per in math mode: Improper `at' size (0.0pt), eplaced by 10pt

[Reissner]

I use siunitx version 28b.
Thus I added the patch https://www.texdev.net/2021/06/30/siunitx-v2-to-v3.

Also I configure with

\usepackage{xfrac}
\usepackage{siunitx}
\sisetup{per-mode=fraction}
\sisetup{fraction-function=\sfrac}

Then all works fine as long as I am in text mode.
Even math modes works frequently, but for example

$[-\qty{11}{\meter\per\second},\qty{11}{\meter\per\second]$

yields an error:

! Improper `at' size (0.0pt), replaced by 10pt. 
<to be read again>
relax

but

$[-\qty{11}{\meter/\second},\qty{11}{\meter/\second]$

works.
It is strange, that in some places even the following works:

$[-{\qty{11}{\meter\per\second}},\qty{11}{\meter\per\second]$

[josephwright]

I will need a complete example: for me

\documentclass{article}
\usepackage{xfrac}
\usepackage{siunitx}%[=v2]
\sisetup{per-mode=fraction}
\sisetup{fraction-function=\sfrac}
\begin{document}
$[-\qty{11}{\meter\per\second},\qty{11}{\meter\per\second}]$
\end{document}

works with the latest release, and if I replace \qty by \SI it also works with the final v2 release.

[Reissner]

I see.
A quite minimal file I found is the following:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{anyfontsize}
\usepackage{xfrac}
\usepackage{siunitx}
\sisetup{per-mode = fraction}
\sisetup{fraction-function=\sfrac}

\ifdefined\qty\else
  \ifdefined\NewCommandCopy
    \NewCommandCopy\qty\SI
  \else
    \NewDocumentCommand\qty{O{}mm}{\SI[#1]{#2}{#3}}
  \fi
\fi
\ifdefined\unit\else
  \ifdefined\NewCommandCopy
    \NewCommandCopy\unit\si
  \else
    \NewDocumentCommand\unit{O{}m}{\si[#1]{#2}}
  \fi
\fi

\begin{document}
Nevertheless, we suggest a range of $[-\qty{11}{\meter\per\second},\qty{11}{\meter\per\second}]$. 
We could spedify a range $[-\qty{90}{\meter\per\second},\qty{90}{\meter/\second}]$. % <----------- this is the line to fail. 
\end{document}

As you can see, i use lualatex.
Independent of this issue, i want to point out, that i need anyfontsize to avoid warnings.
Maybe you should mention that in your documentation.

I wonder what the O{}mm means.. first I thought it shall be 0{}mm.

As you will see, if you remove the line Nevertheless, ...., the problem disappears.
So there seems an influence between the lines.. very strange.

[josephwright]

Aside - you shouldn't be using T1 encoding with LuaTeX.

[josephwright]

Your example compile fine with v3 - I have no plans to make changes to the v2 code unless it breaks with core LaTeX changes.

[Reissner]

So you can reconstruct problem with v2?

By the way? why not T1 with luatex?

[Reissner]

ah, fontspec:
copied the wrong branch
https://github.com/Reissner/latex-maven-plugin/blob/master/maven-latex-plugin/src/main/resources/injections/header.tex

thank you.

[josephwright]

So you can reconstruct problem with v2?

Yes, although my feeling is this is an issue outside of siunitx.

By the way? why not T1 with luatex?

LuaTeX is Unicode, and the font slots are therefore wrong for some parts of T1.

[Reissner]

can you tell me about the meaning of the mysterious {O{}m} in your compatibility definition for v2 users wanting v3 feeling?

[josephwright]

can you tell me about the meaning of the mysterious {O{}m} in your compatibility definition for v2 users wanting v3 feeling?

This is standard LaTeX syntax for \NewDocumentCommand:

• O{} is an optional argument with an empty default value
• m is a mandatory argument

See texdoc usrguide for example.