updatable view set default interact with base rel generated stored columns

hi.
While reviewing virtual generated columns,
I find some issues with the updatable view interacting with stored
generated columns.

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drop table if exists base_tbl cascade;
CREATE TABLE base_tbl (a int, b int GENERATED ALWAYS AS (22) stored, d
int default 22);
create view rw_view1 as select * from base_tbl;
insert into rw_view1(a) values (12) returning *;

alter view rw_view1 alter column b set default 11.1;
insert into rw_view1(a,b,d) values ( 12, default,33) returning *;
insert into rw_view1(a,d) values (12,33) returning *;
insert into rw_view1 default values returning *;

SELECT events & 4 != 0 AS can_upd,
events & 8 != 0 AS can_ins,
events & 16 != 0 AS can_del
FROM pg_catalog.pg_relation_is_updatable('rw_view1'::regclass, false) t(events);
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"alter view rw_view1 alter column b set default 11.1;"
because rw_view1 view relation in ATExecColumnDefault
TupleDesc->attgenerated == '\0',
otherwise it can error out in ATExecColumnDefault.
Now after we set default, we cannot insert any value to rw_view1,
which makes it not updatable.