disallow ALTER VIEW SET DEFAULT when the corresponding base relation column is a generated column

hi.

CREATE TABLE gtest1 (a int, b int GENERATED ALWAYS AS (a * 2) STORED);
CREATE VIEW gtest1v AS SELECT * FROM gtest1;
ALTER VIEW gtest1v ALTER COLUMN b SET DEFAULT 100;

INSERT INTO gtest1v VALUES (8, DEFAULT) returning *;
ERROR: cannot insert a non-DEFAULT value into column "b"
DETAIL: Column "b" is a generated column.

we can make
ALTER VIEW gtest1v ALTER COLUMN b SET DEFAULT 100;
error out,
then
INSERT INTO gtest1v VALUES (8, DEFAULT) returning *;
will work just fine.

obviously,
INSERT INTO gtest1v VALUES (8, 1) returning *;
will fail.

we can do this by in ATExecColumnDefault,
checking if
* gtest1v is updatable view or not
* column b is an updatable column or not
* column b on view corresponding base relation's column is a generated
column or not.

if all these conditions meet then, we error out saying
``cannot alter column \"%s\" on updateable view ``.

what do you think?