add refcounting annotations to sum()

src/runtime/builtin_modules/builtins.cpp

@@ -296,18 +296,19 @@ extern "C" Box* next(Box* iterator, Box* _default) noexcept {
 }
 
 extern "C" Box* sum(Box* container, Box* initial) {
-    assert(0 && "check refcounting");
     if (initial->cls == str_cls)
         raiseExcHelper(TypeError, "sum() can't sum strings [use ''.join(seq) instead]");
 
     static RuntimeICCache<BinopIC, 3> runtime_ic_cache;
     std::shared_ptr<BinopIC> pp = runtime_ic_cache.getIC(__builtin_return_address(0));
 
-    Box* cur = initial;
+    Py_INCREF(initial);
+    auto cur = autoDecref(initial);
     for (Box* e : container->pyElements()) {
+        AUTO_DECREF(e);
         cur = pp->call(cur, e, AST_TYPE::Add);
     }
-    return cur;
+    return incref(cur.get());
 }
 
 extern "C" Box* id(Box* arg) {