add instanceTrunc to old style class, but not expose the API, just let...

add instanceTrunc to old style class, but not expose the API, just let instanceInt call it when the inst don't have __int__ attr

src/runtime/classobj.cpp

@@ -1292,13 +1292,33 @@ Box* instanceInvert(Box* _inst) {
     return runtimeCall(invert_func, ArgPassSpec(0), NULL, NULL, NULL, NULL, NULL);
 }
 
+Box* instanceTrunc(BoxedInstance* _inst) {
+    RELEASE_ASSERT(_inst->cls == instance_cls, "");
+    BoxedInstance* inst = static_cast<BoxedInstance*>(_inst);
+
+    static BoxedString* trunc_str = internStringImmortal("__trunc__");
+    Box* trunc_func = _instanceGetattribute(inst, trunc_str, true);
+
+    return runtimeCall(trunc_func, ArgPassSpec(0), NULL, NULL, NULL, NULL, NULL);
+}
+
 Box* instanceInt(Box* _inst) {
     RELEASE_ASSERT(_inst->cls == instance_cls, "");
     BoxedInstance* inst = static_cast<BoxedInstance*>(_inst);
 
     static BoxedString* int_str = internStringImmortal("__int__");
-    Box* int_func = _instanceGetattribute(inst, int_str, true);
-    return runtimeCall(int_func, ArgPassSpec(0), NULL, NULL, NULL, NULL, NULL);
+    if (PyObject_HasAttr((PyObject*)inst, int_str)) {
+        Box* int_func = _instanceGetattribute(inst, int_str, true);
+        return runtimeCall(int_func, ArgPassSpec(0), NULL, NULL, NULL, NULL, NULL);
+    }
+
+    Box* truncated = instanceTrunc(inst);
+    /* __trunc__ is specified to return an Integral type, but
+       int() needs to return an int. */
+    Box* res = _PyNumber_ConvertIntegralToInt(truncated, "__trunc__ returned non-Integral (type %.200s)");
+    if (!res)
+        throwCAPIException();
+    return res;
 }
 
 Box* instanceLong(Box* _inst) {