MinionPro, microtype and \j - Is there a way to get rid of a strange warning?

Question

When I typeset my free physics textbook, I get a microtype warning

Unknown slot number of character `\j'  

(It happens only for this character, no other one.) I get this message since many years, both on my 2008 tex system and in my new, fully updated 2014 texlive system. The older system says microtype.cfg/183(protrusion), the newer one microtype.cfg/201(protrusion).

The systems use MinionPro, microtype and many other packages with latex2e. How can I get rid of the message? The message only appears when microtype and MinionPro are used together.

Here is a minimal working example that triggers the error:

\documentclass{book} 
\usepackage{MinionPro}
\usepackage{microtype}
\usepackage[T1]{fontenc}

\begin{document}
Hallo.
\end{document} 

In fact, the error disappears if microtype is loaded first. But I read that this is not allowed with MinionPro. What can be done?

Answer 1

EDIT: Thanks to @sebschub this is fixed now.

---

This warning means that microtype, while reading (protrusion) settings, is unable to translate the command \j into a slot number of the font. Since there is indeed no dotless j in MinionPro, this message is harmless and can be ignored -- it's not an error.

Given the harmless nature of this warning, the simplest solution would probably be to just load microtype before MinionPro, which will, as you say, make the message disappear. (I don't see why this should be a problem.) Another solution would be to load microtype with verbose=silent, which will turn all warnings into mere info messages, so that they will only show in the log file, not on the console (but this would apply to all warnings, so relevant ones might go unnoticed).

Now to explain what's happening: MinionPro undeclares the command \j because there is no dotless j in the font (or the slot is used for another glyph). But microtype's default inheritance settings still contain the inheritance pair j = \j. Therefore, when parsing these settings, microtype stumbles over the non-defined \j. This also explains why the message disappears when you load microtype first: at that point, the \j is still defined.

So the proper solution would be to add custom inheritance settings for MinionPro to mt-MinionPro.cfg, containing everything from the default settings (from microtype.cfg) except the j:

\DeclareCharacterInheritance
   { encoding = T1,
     family = {MinionPro-OsF,MinionPro-LF,MinionPro-TOsF,MinionPro-TLF} }
   { A = {\`A,\'A,\^A,\~A,\"A,\r A,\k A,\u A},
     a = {\`a,\'a,\^a,\~a,\"a,\r a,\k a,\u a},
% ...
%     j = {\j},
% ...
     Z = {\'Z,\.Z,\v Z},
     z = {\'z,\.z,\v z}
   }

(If protrusion settings existed for LY1 encoding, its inheritance settings would have to be adjusted accordingly.)