gh-140530: fix a reference leak in an error path for raise exc from cause (#140908)

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.

Lib/test/test_raise.py

@@ -186,18 +186,14 @@ def test_class_cause(self):
             self.fail("No exception raised")
 
     def test_class_cause_nonexception_result(self):
-        class ConstructsNone(BaseException):
-            @classmethod
+        # See https://github.com/python/cpython/issues/140530.
+        class ConstructMortal(BaseException):
             def __new__(*args, **kwargs):
-                return None
-        try:
-            raise IndexError from ConstructsNone
-        except TypeError as e:
-            self.assertIn("should have returned an instance of BaseException", str(e))
-        except IndexError:
-            self.fail("Wrong kind of exception raised")
-        else:
-            self.fail("No exception raised")
+                return ["mortal value"]
+
+        msg = ".*should have returned an instance of BaseException.*"
+        with self.assertRaisesRegex(TypeError, msg):
+            raise IndexError from ConstructMortal
 
     def test_instance_cause(self):
         cause = KeyError()

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);