BUG #15281: Set role does not affect superuser privleges

The following bug has been logged on the website:

Bug reference:      15281
Logged by:          Michal Wozny
Email address:      torcher999@gmail.com
PostgreSQL version: 10.4
Operating system:   Windows Server 2012 R2
Description:

Here are the two scenarios I tested:

1) as a session_user: superuser and current_user: non-superuser I can edit
others' tables
2) as a session_user: non-superuser and current_user: superuser I cannot
edit others' tables

The documentation contradicts 1) explicitly, 2) implicitly:

"when a superuser chooses to SET ROLE to a non-superuser role, they lose
their superuser privileges."
Source: https://www.postgresql.org/docs/current/static/sql-set-role.html


Steps to reproduce:

Preconditions:
create two non-superusers [user1, user2]
create one superuser [user0]
grant user0 to user1
create a table [u2_tab] with owner user2

1)
disconnect
connect as user0
set role user1
rename u2_tab

expected: cannot rename u2_tab
actual: can rename u2_tab

2)
disconnect
connect as user1
set role user0
rename u2_tab

expected: can rename u2_tab
actual: cannot rename u2_tab

SQL steps:

Preconditions:
CREATE USER user0 WITH SUPERUSER
CREATE USER user1
GRANT user0 to user1
CREATE USER user2
CREATE TABLE public.u2_tab
ALTER TABLE public.u2_tab OWNER TO user2

1)
DISCONNECT
CONNECT TO db AS user0
SET ROLE user1
ALTER TABLE public.u2_tab RENAME TO u2_tab1

2)
DISCONNECT
CONNECT TO db AS user1
SET ROLE user0
ALTER TABLE public.u2_tab1 RENAME TO u2_tab2