BUG #14952: COPY fails to fill in IDENTITY column default value

The following bug has been logged on the website:

Bug reference:      14952
Logged by:          Steven Winfield
Email address:      steven.winfield@cantabcapital.com
PostgreSQL version: 10.0
Operating system:   Linux
Description:

COPYing data to a table with an IDENTITY column, where the column's value
isn't specified in the copied input, fails because COPY attempts to insert a
NULL value for the column:

test=# CREATE TABLE identity_test (id bigint GENERATED ALWAYS AS IDENTITY,
name text);
test=# COPY identity_test (name) FROM STDIN;
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself, or an EOF signal.
>> foo
>> \.
ERROR:  null value in column "id" violates not-null constraint
DETAIL:  Failing row contains (null, foo).
CONTEXT:  COPY identity_test, line 1: "foo"

Compare this with a serial column:

test=# CREATE TABLE serial_test (id bigserial, name text);
test=# COPY serial_test (name) FROM STDIN;
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself, or an EOF signal.
>> foo
>> \.
COPY 1


test=# \d identity_test
                     Table "public.identity_test"
 Column |  Type  | Collation | Nullable |           Default
--------+--------+-----------+----------+------------------------------
 id     | bigint |           | not null | generated always as identity
 name   | text   |           |          |

test=# \d serial_test
                            Table "public.serial_test"
 Column |  Type  | Collation | Nullable |                 Default
--------+--------+-----------+----------+-----------------------------------------
 id     | bigint |           | not null |
nextval('serial_test_id_seq'::regclass)
 name   | text   |           |          |