gh-134786: Py_TPFLAGS_MANAGED_WEAKREF and Py_TPFLAGS_MANAGED_DICT must be used only if Py_TPFLAGS_HAVE_GC set

Doc/c-api/typeobj.rst

@@ -1260,7 +1260,7 @@ and :c:data:`PyType_Type` effectively act as defaults.)
       This bit indicates that instances of the class have a :attr:`~object.__dict__`
       attribute, and that the space for the dictionary is managed by the VM.
 
-      If this flag is set, :c:macro:`Py_TPFLAGS_HAVE_GC` should also be set.
+      If this flag is set, :c:macro:`Py_TPFLAGS_HAVE_GC` must also be set.
 
       The type traverse function must call :c:func:`PyObject_VisitManagedDict`
       and its clear function must call :c:func:`PyObject_ClearManagedDict`.
@@ -1278,6 +1278,8 @@ and :c:data:`PyType_Type` effectively act as defaults.)
       This bit indicates that instances of the class should be weakly
       referenceable.
 
+      If this flag is set, :c:macro:`Py_TPFLAGS_HAVE_GC` must also be set.
+
       .. versionadded:: 3.12
 
       **Inheritance:**

Doc/extending/newtypes.rst

@@ -560,6 +560,8 @@ For an object to be weakly referenceable, the extension type must set the
 field. The legacy :c:member:`~PyTypeObject.tp_weaklistoffset` field should
 be left as zero.
 
+If this flag is set, :c:macro:`Py_TPFLAGS_HAVE_GC` should also be set.
+
 Concretely, here is how the statically declared type object would look::
 
    static PyTypeObject TrivialType = {

Doc/whatsnew/3.15.rst

@@ -785,6 +785,14 @@ New features
   (Contributed by Victor Stinner in :gh:`129813`.)
 
 
+Limited C API changes
+---------------------
+
+* If the :c:macro:`Py_TPFLAGS_MANAGED_DICT` and :c:macro:`Py_TPFLAGS_MANAGED_WEAKREF`
+  flags are set then :c:macro:`Py_TPFLAGS_HAVE_GC` must be set too.
+  (Contributed by Sergey Miryanov in :gh:`134786`)
+
+
 Porting to Python 3.15
 ----------------------
 

Include/object.h

@@ -529,7 +529,7 @@ given type object has a specified feature.
 #define Py_TPFLAGS_INLINE_VALUES (1 << 2)
 
 /* Placement of weakref pointers are managed by the VM, not by the type.
- * The VM will automatically set tp_weaklistoffset.
+ * The VM will automatically set tp_weaklistoffset. Implies Py_TPFLAGS_HAVE_GC.
  */
 #define Py_TPFLAGS_MANAGED_WEAKREF (1 << 3)
 

Misc/NEWS.d/next/Core_and_Builtins/2025-06-24-13-12-58.gh-issue-134786.MF0VVk.rst

@@ -0,0 +1,2 @@
+If :c:macro:`Py_TPFLAGS_MANAGED_DICT` and :c:macro:`Py_TPFLAGS_MANAGED_WEAKREF`
+are used, then :c:macro:`Py_TPFLAGS_HAVE_GC` must be used as well.

Objects/typeobject.c

@@ -8893,6 +8893,13 @@ type_ready_preheader(PyTypeObject *type)
                         type->tp_name);
             return -1;
         }
+        if (!(type->tp_flags & Py_TPFLAGS_HAVE_GC)) {
+            PyErr_Format(PyExc_SystemError,
+                        "type %s has the Py_TPFLAGS_MANAGED_DICT flag "
+                        "but not Py_TPFLAGS_HAVE_GC flag",
+                        type->tp_name);
+            return -1;
+        }
         type->tp_dictoffset = -1;
     }
     if (type->tp_flags & Py_TPFLAGS_MANAGED_WEAKREF) {
@@ -8905,6 +8912,13 @@ type_ready_preheader(PyTypeObject *type)
                         type->tp_name);
             return -1;
         }
+        if (!(type->tp_flags & Py_TPFLAGS_HAVE_GC)) {
+            PyErr_Format(PyExc_SystemError,
+                        "type %s has the Py_TPFLAGS_MANAGED_WEAKREF flag "
+                        "but not Py_TPFLAGS_HAVE_GC flag",
+                        type->tp_name);
+            return -1;
+        }
         type->tp_weaklistoffset = MANAGED_WEAKREF_OFFSET;
     }
     return 0;