# microtype + siunitx and \micro - mysterious warnings

This code

\documentclass{scrreprt}

\usepackage{lmodern}
\usepackage[T1]{fontenc}
\usepackage{microtype}
\usepackage{siunitx}

\begin{document}

\SI{10}{\micro\metre}

\end{document}


produces these warnings:

Package microtype Warning: Unknown slot number of character
(microtype)                \textminus '
(microtype)                in font encoding TS1' in protrusion list
(microtype)                textcomp'.

Package microtype Warning: Unknown slot number of character
(microtype)                in font encoding TS1' in protrusion list
(microtype)                textcomp'.

Package microtype Warning: Unknown slot number of character
(microtype)                \textcopyright '
(microtype)                in font encoding TS1' in protrusion list
(microtype)                textcomp'.

Package microtype Warning: Unknown slot number of character
(microtype)                \textregistered '
(microtype)                in font encoding TS1' in protrusion list
(microtype)                textcomp'.

Package microtype Warning: Unknown slot number of character
(microtype)                \textdegree '
(microtype)                in font encoding TS1' in protrusion list
(microtype)                textcomp'.


• This seems only to happen when using siunitx's "\micro" (like µm or µs)
• The errors also come up without fontenc and lmodern packages (but in the real world document where the warnings come up I use them, so I included them in the minimum example)
• loading the textcomp package make the warnings disappear

Questions:

• What exactly do these warnings mean?
• What exactly produces them?
• Should I just load the textcomp package? But it's from 1995 and I think it's obsolete. And I don't need it (appart from making the warnings disappear)
• What is the best solution here?
-

To avoid messing up anything else in a document, siunitx is set up to avoid loading font packages and the like. As such, it uses its 'own' version of the \textminus and \textmu, and sets up a minimal amount of support for that. However, that confuses microtype, as you've seen (it should be harmless). Loading textcomp 'fixes' this as siunitx then uses the textcomp version of the two symbols, and microtype knows what is going on with that. I do recommend loading textcomp with siunitx`, but it's not absolutely required so it's not enforced.