fix a reference leak in raise E from T when T is an exception

subtype for which `T.__new__` does not return an exception instance.

Author: picnixz

Lib/test/test_raise.py

@@ -199,6 +199,14 @@ def __new__(*args, **kwargs):
         else:
             self.fail("No exception raised")
 
+    def test_class_cause_nonexception_result_noleak(self):
+        class ConstructMortal(BaseException):
+            def __new__(*args, **kwargs):
+                return ["mortal value"]
+
+        with self.assertRaises(TypeError):
+            raise IndexError from ConstructMortal
+
     def test_instance_cause(self):
         cause = KeyError()
         try:

Misc/NEWS.d/next/Core_and_Builtins/2025-11-02-12-47-38.gh-issue-140530.S934bp.rst

@@ -0,0 +1,2 @@
+Fix a reference leak when ``raise exc from cause`` fails. Patch by Bénédikt
+Tran.

Python/ceval.c

@@ -2148,6 +2148,7 @@ do_raise(PyThreadState *tstate, PyObject *exc, PyObject *cause)
                               "calling %R should have returned an instance of "
                               "BaseException, not %R",
                               cause, Py_TYPE(fixed_cause));
+                Py_DECREF(fixed_cause);
                 goto raise_error;
             }
             Py_DECREF(cause);