ATTACH PARTITION seems to ignore column generation status

This does not seem good:

regression=# create table pp (a int, b int) partition by range(a);
CREATE TABLE
regression=# create table cc (a int generated always as (b+1) stored, b int);
CREATE TABLE
regression=# alter table pp attach partition cc for values from ('1') to ('10');
ALTER TABLE
regression=# insert into pp values(1,100);
INSERT 0 1
regression=# table pp;
a | b
-----+-----
101 | 100
(1 row)

I'm not sure to what extent it's sensible for partitions to have
GENERATED columns that don't match their parent; but even if that's
okay for payload columns I doubt we want to allow partitioning
columns to be GENERATED.

regards, tom lane