[es6] Implement default parameters

src/parser.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/parser.h"
 
 #include "src/api.h"
 #include "src/ast.h"
 #include "src/ast-literal-reindexer.h"
 #include "src/bailout-reason.h"
@@ skipping 898 matching lines @@
   // Parser - this makes sure that Isolate is not accidentally accessed via
   // ParseInfo during background parsing.
   DCHECK(!info->script().is_null() || info->source_stream() != NULL);
   set_allow_lazy(info->allow_lazy_parsing());
   set_allow_natives(FLAG_allow_natives_syntax || info->is_native());
   set_allow_harmony_arrow_functions(FLAG_harmony_arrow_functions);
   set_allow_harmony_sloppy(FLAG_harmony_sloppy);
   set_allow_harmony_sloppy_function(FLAG_harmony_sloppy_function);
   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);
   set_allow_harmony_new_target(FLAG_harmony_new_target);
   set_allow_strong_mode(FLAG_strong_mode);
   set_allow_legacy_const(FLAG_legacy_const);
   for (int feature = 0; feature < v8::Isolate::kUseCounterFeatureCount;
        ++feature) {
     use_counts_[feature] = 0;
   }
@@ skipping 2974 matching lines @@
 
   if (expr->IsVariableProxy()) {
     // When the formal parameter was originally seen, it was parsed as a
     // VariableProxy and recorded as unresolved in the scope.  Here we undo that
     // parse-time side-effect for parameters that are single-names (not
     // patterns; for patterns that happens uniformly in
     // PatternRewriter::VisitVariableProxy).
     parser_->scope_->RemoveUnresolved(expr->AsVariableProxy());
   }
 
-  AddFormalParameter(parameters, expr, is_rest);
+  Expression* initializer = nullptr;
+  if (!is_rest && parser_->allow_harmony_default_parameters() &&
+      parser_->Check(Token::ASSIGN)) {

[wingo, 2015/08/24 14:10:10]

How on earth does this work?  This function is called after the arrow function
arguments have been parsed, and its purpose is to transform the already-parsed
expression into parameters.  I can't see how this can possibly do the right
thing.

[rossberg, 2015/08/24 14:18:15]

You are right, it doesn't make sense. 8} Not sure what I was thinking.

Any idea why this is never hit by the tests?

+    ExpressionClassifier init_classifier;
+    initializer =
+        parser_->ParseAssignmentExpression(true, &init_classifier, ok);
+    if (!*ok) return;
+    parser_->ValidateExpression(&init_classifier, ok);
+    if (!*ok) return;
+    parameters->is_simple = false;
+  }
+
+  AddFormalParameter(parameters, expr, initializer, is_rest);
 }
 
 
 void ParserTraits::ParseArrowFunctionFormalParameterList(
     ParserFormalParameters* parameters, Expression* expr,
     const Scanner::Location& params_loc,
     Scanner::Location* duplicate_loc, bool* ok) {
   ParseArrowFunctionFormalParameters(parameters, expr, params_loc,
                                      duplicate_loc, ok);
   if (!*ok) return;
@@ skipping 398 matching lines @@
     descriptor.declaration_kind = DeclarationDescriptor::PARAMETER;
     descriptor.parser = this;
     descriptor.declaration_scope = scope_;
     descriptor.scope = scope_;
     descriptor.mode = LET;
     descriptor.is_const = false;
     descriptor.needs_init = true;
     descriptor.declaration_pos = parameter.pattern->position();
     descriptor.initialization_pos = parameter.pattern->position();
     descriptor.init_op = Token::INIT_LET;
+    Expression* initial_value =
+        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);
+      initial_value = factory()->NewConditional(
+          condition, parameter.initializer, initial_value,
+          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(), initial_value);
     PatternRewriter::DeclareAndInitializeVariables(init_block, &descriptor,
                                                    &decl, nullptr, CHECK_OK);
   }
   return init_block;
 }
 
 
 ZoneList<Statement*>* Parser::ParseEagerFunctionBody(
     const AstRawString* function_name, int pos,
     const ParserFormalParameters& parameters, FunctionKind kind,
@@ skipping 145 matching lines @@
   if (reusable_preparser_ == NULL) {
     reusable_preparser_ = new PreParser(zone(), &scanner_, ast_value_factory(),
                                         NULL, stack_limit_);
     reusable_preparser_->set_allow_lazy(true);
 #define SET_ALLOW(name) reusable_preparser_->set_allow_##name(allow_##name());
     SET_ALLOW(natives);
     SET_ALLOW(harmony_arrow_functions);
     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);
     SET_ALLOW(harmony_new_target);
     SET_ALLOW(strong_mode);
 #undef SET_ALLOW
   }
   PreParser::PreParseResult result = reusable_preparser_->PreParseLazyFunction(
       language_mode(), function_state_->kind(), logger, bookmark);
   if (pre_parse_timer_ != NULL) {
@@ skipping 1505 matching lines @@
 Expression* Parser::SpreadCallNew(Expression* function,
                                   ZoneList<v8::internal::Expression*>* args,
                                   int pos) {
   args->InsertAt(0, function, zone());
 
   return factory()->NewCallRuntime(
       ast_value_factory()->reflect_construct_string(), NULL, args, pos);
 }
 }  // namespace internal
 }  // namespace v8