py/objtype: Allow mp_instance_cast_to_native_base to take native obj.

And rename it to mp_obj_cast_to_native_base() to indicate this.  This
allows users of this function to easily support native and native-subclass
objects in the same way (by just passing the object through this function).

py/obj.h

@@ -728,7 +728,7 @@ mp_obj_t mp_obj_new_memoryview(byte typecode, size_t nitems, void *items);
 const mp_obj_type_t *mp_obj_get_type(mp_const_obj_t o_in);
 const char *mp_obj_get_type_str(mp_const_obj_t o_in);
 bool mp_obj_is_subclass_fast(mp_const_obj_t object, mp_const_obj_t classinfo); // arguments should be type objects
-mp_obj_t mp_instance_cast_to_native_base(mp_const_obj_t self_in, mp_const_obj_t native_type);
+mp_obj_t mp_obj_cast_to_native_base(mp_obj_t self_in, mp_const_obj_t native_type);
 
 void mp_obj_print_helper(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t kind);
 void mp_obj_print(mp_obj_t o, mp_print_kind_t kind);

py/objtuple.c

@@ -109,7 +109,7 @@ STATIC mp_obj_t tuple_cmp_helper(mp_uint_t op, mp_obj_t self_in, mp_obj_t anothe
     mp_obj_tuple_t *self = MP_OBJ_TO_PTR(self_in);
     if (another_type->getiter != mp_obj_tuple_getiter) {
         // Slow path for user subclasses
-        another_in = mp_instance_cast_to_native_base(another_in, MP_OBJ_FROM_PTR(&mp_type_tuple));
+        another_in = mp_obj_cast_to_native_base(another_in, MP_OBJ_FROM_PTR(&mp_type_tuple));
         if (another_in == MP_OBJ_NULL) {
             return MP_OBJ_NULL;
         }

py/objtype.c

@@ -1389,13 +1389,17 @@ STATIC mp_obj_t mp_builtin_isinstance(mp_obj_t object, mp_obj_t classinfo) {
 
 MP_DEFINE_CONST_FUN_OBJ_2(mp_builtin_isinstance_obj, mp_builtin_isinstance);
 
-mp_obj_t mp_instance_cast_to_native_base(mp_const_obj_t self_in, mp_const_obj_t native_type) {
+mp_obj_t mp_obj_cast_to_native_base(mp_obj_t self_in, mp_const_obj_t native_type) {
     const mp_obj_type_t *self_type = mp_obj_get_type(self_in);
-    if (!mp_obj_is_subclass_fast(MP_OBJ_FROM_PTR(self_type), native_type)) {
+
+    if (MP_OBJ_FROM_PTR(self_type) == native_type) {
+        return self_in;
+    } else if (!mp_obj_is_subclass_fast(MP_OBJ_FROM_PTR(self_type), native_type)) {
         return MP_OBJ_NULL;
+    } else {
+        mp_obj_instance_t *self = (mp_obj_instance_t*)MP_OBJ_TO_PTR(self_in);
+        return self->subobj[0];
     }
-    mp_obj_instance_t *self = (mp_obj_instance_t*)MP_OBJ_TO_PTR(self_in);
-    return self->subobj[0];
 }
 
 /******************************************************************************/