bug#71887: Raise an error when extending a service type that doesn't support extensions

[Richard Sent <richard@freakingpenguin.com>]

Hi Guix,

At present it is possible to extend service-types that do not implement
compose or extend methods, resulting in surprising behavior that's hard
to debug [1]. We should throw an error when a service-type that does not
have compose or extend fields is extended.

--8<---------------cut here---------------start------------->8---
(use-modules (gnu home)
             (gnu home services)
             (gnu home services desktop)
             (gnu services))

(home-environment
 (services
  (list
   ;; home-redshift-service-type does not support extensions
   (simple-service 'this-should-error home-redshift-service-type
                   "According to all known laws of aviation..."))))
--8<---------------cut here---------------end--------------->8---


[1]: https://lists.gnu.org/archive/html/help-guix/2024-07/msg00000.html

-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.