 5e3db128ada4d98a13861b6fcd4bfa37551fdf57..8b68a1dbad498467268702ac7645d6a4300f7a68 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -876,6 +876,7 @@ 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_optional_params(FLAG_harmony_optional_params);
   set_allow_strong_mode(FLAG_strong_mode);
   for (int feature = 0; feature < v8::Isolate::kUseCounterFeatureCount;
        ++feature) {
@@ -1139,13 +1140,18 @@ FunctionLiteral* Parser::ParseLazy(Isolate* isolate, ParseInfo* info,
       scope->set_start_position(shared_info->start_position());
       FormalParameterErrorLocations error_locs;
       bool has_rest = false;
+      bool has_initializers = false;
+      ZoneList<Expression*>* initializers =
+          new (zone()) ZoneList<Expression*>(0, zone());
       if (Check(Token::LPAREN)) {
         // '(' StrictFormalParameters ')'
-        ParseFormalParameterList(scope, &error_locs, &has_rest, &ok);
+        ParseFormalParameterList(scope, &error_locs, initializers,
+                                 &has_initializers, &has_rest, &ok);
         if (ok) ok = Check(Token::RPAREN);
       } else {
         // BindingIdentifier
-        ParseFormalParameter(scope, &error_locs, has_rest, &ok);
+        ParseFormalParameter(scope, &error_locs, nullptr, nullptr, has_rest,

[wingo, 2015/05/11 15:00:48]

nit: would be easier to read with named variables for these nullptrs

[caitp (gmail), 2015/05/11 15:34:24]

Acknowledged.

+                             &ok);
       }
 
       if (ok) {
@@ -2069,6 +2075,11 @@ Variable* Parser::Declare(Declaration* declaration, bool resolve, bool* ok) {
 }
 
 
+void Parser::ShadowParametersForExpressions(Scope* scope) {
+  scope->ShadowParametersForExpressions();
+}
+
+
 // Language extension which is only enabled for source files loaded
 // through the API's extension mechanism.  A native function
 // declaration is resolved by looking up the function through a
@@ -3527,6 +3538,109 @@ Statement* Parser::DesugarLexicalBindingsInForStatement(
 }
 
 
+ZoneList<Statement*>* Parser::DesugarInitializeParameters(
+    Scope* scope, bool has_initializers, ZoneList<Expression*>* initializers) {
+  DCHECK(scope->is_function_scope());
+
+  if (has_initializers) {
+    // If hasParameterExpressions for the function is true, each parameter is

[wingo, 2015/05/11 15:00:48]

has_initializers I guess?  I know hasParameterExpressions is the spec language;
is has_initializers the same?

[caitp (gmail), 2015/05/11 15:34:24]

HasParameterExpressions is also true for Object binding patterns with computed
property keys, so the language will have to change eventually

+    // 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
+    //
+
+    ShadowParametersForExpressions(scope);
+    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);
+
+      // Lexically declare the initialized variable
+      static const VariableMode mode = LET;
+      VariableProxy* proxy =
+          NewUnresolved(scope->parameter(i)->raw_name(), mode);
+      VariableDeclaration* declaration = factory()->NewVariableDeclaration(
+          proxy, mode, scope, RelocInfo::kNoPosition);
+      bool ok = true;
+      proxy = factory()->NewVariableProxy(Declare(declaration, true, &ok), pos);
+      DCHECK(ok);

[wingo, 2015/05/11 15:00:48]

Seems that Declare can only fail for a var redeclaration, which is impossible
here, OK.  Still, a comment would save the reader some time :)

[caitp (gmail), 2015/05/11 15:34:24]

Acknowledged.

+      proxy->var()->set_maybe_assigned();
+
+      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());
+
+      // TODO(caitp): ensure proper TDZ behaviour --- need hole-check for
+      // all parameter bindings, including ones without initializers
+      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);
+
+        body->Add(
+            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);
+        body->Add(
+            factory()->NewExpressionStatement(assign, RelocInfo::kNoPosition),
+            zone());
+        proxy->var()->set_initializer_position(pos);
+      }
+    }
+    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 ::
@@ -3937,7 +4051,8 @@ 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) {
     // Arrow function parameter lists are parsed as StrictFormalParameters,
@@ -4067,8 +4182,12 @@ FunctionLiteral* Parser::ParseFunctionLiteral(
     Expect(Token::LPAREN, CHECK_OK);
     int start_position = scanner()->location().beg_pos;
     scope_->set_start_position(start_position);
+    ZoneList<Expression*>* initializers =
+        new (zone()) ZoneList<Expression*>(0, zone());
+    bool has_initializers = false;
     num_parameters =
-        ParseFormalParameterList(scope, &error_locs, &has_rest, CHECK_OK);
+        ParseFormalParameterList(scope, &error_locs, initializers,
+                                 &has_initializers, &has_rest, CHECK_OK);
     Expect(Token::RPAREN, CHECK_OK);
     int formals_end_position = scanner()->location().end_pos;
 
@@ -4169,8 +4288,21 @@ FunctionLiteral* Parser::ParseFunctionLiteral(
       }
     }
     if (!is_lazily_parsed) {
-      body = ParseEagerFunctionBody(function_name, pos, fvar, fvar_init_op,
-                                    kind, CHECK_OK);
+      body = DesugarInitializeParameters(scope, has_initializers, initializers);
+      if (has_initializers) {
+        // 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);
+        scope->set_end_position(function_body_scope->end_position());
+        body->AddAll(*inner_body, 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();
@@ -4421,6 +4553,8 @@ PreParser::PreParseResult Parser::ParseLazyFunctionBodyWithPreParser(
         allow_harmony_spreadcalls());
     reusable_preparser_->set_allow_harmony_destructuring(
         allow_harmony_destructuring());
+    reusable_preparser_->set_allow_harmony_optional_params(
+        allow_harmony_optional_params());
     reusable_preparser_->set_allow_strong_mode(allow_strong_mode());
   }
   PreParser::PreParseResult result = reusable_preparser_->PreParseLazyFunction(