[es6] Implement default parameters

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index 45202cb3a82b922c7d487061cfff03578cc3a3c9..bfe84cb1f4a0d46da2da8a7cc60d8db1aae37744 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -914,6 +914,7 @@ Parser::Parser(ParseInfo* info)
   set_allow_harmony_sloppy(FLAG_harmony_sloppy);
   set_allow_harmony_sloppy_let(FLAG_harmony_sloppy_let);
   set_allow_harmony_rest_parameters(FLAG_harmony_rest_parameters);
+  set_allow_harmony_default_parameters(FLAG_harmony_default_parameters);
   set_allow_harmony_spreadcalls(FLAG_harmony_spreadcalls);
   set_allow_harmony_destructuring(FLAG_harmony_destructuring);
   set_allow_harmony_spread_arrays(FLAG_harmony_spread_arrays);
@@ -1196,8 +1197,7 @@ FunctionLiteral* Parser::ParseLazy(Isolate* isolate, ParseInfo* info,
           if (ok) ok = Check(Token::RPAREN);
         } else {
           // BindingIdentifier
-          const bool is_rest = false;
-          ParseFormalParameter(is_rest, &formals, &formals_classifier, &ok);
+          ParseFormalParameter(&formals, &formals_classifier, &ok);
           if (ok) {
             DeclareFormalParameter(
                 formals.scope, formals.at(0), formals.is_simple,
@@ -3905,7 +3905,8 @@ void ParserTraits::ParseArrowFunctionFormalParameters(
     parser_->scope_->RemoveUnresolved(expr->AsVariableProxy());
   }
 
-  AddFormalParameter(parameters, expr, is_rest);
+  // TODO(rossberg): allow initializers

[rossberg, 2015/08/12 15:20:27]

Need to come up with a test that actually triggers this.

+  AddFormalParameter(parameters, expr, nullptr, is_rest);
 }
 
 
@@ -4324,9 +4325,21 @@ Block* Parser::BuildParameterInitializationBlock(
     descriptor.declaration_pos = parameter.pattern->position();
     descriptor.initialization_pos = parameter.pattern->position();
     descriptor.init_op = Token::INIT_LET;
+    Expression* param =

[adamk, 2015/08/12 18:22:59]

I think this should be called "initializer"

[rossberg, 2015/08/13 11:06:36]

That would confuse it with the actual initializer expression. I renamed it to
initial_value instead, is that okay?

+        factory()->NewVariableProxy(parameters.scope->parameter(i));
+    if (parameter.initializer != nullptr) {
+      // IS_UNDEFINED($param) ? initializer : $param
+      auto condition = factory()->NewCompareOperation(
+          Token::EQ_STRICT,
+          factory()->NewVariableProxy(parameters.scope->parameter(i)),
+          factory()->NewUndefinedLiteral(RelocInfo::kNoPosition),
+          RelocInfo::kNoPosition);
+      param = factory()->NewConditional(
+          condition, parameter.initializer, param, RelocInfo::kNoPosition);
+      descriptor.initialization_pos = parameter.initializer->position();
+    }
     DeclarationParsingResult::Declaration decl(
-        parameter.pattern, parameter.pattern->position(),
-        factory()->NewVariableProxy(parameters.scope->parameter(i)));
+        parameter.pattern, parameter.pattern->position(), param);

[adamk, 2015/08/12 18:22:59]

The above-suggested renaming would make the callsite clearer.

[rossberg, 2015/08/13 11:06:36]

Done.

     PatternRewriter::DeclareAndInitializeVariables(init_block, &descriptor,
                                                    &decl, nullptr, CHECK_OK);
   }
@@ -4492,6 +4505,7 @@ PreParser::PreParseResult Parser::ParseLazyFunctionBodyWithPreParser(
     SET_ALLOW(harmony_sloppy);
     SET_ALLOW(harmony_sloppy_let);
     SET_ALLOW(harmony_rest_parameters);
+    SET_ALLOW(harmony_default_parameters);
     SET_ALLOW(harmony_spreadcalls);
     SET_ALLOW(harmony_destructuring);
     SET_ALLOW(harmony_spread_arrays);