BUG #15168: "pg_isready -d" effectively ignores given database name

The following bug has been logged on the website:

Bug reference: 15168
Logged by: Jake
Email address: jakelist(at)zoho(dot)com
PostgreSQL version: 9.5.10
Operating system: Fedora 25
Description:

* Using Bash
* Tried to update as recommended. 9.6.12 does not appear to be available for
Fedora 25.

$ cd /run/media/jthomas/BackupOne/development/snowdrift

$ export PGDATA="$PWD"/.postgres-work
$ export PGHOST="$PGDATA"
$ export PGDATABASE="snowdrift"

$ pg_ctl initdb
$ pg_ctl start -w -o "-F -h '' -k "$PGHOST"" -l "$PGDATA"/log

# As you would expect; "pg_ctl initdb" created database "postgres":
$ pg_isready -d postgres
/run/media/jthomas/BackupOne/development/snowdrift/.postgres-work:5432 -
accepting connections

# Not expected, database "jibberish" does not exist:
$ pg_isready -d jibberish
/run/media/jthomas/BackupOne/development/snowdrift/.postgres-work:5432 -
accepting connections

# Just to confirm:
$ psql -l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access
privileges
-----------+---------+----------+-------------+-------------+---------------------
postgres | jthomas | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
template0 | jthomas | UTF8 | en_US.UTF-8 | en_US.UTF-8 | =c/jthomas
+
| | | | |
jthomas=CTc/jthomas
template1 | jthomas | UTF8 | en_US.UTF-8 | en_US.UTF-8 | =c/jthomas
+
| | | | |
jthomas=CTc/jthomas
(3 rows)

# No database "jibberish"

Note: https://www.postgresql.org/docs/10/static/bug-reporting.html , 5.3
portrays the mailing list as the preferred method, but then the pgsql-bugs
page
asks that the bug reporting form be used instead.