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@zoho.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.