How can I use \SI from siunitx inside \replaced from changes?

Question

I'm trying to record some changes using the changes package - I've already done a lot of the text, and had to work around display maths but I've got a simple value typeset using siunitx, and changing that value doesn't appear to work:

\documentclass{article}
\usepackage{siunitx}
\usepackage{changes}
\begin{document}
    \replaced{\SI{1}{\volt}}{\SI{2}{\volt}}
\end{document}

gives:

! Extra }, or forgotten \endgroup.
\UL@stop ...z@ \else \UL@putbox \fi \else \egroup 
                                                  \egroup \UL@putbox \fi \if...
l.5 ...\SI{1}{\volt}}{\SI{2}{\volt}}

If I continue to compile (getting the same error again and 2 missing } inserteds, the output is actually exactly as expected:

adding \protect before the \SI gives exactly the same error, and of course trying \SI{\replaced{1}{2}}{\volt} was never going to work:

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! siunitx error: "invalid-token-in-number"
! 
! Invalid token '\let ' in numerical input.
! 
! See the siunitx documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

Is there some workaround that will get this to run without errors? I don't really want to run in nonstopmode in case of "real" errors, and besides, I used a build script that's designed to stop on errors.

Answer 1

The problem here is actually an interaction with the \sout command from the ulem package. Essentially, there is a clash over the insertion of a \penalty between the number and the unit: ulem alters how this expects to work and that doesn't agree with the box/grouping in siunitx. I've not traced it all back to a minimal example of the low-level code, but that probably wouldn't help in any case (at least without quite a bit of effort). What you can do is simply disable the insertion of a penalty. There isn't an interface for this (one of the few things not in the options for siunitx!) so at the moment a low-level redefinition is required

\documentclass{article}
\usepackage{siunitx}
\usepackage{changes}
\ExplSyntaxOn
\cs_set_eq:NN \__siunitx_unit_output_number_sep:
  \__siunitx_unit_output_number_sep_aux:
\ExplSyntaxOff
\begin{document}
    \replaced{\SI{1}{\volt}}{\SI{2}{\volt}}
\end{document}

Answer 2

You could define your own deletion markup no depending on the ulem package, e.g.:

\usepackage{changes}
\setdeletedmarkup{---#1---}

Downside of this approach is that the deleted markup does not match expectations of being striked out.

See example in https://gitlab.com/ekleinod/changes/tree/master/testfiles/t61-siunitx

As I cannot answer the duplicate question Undocumented incompatibility between changes and siunitx? here is the link for the corresponding issue tracker Incompatibility with siunitx and maybe other packages

Disclaimer: I am the author of the package.

Answer 3

An easy workaround for this is to replace \SI{x}{y} for \num{x}\,\si{y}, which should produce an equivalent result in most cases and doesn't produce the error:

\documentclass{article}
\usepackage{siunitx}
\usepackage{changes}
\begin{document}
    \replaced{\SI{1}{\volt}}{\num{2}\,\si{\volt}}
\end{document}

Answer 4

Wrap \SI inside a \mbox:
\mbox{\SI{}}.
Take into account that the word will continue along the line (no breakup).