Subject: 27.1; defcustom for mail-user-agent is too strict?

From: David Bremner <[email-masked]>
Date: Tue, 07 Sep 2021 14:58:41 -0300

The defcustom for mail-user-agent looks like

:type '(radio (function-item :tag "Message package"
			       :format "%t\n"
			       message-user-agent)
;; [...] snip                               
		(function-item :tag "Message with full Gnus features"
			       :format "%t\n"
			       gnus-user-agent)
		(function :tag "Other"))

This means that a symbol without a function definition cannot be set
(using customize) as mail-user-agent. This seem inconsistent with the
docstring of define-mail-user-agent, 

    SYMBOL can be any Lisp symbol.  Its function definition and/or
    value as a variable do not matter for this usage; we use only certain
    properties on its property list, to encode the rest of the arguments.

and with existing mail user agents such as message-user-agent and
gnus-user-agent which don't have function definitions.

As far as I know, this defcustom is still enforcing that the symbol is a
function in emacs 28.x.

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0)
 of 2021-03-27, modified by Debian built on x86-conova-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
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