Strange behavior with using lmodern together with siunitx package

Question

I would like to use the siunitx package in my document, but when I try to print an angle with the command \ang{90}, I've received

90ř

instead of

90°

Gradual reduction of the code, I found that the reason is the use of the package lmodern. How can this problem be solved? It seems that this behavior is similar to the description in the question: siunitx: µ doesn't work

MNWE:

\documentclass{scrbook}
\usepackage{xltxtra}
\usepackage{lmodern}
\usepackage{siunitx}

\begin{document}
 Angle is \ang{90} or \(\ang[number-angle-product = \,]{2.67}\)
\end{document}

Answer 1

The set up in siunitx for fonts tries to cover a range of 'common' cases. Here, with the Unicode engine XeTeX I'm 'expecting' you to use fontspec (perhaps with unicode-math). If you'd just loaded lmodern things would look OK (but see note below), but adding xltxtra before lmodern has confused the 'guesses' entirely: it's a far-from typical set up. Once you go 'off piste' you do have to set such things yourself: siunitx can't predict every possible combination of font choices. (Loading fontspec, which xltxtra does, uses the OpenType form of Latin Modern so I'm not really sure what you are aiming for!)

In detail, what happens here is that siunitx detects fontspec so switches the standard definition for the degree symbol to use the Unicode codepoint (decimal 176). When you then load lmodern later, this position is no longer the right glyph as the OT1-encoded font you end up with math mode has something different in that slot.

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Whilst one can load lmodern with XeTeX, the font encoding (OT1 or T1) is wrong: some slots are incorrect and will hyphenate badly. You really should consider using fontspec here.