# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1235846415 -3600
# Node ID 55c8334f8ef15e24049a3a03a6aa8490fbb8bab5
# Parent  ae20cd4d0679a04b593dd1a0126921cd421c5144
Attempt to enforce proper checks on attribute access with context overriding.
Attempt to make _CheckInstance work correctly.

diff -r ae20cd4d0679 -r 55c8334f8ef1 docs/assignment.txt
--- a/docs/assignment.txt	Sat Feb 28 01:49:33 2009 +0100
+++ b/docs/assignment.txt	Sat Feb 28 19:40:15 2009 +0100
@@ -61,7 +61,8 @@
   null context                      preserved
   other context (instance)          preserved
 
-  LoadAttrIndex must therefore check whether the context must be overridden
+  LoadAddressContext and LoadAttrIndex must therefore check whether the
+  context must be overridden
 
   Since the object table encodes sufficient information (an instance must be
   compatible to access the class attribute, and compatibility information is
diff -r ae20cd4d0679 -r 55c8334f8ef1 rsvp.py
--- a/rsvp.py	Sat Feb 28 01:49:33 2009 +0100
+++ b/rsvp.py	Sat Feb 28 19:40:15 2009 +0100
@@ -320,9 +320,9 @@
         self.value = self.load(self.operand)
 
     def LoadAddressContext(self):
-        value = self.load(self.operand)
-        # Replace the context with the current value.
-        self.value = self.value[1], value[1]
+        context, ref = self.load(self.operand)
+        inst_context, inst_ref = self.value
+        self.value = self._LoadAddressContext(context, ref, inst_context, inst_ref)
 
     def StoreAddress(self):
         # Preserve context.
@@ -356,10 +356,7 @@
         if attr_index == self.operand:
             if class_attr:
                 loaded_context, loaded_ref = self.load(offset) # offset is address of class attribute
-                if replace_context:
-                    self.value = ref, loaded_ref # classes can also replace the context if compatible
-                    return
-                self.value = loaded_context, loaded_ref
+                self.value = self._LoadAddressContext(loaded_context, loaded_ref, context, ref)
             else:
                 self.value = self.load(ref + offset)
         else:
@@ -549,6 +546,11 @@
         data = self.load(ref)
         target_data = self.load(cls)
 
+        # Insist on a class.
+
+        if target_data.instance:
+            return 0
+
         # Find the table entry for the descendant.
 
         element = self.objlist[target_data.classcode + data.attrcode]
@@ -562,6 +564,16 @@
         self.save(addr, data)
         return addr
 
+    def _LoadAddressContext(self, context, ref, inst_context, inst_ref):
+
+        # Check the instance context against the target's context.
+
+        if self._CheckInstance(inst_ref, context):
+            # Replace the context with the instance.
+            return inst_ref, ref
+        else:
+            return context, ref
+
     # Native function implementations.
 
     def builtins_object_init(self):