Allow lexically declared "arguments" in function scope in sloppy mode.

src/ast/scopes.cc

Index: src/ast/scopes.cc
diff --git a/src/ast/scopes.cc b/src/ast/scopes.cc
index 73c243b6250c21e4adccb99288be13b8b803707e..f34e3e0ee85a99e6e172eade4ab32519bd55742b 100644
--- a/src/ast/scopes.cc
+++ b/src/ast/scopes.cc
@@ -436,15 +436,34 @@ void DeclarationScope::DeclareThis(AstValueFactory* ast_value_factory) {
   receiver_ = var;
 }
 
-void DeclarationScope::DeclareDefaultFunctionVariables(
-    AstValueFactory* ast_value_factory) {
+void DeclarationScope::DeclareArguments(AstValueFactory* ast_value_factory) {
   DCHECK(is_function_scope());
   DCHECK(!is_arrow_scope());
+
+  // Check if there's lexically declared variable named arguments to avoid
+  // redeclaration.
+  // See https://tc39.github.io/ecma262/#sec-functiondeclarationinstantiation,

[adamk, 2016/09/01 17:36:26]

The style we've mostly been going for here has been to reduce the entire part of
the URL before the "#" to "ES", e.g.,

"See ES#some-section-anchor, step 20"

[lpy, 2016/09/01 20:00:55]

Done.

+  // step 20.
+  Variable* arg_variable = LookupLocal(ast_value_factory->arguments_string());
+  if (arg_variable != nullptr && IsLexicalVariableMode(arg_variable->mode())) {
+    return;
+  }
+
   // Declare 'arguments' variable which exists in all non arrow functions.
   // Note that it might never be accessed, in which case it won't be
   // allocated during variable allocation.
-  arguments_ = Declare(zone(), this, ast_value_factory->arguments_string(), VAR,
-                       Variable::ARGUMENTS, kCreatedInitialized);
+  if (arg_variable == nullptr) {
+    arguments_ = Declare(zone(), this, ast_value_factory->arguments_string(),
+                         VAR, Variable::ARGUMENTS, kCreatedInitialized);
+  } else {
+    arguments_ = arg_variable;
+  }
+}
+
+void DeclarationScope::DeclareDefaultFunctionVariables(
+    AstValueFactory* ast_value_factory) {
+  DCHECK(is_function_scope());
+  DCHECK(!is_arrow_scope());
 
   new_target_ = Declare(zone(), this, ast_value_factory->new_target_string(),
                         CONST, Variable::NORMAL, kCreatedInitialized);
@@ -1501,9 +1520,6 @@ void DeclarationScope::AllocateParameterLocals() {
       // allocate the arguments object by nulling out arguments_.
       arguments_ = nullptr;
     }
-
-  } else {
-    DCHECK(is_arrow_scope());
   }
 
   // The same parameter may occur multiple times in the parameters_ list.