{"ast_node_type":"other","tokens":[{"end":43,"kind":"keyword","start":0,"text":"(None,"}]}
                    (None, None) => return,
                    (Some(a), Some(b)) => assert_assignable(fx, a, b, limit - 1),
                    (Some(_), None) | (None, Some(_)) => panic!("{:#?}/{:#?}", from_ty, to_ty),