BUG #15835: Errors altering data type of the column used in partial exclusion constraint

The following bug has been logged on the website:

Bug reference:      15835
Logged by:          Yaroslav Schekin
Email address:      ladayaroslav@yandex.ru
PostgreSQL version: 11.3
Operating system:   Any
Description:

Executing the below code:

CREATE TABLE t(some_id int);
ALTER TABLE t ADD EXCLUDE USING btree(some_id WITH =) WHERE (some_id IS NOT
NULL);
ALTER TABLE t ALTER COLUMN some_id TYPE bigint;

Produces the following errors (per PostgreSQL version):
9.4.19, 9.5.15, 9.6.13: ERROR:  could not open relation with OID 195837
10.8: ERROR: cache lookup failed for relation 630589
11.3, 12beta1: ERROR: relation "t_some_id_excl" already exists