gh-131670: Fix crash in anext() when __anext__ is sync and raises (#131682)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: sobolevn

Lib/test/test_asyncgen.py

@@ -1169,6 +1169,26 @@ async def run():
 
         self.loop.run_until_complete(run())
 
+    def test_sync_anext_raises_exception(self):
+        # See: https://github.com/python/cpython/issues/131670
+        msg = 'custom'
+        for exc_type in [
+            StopAsyncIteration,
+            StopIteration,
+            ValueError,
+            Exception,
+        ]:
+            exc = exc_type(msg)
+            with self.subTest(exc=exc):
+                class A:
+                    def __anext__(self):
+                        raise exc
+
+                with self.assertRaisesRegex(exc_type, msg):
+                    anext(A())
+                with self.assertRaisesRegex(exc_type, msg):
+                    anext(A(), 1)
+
     def test_async_gen_asyncio_anext_stopiteration(self):
         async def foo():
             try:

Misc/NEWS.d/next/Core_and_Builtins/2025-03-24-19-38-53.gh-issue-131670.IffOZj.rst

@@ -0,0 +1 @@
+Fix :func:`anext` failing on sync :meth:`~object.__anext__` raising an exception.

Python/bltinmodule.c

@@ -1837,6 +1837,9 @@ builtin_anext_impl(PyObject *module, PyObject *aiterator,
     }
 
     awaitable = (*t->tp_as_async->am_anext)(aiterator);
+    if (awaitable == NULL) {
+        return NULL;
+    }
     if (default_value == NULL) {
         return awaitable;
     }