microtype v3.0a is not compatible with book class

[akiyks]

Description

microtype v3.0a stopped working with a document of the book class, which has a title page and a footnote, with hyperref.

_Please also indicate the engine(s) with which the issue occurs: pdfTeX

Minimal example demonstrating the issue (works with v3.0)

\documentclass{book}
\usepackage{lmodern}
\usepackage{hyperref}
\usepackage{microtype}

\title{Test of microtype}
\author{A microtype User}

\begin{document}

\frontmatter

\maketitle

\mainmatter

\chapter{Test 1}

Xxxx yyyy zzzz.\footnote{
  Aaaa bbbb cccc.}

\end{document}

With v3.0a, pdflatex stops with the error of:

[...]
(./main.aux) (/usr/local/texlive/2021/texmf-dist/tex/latex/hyperref/nameref.sty
(/usr/local/texlive/2021/texmf-dist/tex/latex/refcount/refcount.sty)
(/usr/local/texlive/2021/texmf-dist/tex/generic/gettitlestring/gettitlestring.s
ty)) (/usr/local/texlive/2021/texmf-dist/tex/latex/microtype/mt-cmr.cfg)
Runaway argument?
{\scantokens {\LARGE \@title 
! Paragraph ended before \MT@prot@l was complete.
<to be read again> 
                   \par 
l.14 
     
? 

Can you look into this?

BR, Akira

[schlcht]

Many thanks for the report. Could you try with:

\makeatletter
\long\def\MT@prot@l#1{%
  \MT@get@prot{#1}{left}%
  \leavevmode
  #1}
\def\MT@prot@group@@{%
    \catcode`\\=0
    \catcode`\^^M=10
    \makeatletter
    {\expandafter\MT@prot@l\expandafter
      {\expandafter\scantokens\expandafter{\the\MT@toks\noexpand}}}%
  \endgroup}
\makeatother

(BTW: Are you sure that hyperref and the footnote are involved? I get the error only at \maketitle)

[akiyks]

Ah, I thought it was minimal, but apparently wasn't.
hyperref and footnote aren't involved here.

Your fix to v3.0a works fine.
Thank you for the quick fix. Much appreciated!

[davidcarlisle]

I haven't traced this fully but \scantokens here seems a bit dangerous, adding \makeatletter makes package code safe again but may break code that originally came from the document with @ catcode12, it will also I fail if any expl3 code has been picked up as the expl3 catcode regime will be lost in the rescan. you can't add \ExplSyntaxOn to match \makeatletter as that will prevent _ as subscript and lose all spaces.

[schlcht]

@davidcarlisle Thanks for your comment, I think I found a way to avoid the rescanning while still allowing verbatim material etc.:

\def\MT@prot@group#{\bgroup\afterassignment\MT@prot@get@firstchar\let\MT@temp= }

[akiyks]

FWIW, current dev branch (d9a2315) works with my (not the minimal) document.