BUG #16446: ERROR: virtual tuple table slot does not have system attributes on insert to partitioned table

The following bug has been logged on the website:

Bug reference: 16446
Logged by: Георгий Драк
Email address: sonicgd(at)gmail(dot)com
PostgreSQL version: 12.2
Operating system: Debian 10.3
Description:

Hello. I'm catch error "virtual tuple table slot does not have system
attributes" when inserting row into partitioned table with RETURNING xmin;

Reproduction.

1. Create schema
CREATE TABLE "tmp"
(
id bigint generated always as identity,
date timestamptz not null,
foo int not null,
PRIMARY KEY ("id", "date")
)
PARTITION BY RANGE ("date");
CREATE TABLE "tmp_2020" PARTITION OF "tmp" FOR VALUES FROM ('2020-01-01') TO
('2021-01-01');

2. Execute query
INSERT INTO "tmp" ("date", "foo")
VALUES (NOW(), 1)
RETURNING id, xmin;

3. Result - ERROR: virtual tuple table slot does not have system
attributes

4. Expected result - id and xmin of inserted row.