[luatex] Picking defaults for axes on variable fonts

[Sergio Callegari]

Hi,

I have noticed that luatex now supports variable fonts and I've started 
experimenting. I am asking a question here but I am unsure if it is 
actually tied to luatex or luaotfload or maybe fontspec. In case, please 
be so kind to address me to the correct forum.

I have noticed that luatex may get very unhappy unless you explicitly 
set values for the axes.  For instance see the comments on 
https://tex.stackexchange.com/questions/355104/tex-luatex-xetex-fontspec-support-for-opentype-variable-fonts.

If you do

\documentclass[a4paper,12pt]{article}
\usepackage{fontspec}
\setmainfont{Source Serif 4 Variable}
\begin{document}
The quick brown fox jumps over the lazy dog.
\end{document}

then this fails with

  error:  (file .fonts/source-serif-4.004/VAR/SourceSerif4Variab
le-Roman.otf) (cff): Type2 Charstring Parser: subroutine called but no 
subrouti
ne found
!  ==> Fatal error occurred, no output PDF file produced!

To make the code work you need to add a

\defaultfontfeatures{RawFeature={+axis={wght=100}}}

Is this expected?

Shouldn't the font selection mechanism be able to determine a suitable 
weight for the font if it is not specified? If I am correct the font 
fvar table should be able to provide a defaultValue for each axis.

Why does this happen with the wght axis, but not for the opsz axis?

Thanks for considering this matter,

Best,

Sergio