objtype: Don't treat inheritance from "object" as from native type.

"object" type in MicroPython currently doesn't implement any methods, and
hopefully, we'll try to stay like that for as long as possible. Even if we
have to add something eventually, look up from there might be handled in
adhoc manner, as last resort (that's not compliant with Python3 MRO, but
we're already non-compliant). Hence: 1) no need to spend type trying to
lookup anything in object; 2) no need to allocate subobject when explicitly
inheriting from object; 3) and having multiple bases inheriting from object
is not a case of incompatible multiple inheritance.

py/objtype.c

@@ -67,7 +67,12 @@ STATIC int instance_count_native_bases(const mp_obj_type_t *type, const mp_obj_t
     int count = 0;
     for (uint i = 0; i < len; i++) {
         assert(MP_OBJ_IS_TYPE(items[i], &mp_type_type));
-        if (is_native_type((const mp_obj_type_t *)items[i])) {
+        const mp_obj_type_t *bt = (const mp_obj_type_t *)items[i];
+        if (bt == &mp_type_object) {
+            // Not a "real" type
+            continue;
+        }
+        if (is_native_type(bt)) {
             *last_native_base = items[i];
             count++;
         } else {
@@ -144,7 +149,12 @@ STATIC void mp_obj_class_lookup(mp_obj_instance_t *o, const mp_obj_type_t *type,
         }
         for (uint i = 0; i < len - 1; i++) {
             assert(MP_OBJ_IS_TYPE(items[i], &mp_type_type));
-            mp_obj_class_lookup(o, (mp_obj_type_t*)items[i], attr, meth_offset, dest);
+            mp_obj_type_t *bt = (mp_obj_type_t*)items[i];
+            if (bt == &mp_type_object) {
+                // Not a "real" type
+                continue;
+            }
+            mp_obj_class_lookup(o, bt, attr, meth_offset, dest);
             if (dest[0] != MP_OBJ_NULL) {
                 return;
             }
@@ -153,6 +163,10 @@ STATIC void mp_obj_class_lookup(mp_obj_instance_t *o, const mp_obj_type_t *type,
         // search last base (simple tail recursion elimination)
         assert(MP_OBJ_IS_TYPE(items[len - 1], &mp_type_type));
         type = (mp_obj_type_t*)items[len - 1];
+        if (type == &mp_type_object) {
+            // Not a "real" type
+            return;
+        }
     }
 }
 

tests/basics/subclass-native5.py

@@ -0,0 +1,12 @@
+# Subclass from 2 bases explicitly subclasses from object
+
+class Base1(object):
+    pass
+
+class Base2(object):
+    pass
+
+class Sub(Base1, Base2):
+    pass
+
+o = Sub()