Add type.__instancecheck__ and __subclasscheck__

We supported classes that overrode them, but didn't provide a
default implementation in case people called it by hand.

The tricky part is making sure that these new additions don't
mess with our "does this class override __instancecheck__" optimizations.

src/capi/abstract.cpp

@@ -185,6 +185,10 @@ extern "C" int PyObject_IsInstance(PyObject* inst, PyObject* cls) noexcept {
     return recursive_isinstance(inst, cls);
 }
 
+extern "C" int _PyObject_RealIsInstance(PyObject* inst, PyObject* cls) noexcept {
+    return recursive_isinstance(inst, cls);
+}
+
 
 extern "C" int PyObject_IsSubclass(PyObject* derived, PyObject* cls) noexcept {
     static PyObject* name = NULL;

src/capi/typeobject.cpp

@@ -1924,6 +1924,18 @@ static const slotdef* update_one_slot(BoxedClass* type, const slotdef* p) noexce
                     descr = NULL;
             }
 
+            static BoxedString* instancecheck_str = getStaticString("__instancecheck__");
+            if (p->name_strobj == instancecheck_str) {
+                if (descr == type_cls->getattr(instancecheck_str))
+                    descr = NULL;
+            }
+
+            static BoxedString* subclasscheck_str = getStaticString("__subclasscheck__");
+            if (p->name_strobj == subclasscheck_str) {
+                if (descr == type_cls->getattr(subclasscheck_str))
+                    descr = NULL;
+            }
+
             static BoxedString* getattribute_str = getStaticString("__getattribute__");
             if (p->name_strobj == getattribute_str) {
                 if (type->tp_getattr == NULL && descr && descr->cls == &PyWrapperDescr_Type

src/runtime/types.cpp

@@ -1400,6 +1400,28 @@ static int type_sub_set_dict(BORROWED(Box*) obj, BORROWED(Box*) val, void* conte
     return 0;
 }
 
+static PyObject* type___instancecheck__(PyObject* type, PyObject* inst) noexcept {
+    switch (_PyObject_RealIsInstance(inst, type)) {
+        case -1:
+            return NULL;
+        case 0:
+            Py_RETURN_FALSE;
+        default:
+            Py_RETURN_TRUE;
+    }
+}
+
+static PyObject* type___subclasscheck__(PyObject* type, PyObject* inst) noexcept {
+    switch (_PyObject_RealIsSubclass(inst, type)) {
+        case -1:
+            return NULL;
+        case 0:
+            Py_RETURN_FALSE;
+        default:
+            Py_RETURN_TRUE;
+    }
+}
+
 extern "C" void PyType_SetDict(PyTypeObject* type, STOLEN(PyObject*) dict) noexcept {
     type_sub_set_dict(type, dict, NULL);
     Box* old_dict = type->tp_dict;
@@ -4399,6 +4421,12 @@ void setupRuntime() {
     type_cls->giveAttrDescriptor("__abstractmethods__", (getter)type_abstractmethods, (setter)type_set_abstractmethods);
 
     type_cls->giveAttr("__call__", new BoxedFunction(typeCallObj));
+    type_cls->giveAttr(
+        "__subclasscheck__",
+        new BoxedFunction(BoxedCode::create((void*)type___subclasscheck__, BOXED_BOOL, 2, "type.__subclasscheck__")));
+    type_cls->giveAttr(
+        "__instancecheck__",
+        new BoxedFunction(BoxedCode::create((void*)type___instancecheck__, BOXED_BOOL, 2, "type.__instancecheck__")));
 
     type_cls->giveAttr(
         "__new__", new BoxedFunction(BoxedCode::create((void*)typeNewGeneric, UNKNOWN, 4, false, false, "type.__new__"),

test/tests/subclasscheck_test.py

+assert hasattr(object, "__subclasscheck__")
+assert hasattr(object, "__instancecheck__")
+
 class M(type):
     def __instancecheck__(self, rhs):
         print "M.instancecheck",