gh-112217: Add check to call result for do_raise() where cause is a type.

Lib/test/test_raise.py

@@ -185,6 +185,20 @@ def test_class_cause(self):
         else:
             self.fail("No exception raised")
 
+    def test_class_cause_nonexception_result(self):
+        class ConstructsNone(BaseException):
+            @classmethod
+            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")
+
     def test_instance_cause(self):
         cause = KeyError()
         try:

Misc/NEWS.d/next/Core and Builtins/2023-11-27-18-55-30.gh-issue-112217.SwFLMj.rst

@@ -0,0 +1 @@
+Add check for the type of ``__cause__`` returned from calling the type ``T`` in ``raise from T``.

Python/ceval.c

@@ -1920,6 +1920,13 @@ do_raise(PyThreadState *tstate, PyObject *exc, PyObject *cause)
             fixed_cause = _PyObject_CallNoArgs(cause);
             if (fixed_cause == NULL)
                 goto raise_error;
+            if (!PyExceptionInstance_Check(fixed_cause)) {
+                _PyErr_Format(tstate, PyExc_TypeError,
+                              "calling %R should have returned an instance of "
+                              "BaseException, not %R",
+                              cause, Py_TYPE(fixed_cause));
+                goto raise_error;
+            }
             Py_DECREF(cause);
         }
         else if (PyExceptionInstance_Check(cause)) {