Change GN to disallow reading args defined in the same declare_args() call.

tools/gn/scope.h

Index: tools/gn/scope.h
diff --git a/tools/gn/scope.h b/tools/gn/scope.h
index b87afcd6f9c756178c0ef0c4c76057d857faa868..c10ead48e2af18770eb0c22b6f669b6df4a92efe 100644
--- a/tools/gn/scope.h
+++ b/tools/gn/scope.h
@@ -112,7 +112,7 @@ class Scope {
 
   const Settings* settings() const { return settings_; }
 
-  // See the const_/mutable_containing_ var declaraions below. Yes, it's a
+  // See the const_/mutable_containing_ var declarations below. Yes, it's a
   // bit weird that we can have a const pointer to the "mutable" one.
   Scope* mutable_containing() { return mutable_containing_; }
   const Scope* mutable_containing() const { return mutable_containing_; }
@@ -140,6 +140,11 @@ class Scope {
   const Value* GetValue(const base::StringPiece& ident,
                         bool counts_as_used);
   const Value* GetValue(const base::StringPiece& ident) const;
+  const Value* GetValueWithScope(const base::StringPiece& ident,
+                                 const Scope** found_in_scope) const;
+  const Value* GetValueWithScope(const base::StringPiece& ident,
+                                 bool counts_as_used,
+                                 const Scope** found_in_scope);
 
   // Returns the requested value as a mutable one if possible. If the value
   // is not found in a mutable scope, then returns null. Note that the value
@@ -375,4 +380,9 @@ class Scope {
   DISALLOW_COPY_AND_ASSIGN(Scope);
 };
 
+// This key is set as a scope property on the scope of a declare_args() block,
+// in order to prevent reading a variable defined earlier in the same call
+// (see `gn help declare_args` for more).
+extern const int kInDeclareArgsKey;
+
 #endif  // TOOLS_GN_SCOPE_H_