Inherit Py_TPFLAGS_CHECKTYPES.

src/capi/typeobject.cpp

@@ -3235,9 +3235,6 @@ extern "C" int PyType_Ready(PyTypeObject* cls) noexcept {
                        | Py_TPFLAGS_TUPLE_SUBCLASS | Py_TPFLAGS_STRING_SUBCLASS | Py_TPFLAGS_UNICODE_SUBCLASS
                        | Py_TPFLAGS_DICT_SUBCLASS | Py_TPFLAGS_BASE_EXC_SUBCLASS | Py_TPFLAGS_TYPE_SUBCLASS;
     RELEASE_ASSERT((cls->tp_flags & ~ALLOWABLE_FLAGS) == 0, "");
-    if (cls->tp_as_number) {
-        // RELEASE_ASSERT(cls->tp_flags & Py_TPFLAGS_CHECKTYPES, "Pyston doesn't yet support non-checktypes behavior");
-    }
 
     RELEASE_ASSERT(cls->tp_free == NULL || cls->tp_free == PyObject_Del || cls->tp_free == PyObject_GC_Del, "");
     RELEASE_ASSERT(cls->tp_is_gc == NULL, "");

src/runtime/objmodel.cpp

@@ -429,6 +429,13 @@ BoxedClass::BoxedClass(BoxedClass* base, gcvisit_func gc_visit, int attrs_offset
     if (base && (base->tp_flags & Py_TPFLAGS_HAVE_NEWBUFFER))
         tp_flags |= Py_TPFLAGS_HAVE_NEWBUFFER;
 
+    // From CPython: It's a new-style number unless it specifically inherits any
+    // old-style numeric behavior.
+    if (base) {
+        if ((base->tp_flags & Py_TPFLAGS_CHECKTYPES) || (base->tp_as_number == NULL))
+            tp_flags |= Py_TPFLAGS_CHECKTYPES;
+    }
+
     tp_base = base;
 
     if (tp_base) {