1 files changed, 26 insertions, 0 deletions
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 1980fcb..0801d9f 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -3676,6 +3676,32 @@ PyType_FromMetaclass(PyTypeObject *metaclass, PyObject *module,
         goto finally;
     }
 
+    /* If this is an immutable type, check if all bases are also immutable,
+     * and (for now) fire a deprecation warning if not.
+     * (This isn't necessary for static types: those can't have heap bases,
+     * and only heap types can be mutable.)
+     */
+    if (spec->flags & Py_TPFLAGS_IMMUTABLETYPE) {
+        for (int i=0; i<PyTuple_GET_SIZE(bases); i++) {
+            PyTypeObject *b = (PyTypeObject*)PyTuple_GET_ITEM(bases, i);
+            if (!b) {
+                goto finally;
+            }
+            if (!_PyType_HasFeature(b, Py_TPFLAGS_IMMUTABLETYPE)) {
+                if (PyErr_WarnFormat(
+                    PyExc_DeprecationWarning,
+                    0,
+                    "Creating immutable type %s from mutable base %s is "
+                    "deprecated, and slated to be disallowed in Python 3.14.",
+                    spec->name,
+                    b->tp_name))
+                {
+                    goto finally;
+                }
+            }
+        }
+    }
+
     /* Calculate the metaclass */
 
     if (!metaclass) {