In the guix/scripts/system.scm file we do not check the value while parsing the
flag:
--8<---cut here---start->8---
(option '("on-error") #t #f
(lambda (opt name arg result)
(alist-cons 'on-error (string->symbol arg)
result)))
--8<---cut here---end--->8---
and then blindly pass it to load*:
--8<---cut here---start->8---
(load* file %user-module
#:on-error (assoc-ref opts 'on-error))
--8<---cut here---end--->8---
and load* uses it in a case that only gets called when an actual error occurs
and treats the correct symbols but has a default clause that silently ignores
values other than debug and backtrace:
--8<---cut here---start->8---
(case on-error
((debug)
...)
((backtrace)
...)
(else
#t))
--8<---cut here---end--->8---
meaning that for example a typo such as `--on-error=stacktrace`, gets treated
as if the flag was not passed at all.
Minimum replication:
--8<---cut here---start->8---
guix system build <(echo x) --on-error=stacktrace
guix system build <(echo x) --on-error=backtrace
--8<---cut here---end--->8---
I'm not sure where the check should be done, nor what would be an acceptable
way to not duplicate the list of valid values between guix/ui.scm and
guix/scripts/system.scm
