Received: from malur.postgresql.org ([217.196.149.56])
	by arkaria.postgresql.org with esmtps  (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
	(Exim 4.96)
	(envelope-from <pgsql-hackers-owner+archive@lists.postgresql.org>)
	id 1vlGll-009RWz-2J
	for pgsql-hackers@arkaria.postgresql.org;
	Thu, 29 Jan 2026 01:24:26 +0000
Received: from localhost ([127.0.0.1] helo=malur.postgresql.org)
	by malur.postgresql.org with esmtp (Exim 4.96)
	(envelope-from <pgsql-hackers-owner+archive@lists.postgresql.org>)
	id 1vlGlk-005CIU-2q
	for pgsql-hackers@arkaria.postgresql.org;
	Thu, 29 Jan 2026 01:24:25 +0000
Received: from magus.postgresql.org ([2a02:c0:301:0:ffff::29])
	by malur.postgresql.org with esmtps  (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
	(Exim 4.96)
	(envelope-from <tgl@sss.pgh.pa.us>)
	id 1vlGlk-005CIK-1t
	for pgsql-hackers@lists.postgresql.org;
	Thu, 29 Jan 2026 01:24:24 +0000
Received: from sss.pgh.pa.us ([68.162.161.243])
	by magus.postgresql.org with esmtps  (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
	(Exim 4.98.2)
	(envelope-from <tgl@sss.pgh.pa.us>)
	id 1vlGli-00000000yvD-28Gl
	for pgsql-hackers@postgresql.org;
	Thu, 29 Jan 2026 01:24:24 +0000
Received: from sss1.sss.pgh.pa.us (localhost [127.0.0.1])
	by sss.pgh.pa.us (8.15.2/8.15.2) with ESMTP id 60T1OJIO1127585;
	Wed, 28 Jan 2026 20:24:19 -0500
From: Tom Lane <tgl@sss.pgh.pa.us>
To: jian he <jian.universality@gmail.com>
cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: ERROR: failed to find conversion function from unknown to text
In-reply-to: <CACJufxHu0sXO8791FDcNXp2bFnE89jyuGkJbLCQkhgWq6XuNLg@mail.gmail.com>
References: <CACJufxHu0sXO8791FDcNXp2bFnE89jyuGkJbLCQkhgWq6XuNLg@mail.gmail.com>
Comments: In-reply-to jian he <jian.universality@gmail.com>
	message dated "Thu, 29 Jan 2026 09:18:49 +0800"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <1127583.1769649859.1@sss.pgh.pa.us>
Date: Wed, 28 Jan 2026 20:24:19 -0500
Message-ID: <1127584.1769649859@sss.pgh.pa.us>
List-Id: <pgsql-hackers.lists.postgresql.org>
List-Help: <https://lists.postgresql.org/manage/>
List-Subscribe: <https://lists.postgresql.org/manage/>
List-Post: <mailto:pgsql-hackers@lists.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@lists.postgresql.org>
List-Archive: <https://www.postgresql.org/list/pgsql-hackers>
Archived-At: <https://www.postgresql.org/message-id/1127584.1769649859%40sss.pgh.pa.us>
Precedence: bulk

jian he <jian.universality@gmail.com> writes:
> select cast(NULL::text as unknown);
> ERROR:  failed to find conversion function from unknown to text

Is there a good reason why that shouldn't be an error?
I certainly don't like the cast pathway you suggest adding
to make it not be one --- that seems likely to cause lots of
not-very-desirable behaviors.

			regards, tom lane