gh-159294: Fix crash when creating an ExceptionGroup fails with MemoryError

BaseExceptionGroup_new() passed the result of BaseException_new() to
PyBaseExceptionGroupObject_CAST() before checking it for NULL. The cast
asserts the type of its argument, so a debug build dereferenced NULL
instead of raising MemoryError.

Author: christianaurichzm

Lib/test/test_exception_group.py

@@ -1,6 +1,7 @@
 import collections
 import types
 import unittest
+from test import support
 from test.support import (skip_emscripten_stack_overflow,
                           skip_wasi_stack_overflow, run_with_limited_c_stack,
                           exceeds_recursion_limit)
@@ -130,6 +131,15 @@ class MyBEG(BaseExceptionGroup):
             type(MyBEG("eg", [ValueError(12), KeyboardInterrupt(42)])),
             MyBEG)
 
+    @support.nomemtest
+    def test_no_memory(self):
+        import _testcapi
+        # A tuple of exceptions is not copied, so the first allocation
+        # is the one of the exception group itself.
+        excs = (ValueError(1),)
+        with self.assertRaises(MemoryError):
+            _testcapi.call_with_nomemory(0, 0, ExceptionGroup, "eg", excs)
+
 
 class StrAndReprTests(unittest.TestCase):
     def test_ExceptionGroup(self):

Misc/NEWS.d/next/Core_and_Builtins/2026-10-13-06-23-56.gh-issue-159294.9LTsNA.rst

@@ -0,0 +1,3 @@
+Fix a crash in a debug build when creating an :exc:`ExceptionGroup` or
+:exc:`BaseExceptionGroup` fails with :exc:`MemoryError`. Patch by Christian
+Aurich Zanettini Martins.

Objects/exceptions.c

@@ -974,11 +974,11 @@ BaseExceptionGroup_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
          */
         cls = (PyTypeObject*)PyExc_BaseExceptionGroup;
     }
-    PyBaseExceptionGroupObject *self =
-        PyBaseExceptionGroupObject_CAST(BaseException_new(cls, args, kwds));
-    if (!self) {
+    PyObject *op = BaseException_new(cls, args, kwds);
+    if (!op) {
         goto error;
     }
+    PyBaseExceptionGroupObject *self = PyBaseExceptionGroupObject_CAST(op);
 
     self->msg = Py_NewRef(message);
     self->excs = exceptions;