How can I use fontspec to set a sans heading font with mathdesign

Question

I like the mathdesign package, but would like to use Myriad Pro section headings in LuaLaTeX. Is this possible, or do I have to convert an OTF font to T1? (My ignorance of font issues in *TeX is boundless.) The \setsansfont command and mathdesign packages seem not to work together, though there is no mention of fontspec within mathdesign. Here's a MWE that shows what doesn't work when running through LuaLaTeX.

 \documentclass{article}
 \usepackage [english] {babel}
 \usepackage {blindtext}
 \usepackage {titlesec}
 \usepackage {etoolbox}
 \usepackage {ifluatex}
 \titleformat{\section}{\Large\sffamily\bfseries}{\thesection}{1em}{}

 \ifbool{luatex} { 
   \usepackage {fontspec}
   \setsansfont [Ligatures=TeX,Scale=MatchLowercase] {Myriad Pro}
   \usepackage [charter] {mathdesign}
 }{\usepackage [T1] {fontenc}
   \usepackage [charter] {mathdesign}
   \renewcommand*{\sfdefault}{phv} % Adobe Helvetica 
 }

 \begin{document}
 \section {A section}
 \blindmathpaper
 \end{document}

The beginning of that document looks as follows (note the lack of sans serif section heading)

If I load the mathdesign package before fontspec I get the correct section heading font, but the rmdefault font switches to CM.

Interestingly, the math font remains Charter in all cases.

Answer 1

You have to set also the correct font encoding in order to choose the correct font (for the LuaLaTeX version):

\titleformat{\section}
  {\fontencoding{EU2}\Large\sffamily\bfseries}
  {\thesection}
  {1em}
  {}

\documentclass{article}
\usepackage[english]{babel}
\usepackage{blindtext}
\usepackage{titlesec}
\usepackage{ifluatex}

\ifluatex
  \usepackage {fontspec}
  \setsansfont [Ligatures=TeX,Scale=MatchLowercase] {Myriad Pro}
  \usepackage[T1]{fontenc}
  \usepackage[charter]{mathdesign}
  \titleformat{\section}
    {\fontencoding{EU2}\Large\sffamily\bfseries}
    {\thesection}
    {1em}
    {}
\else
  \usepackage[T1]{fontenc}
  \usepackage[charter]{mathdesign}
  \renewcommand*{\sfdefault}{phv} % Adobe Helvetica
  \titleformat{\section}
    {\Large\sffamily\bfseries}
    {\thesection}
    {1em}
    {}
\fi

\begin{document}

\section {A section}
\blindmathpaper
\end{document}

Just for completeness, here is a LuaTeX only version with the fonts Herbert pointed at in a comment:

\documentclass{article}
\usepackage[english]{babel}
\usepackage{blindtext}
\usepackage{titlesec}

\usepackage[charter]{mathdesign}
\usepackage[no-math]{fontspec}
\setmainfont[Ligatures=TeX,
             BoldFont={CharterITC Bold BT},
             BoldItalicFont={CharterITC Bold Italic BT},
            ]{CharterITC BT}
\setsansfont[Ligatures=TeX,Scale=MatchLowercase]{Myriad Pro}

\titleformat{\section}{\Large\sffamily\bfseries}{\thesection}{1em}{}

\begin{document}

\textbf{Abc}

\section {A section}
\blindmathpaper
\end{document}

Your way to specify the fonts may need to be different (here I put the fonts in the system fonts). Notice that one has to specify by hand the bold and bold italic versions, because the defaults are "Black", which fontspec doesn't understand. I added a boldface word just as an example.