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

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

Lib/test/test_asyncgen.py

@@ -1169,6 +1169,26 @@ class AsyncGenAsyncioTest(unittest.TestCase):
 
         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: