Incorrect type in pg_cast causes Postmaster to core dump

Hi,

I was setting up an implicit type cast for an application that was
inserting a boolean into a numeric field, but I used the wrong return type
from the function... and this caused the Postmaster to core dump:

[postgres(at)efm1 ~]$ *cat /etc/redhat-release*
CentOS Linux release 7.3.1611 (Core)

[postgres(at)efm1 ~]$ *pg_ctl -c start*
server starting
[postgres(at)efm1 ~]$ < 2017-04-21 11:10:35.287 BST > LOG: redirecting log
output to logging collector process
< 2017-04-21 11:10:35.287 BST > HINT: Future log output will appear in
directory "pg_log".

[postgres(at)efm1 ~]$ *psql*
psql (9.6.2)
Type "help" for help.

postgres=# *select version();*
version
----------------------------------------------------------------------------------------------------------
PostgreSQL 9.6.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5
20150623 (Red Hat 4.8.5-11), 64-bit
(1 row)

postgres=# *CREATE FUNCTION bool_to_num (boolean) RETURNS integer*
postgres-# * AS 'SELECT CASE WHEN $1 = true THEN 1 ELSE 0 END;'*
postgres-# * LANGUAGE SQL*
postgres-# * IMMUTABLE*
postgres-# * RETURNS NULL ON NULL INPUT;*
CREATE FUNCTION

postgres=# *select oid from pg_proc where proname = 'bool_to_num';*
oid
-------
16384
(1 row)

postgres=# *SELECT oid, typname FROM pg_type WHERE typname IN ('bool',
'numeric');*
oid | typname
------+---------
16 | bool
1700 | numeric
(2 rows)

postgres=# *INSERT INTO pg_cast (castsource, casttarget, castfunc,
castcontext, castmethod) VALUES (16,1700,16384,'a','f');*
INSERT 16385 1

postgres=# *create table bool_test(test_column numeric(22,0));*
CREATE TABLE

postgres=# *insert into bool_test(test_column) values(true);*
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.

[postgres(at)efm1 ~]$ *ls -al $PGDATA/core.27553*
-rw-------. 1 postgres postgres 152203264 Apr 21 11:14
/var/lib/pgsql/9.6/data/core.27553

[postgres(at)efm1 ~]$ *file $PGDATA/core.27553*
/var/lib/pgsql/9.6/data/core.27553: ELF 64-bit LSB core file x86-64,
version 1 (SYSV), SVR4-style, from 'postgres: postgres postgres', real uid:
1002, effective uid: 1002, real gid: 1002, effective gid: 1002, execfn:
'/usr/pgsql-9.6/bin/postgres', platform: 'x86_64'

I realise that my types were wrong (function returned an "integer", and
should have returned a "numeric"), and I've fixed that and now it's working
fine... but how can a type mismatch cause the entire cluster to crash?

Regards,
Greg Clough.