Pass NULL instead of None for descriptors

To specify the lack of the second argument.  This improves compatibility
when applying descriptors to the None object itself.

This was showing up since we were trying to call None.__format__

src/runtime/objmodel.cpp

@@ -1764,63 +1764,34 @@ Box* getattrInternalGeneric(Box* obj, BoxedString* attr, GetattrRewriteArgs* rew
                 }
 
                 // Lookup __get__
-                RewriterVar* r_get = NULL;
-                Box* local_get;
+                descrgetfunc local_get = val->cls->tp_descr_get;
                 if (rewrite_args) {
-                    RewriterVar* r_val_cls = r_val->getAttr(offsetof(Box, cls), Location::any());
-                    GetattrRewriteArgs grewrite_args(rewrite_args->rewriter, r_val_cls, Location::any());
-                    local_get = typeLookup(val->cls, get_str, &grewrite_args);
-                    if (!grewrite_args.out_success) {
-                        rewrite_args = NULL;
-                    } else if (local_get) {
-                        r_get = grewrite_args.out_rtn;
-                    }
-                } else {
-                    local_get = typeLookup(val->cls, get_str, NULL);
+                    RewriterVar* r_cls = r_val->getAttr(offsetof(Box, cls));
+                    r_cls->addAttrGuard(offsetof(BoxedClass, tp_descr_get), (intptr_t)local_get);
                 }
 
-                // Call __get__(val, None, obj)
-                if (local_get) {
-                    Box* res;
-
-                    if (for_call) {
-#if STAT_CALLATTR_DESCR_ABORTS
-                        if (rewrite_args) {
-                            std::string attr_name = "num_callattr_descr_abort";
-                            Stats::log(Stats::getStatCounter(attr_name));
-                            logByCurrentPythonLine(attr_name);
-                        }
-#endif
-
-                        rewrite_args = NULL;
-                        REWRITE_ABORTED("");
-                    }
-
+                if (!local_get) {
                     if (rewrite_args) {
-                        CallRewriteArgs crewrite_args(rewrite_args->rewriter, r_get, rewrite_args->destination);
-                        crewrite_args.arg1 = r_val;
-                        crewrite_args.arg2 = rewrite_args->rewriter->loadConst((intptr_t)None, Location::any());
-                        crewrite_args.arg3 = rewrite_args->obj;
-                        res = runtimeCallInternal<CXX>(local_get, &crewrite_args, ArgPassSpec(3), val, None, obj, NULL,
-                                                       NULL);
-                        if (!crewrite_args.out_success) {
-                            rewrite_args = NULL;
-                        } else {
-                            rewrite_args->out_success = true;
-                            rewrite_args->out_rtn = crewrite_args.out_rtn;
-                        }
-                    } else {
-                        res = runtimeCallInternal<CXX>(local_get, NULL, ArgPassSpec(3), val, None, obj, NULL, NULL);
+                        rewrite_args->out_rtn = r_val;
+                        rewrite_args->out_success = true;
                     }
-                    return res;
+                    return val;
                 }
 
-                // If there was no local __get__, just return val
+                // Call __get__(val, None, obj)
+                Box* r = local_get(val, NULL, obj);
+                if (!r)
+                    throwCAPIException();
+
                 if (rewrite_args) {
-                    rewrite_args->out_rtn = r_val;
+                    rewrite_args->out_rtn = rewrite_args->rewriter->call(
+                        true, (void*)local_get, r_val, rewrite_args->rewriter->loadConst(0, Location::forArg(1)),
+                        rewrite_args->obj);
+                    rewrite_args->rewriter->checkAndThrowCAPIException(rewrite_args->out_rtn);
                     rewrite_args->out_success = true;
                 }
-                return val;
+
+                return r;
             }
         }
     }