1 files changed, 26 insertions, 0 deletions
diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index df03556..f7625ab 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -5528,6 +5528,32 @@ ast_type_replace_check(PyObject *self,
             Py_DECREF(unused);
         }
     }
+
+    // Discard fields from 'expecting' that default to None
+    PyObject *field_types = NULL;
+    if (PyObject_GetOptionalAttr((PyObject*)Py_TYPE(self),
+                                 &_Py_ID(_field_types),
+                                 &field_types) < 0)
+    {
+        Py_DECREF(expecting);
+        return -1;
+    }
+    if (field_types != NULL) {
+        Py_ssize_t pos = 0;
+        PyObject *field_name, *field_type;
+        while (PyDict_Next(field_types, &pos, &field_name, &field_type)) {
+            if (_PyUnion_Check(field_type)) {
+                // optional field
+                if (PySet_Discard(expecting, field_name) < 0) {
+                    Py_DECREF(expecting);
+                    Py_DECREF(field_types);
+                    return -1;
+                }
+            }
+        }
+        Py_DECREF(field_types);
+    }
+
     // Now 'expecting' contains the fields or attributes
     // that would not be filled inside ast_type_replace().
     Py_ssize_t m = PySet_GET_SIZE(expecting);