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@gmail.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.