BUG #16441: Cannot multi-insert into generated column with DEFAULT value

The following bug has been logged on the website:

Bug reference:      16441
Logged by:          Colin Darie
Email address:      colin@darie.eu
PostgreSQL version: 12.3
Operating system:   macos
Description:

Multiple inserts of DEFAULT value for generated columns are failing. Single
inserts are OK.


CREATE TABLE squares (num INTEGER NOT NULL, square INTEGER GENERATED ALWAYS
AS (num * num) STORED);


INSERT INTO squares(num, square) VALUES (1, DEFAULT);
=> INSERT 0 1
INSERT INTO squares(num, square) VALUES (2, DEFAULT), (3, DEFAULT);
=> ERROR:  cannot insert into column "square"
=> DETAIL:  Column "square" is a generated column.


Tested with postgresql 12.2 and 12.3 on macos.