gh-128078: Simplify logic in _PyGen_SetStopIterationValue (follow-up to gh-128780)

Lib/test/test_asyncgen.py

@@ -1152,6 +1152,24 @@ async def run():
 
         self.loop.run_until_complete(run())
 
+    def test_async_gen_asyncio_anext_tuple_no_exceptions(self):
+        # StopAsyncIteration exceptions should be cleared.
+        # See: https://github.com/python/cpython/issues/128078.
+
+        async def foo():
+            if False:
+                yield (1, 2)
+
+        async def run():
+            it = foo().__aiter__()
+            with self.assertRaises(StopAsyncIteration):
+                await it.__anext__()
+            a, b = await anext(it, ('a', 'b'))
+            self.assertEqual(a, 'a')
+            self.assertEqual(b, 'b')
+
+        self.loop.run_until_complete(run())
+
     def test_async_gen_asyncio_anext_stopiteration(self):
         async def foo():
             try:

Misc/NEWS.d/next/Core_and_Builtins/2024-12-27-13-07-21.gh-issue-128078.xV1jpJ.rst

@@ -0,0 +1,2 @@
+Fix a :exc:`SystemError` when using :func:`anext` with a default tuple value.
+Patch by Bénédikt Tran.

Objects/genobject.c

@@ -642,6 +642,22 @@ _PyGen_SetStopIterationValue(PyObject *value)
         PyErr_SetObject(PyExc_StopIteration, value);
         return 0;
     }
+
+    // Since _PyGen_SetStopIterationValue() is meant to create
+    // a StopItertation or substitute one for a StopAsyncItertation
+    // an exception of another type should not already be set.
+    PyObject *run_ex = PyErr_Occurred();
+    if (run_ex) {
+        if (!PyErr_GivenExceptionMatches(run_ex, PyExc_StopAsyncIteration)) {
+            // Replace existing bad exception with a SystemError instead.
+            PyErr_Format(PyExc_SystemError,
+                         "%s:%d: unexpected caller exception: %R",
+                         __FILE__, __LINE__, run_ex);
+            return -1;
+        }
+        PyErr_Clear();
+    }
+
     /* Construct an exception instance manually with
      * PyObject_CallOneArg and pass it to PyErr_SetObject.
      *