[es6] implement default parameters via desugaring

src/scopes.cc

Index: src/scopes.cc
diff --git a/src/scopes.cc b/src/scopes.cc
index a5af8c015783152c94ae3b24a9cf6c137bb93dd6..34cda67598ad91c86d8181de9dae7880406e02ec 100644
--- a/src/scopes.cc
+++ b/src/scopes.cc
@@ -77,6 +77,7 @@ Scope::Scope(Zone* zone, Scope* outer_scope, ScopeType scope_type,
       internals_(4, zone),
       temps_(4, zone),
       params_(4, zone),
+      param_positions_(4, zone),
       unresolved_(16, zone),
       decls_(4, zone),
       module_descriptor_(
@@ -100,6 +101,7 @@ Scope::Scope(Zone* zone, Scope* inner_scope, ScopeType scope_type,
       internals_(4, zone),
       temps_(4, zone),
       params_(4, zone),
+      param_positions_(4, zone),
       unresolved_(16, zone),
       decls_(4, zone),
       module_descriptor_(NULL),
@@ -126,6 +128,7 @@ Scope::Scope(Zone* zone, Scope* inner_scope,
       internals_(0, zone),
       temps_(0, zone),
       params_(0, zone),
+      param_positions_(0, zone),
       unresolved_(0, zone),
       decls_(0, zone),
       module_descriptor_(NULL),
@@ -449,7 +452,7 @@ Variable* Scope::Lookup(const AstRawString* name) {
 
 
 Variable* Scope::DeclareParameter(const AstRawString* name, VariableMode mode,
-                                  bool is_rest, bool* is_duplicate) {
+                                  bool is_rest, bool* is_duplicate, int pos) {
   DCHECK(!already_resolved());
   DCHECK(is_function_scope());
   Variable* var = variables_.Declare(this, name, mode, Variable::NORMAL,
@@ -462,6 +465,7 @@ Variable* Scope::DeclareParameter(const AstRawString* name, VariableMode mode,
   // TODO(wingo): Avoid O(n^2) check.
   *is_duplicate = IsDeclaredParameter(name);
   params_.Add(var, zone());
+  param_positions_.Add(pos, zone());

[wingo, 2015/05/11 15:02:51]

Don't we require already that parameters are declared left-to-right?  In that
case the "pos" argument is unneeded.  Just a thought.

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

It's needed because they end up as lexical declarations, which require a
position. The hole-checking logic uses it too, so it needs to be accurate

   return var;
 }
 
@@ -491,6 +495,18 @@ Variable* Scope::DeclareDynamicGlobal(const AstRawString* name) {
 }
 
 
+void Scope::ShadowParametersForExpressions() {
+  // Parameters have expressions --- ensure that the declared parameters can
+  // never be resolved
+  DCHECK(is_function_scope());
+  for (int i = 0; i < num_parameters(); ++i) {
+    Variable* p = parameter(i);
+    const AstRawString* name = p->raw_name();
+    variables_.Remove(const_cast<AstRawString*>(name), name->hash());
+  }
+}
+
+
 void Scope::RemoveUnresolved(VariableProxy* var) {
   // Most likely (always?) any variable we want to remove
   // was just added before, so we search backwards.