hyperref warns when using cleveref in \section

Question

I encounter a small but annoying problem with the following MWE

\documentclass{article}

\usepackage{hyperref,cleveref}
\newtheorem{thm}{Theorem}

\begin{document}

\begin{thm}\label{good}
    Cleveref is good
\end{thm}

\section{Proof of \Cref{good}}
    It is trivial.

\end{document}

The compiler (pdflatex) complains

Package hyperref Warning: Token not allowed in a PDF string (PDFDocEncoding):
(hyperref)                removing `\@ifnextchar' on input line 13.

Also the PDF TOC shows Proof of good instead of Proof Theorem 1. Is their any solution other than saying \section{Proof of Theorem \ref{...}}? After all, the whole point of using cleveref is not to repeat the fact that good is a theorem.

PS1. I also want a solution for amsart or more general document classes. (That is, the order of \usepackages is more or less fixed)

PS2. Feel free to mark duplicate. Although other MWEs I saw on this site are far from minimal.

Answer 1

As I said in the comment, the issue is that cleveref's machinery is not expandable (the \@ifnextchar that hyperref complains about, for example, isn't expandable), so it can't be turned into a PDF string.

The crossreftools package provides an expandable alternative for cross referencing commands. For cleverref you can replace \cref by \crtcref and \Cref by \crtCref.

However, as you noted in your comment, only using that breaks badly. The reason is that cleverref redefines some internals of the theorems and inserts some \MakeUppercase and \MakeLowercase (which aren't expandable). The issue boils down to the fact that these commands end up in the cross reference code, which is expected to contain only text. When hyperref tries to expand that, chaos ensues.

I tried to find a proper fix, but most of them led to a can of worms of code which would then be incompatible with everything. An easier way out, which preserves cleveref's functionality and leaves the cross reference text expandable, is to use expl3's case changing functions. I used etoolbox's \patchcmd to replace the two occurrences of \MakeUppercase and one of \MakeLowercase by \text_uppercase:n and \text_uppercase:n, respectively, from the commands \@othm, \@xnthm, and \@ynthm. After patching that, the expandability problem is no more :-)

The pdf:

and the pdf bookmark file:

\BOOKMARK [1][-]{section.1}{Proof of Theorem 1}{}% 1

Code:

\documentclass{article}

\usepackage{expl3,etoolbox}

\usepackage{hyperref,cleveref}
\usepackage{crossreftools}

% This patch must be after loading cleverref and before defining a new theorem
\ExplSyntaxOn
\makeatletter
\clist_map_inline:nn { \@othm, \@xnthm, \@ynthm }
  {
    \patchcmd #1 { \MakeUppercase } { \text_uppercase:n } { } { \FAILED: }
    \patchcmd #1 { \MakeUppercase } { \text_uppercase:n } { } { \FAILED: }
    \patchcmd #1 { \MakeLowercase } { \text_lowercase:n } { } { \FAILED: }
  }
\makeatother
\ExplSyntaxOff

\newtheorem{thm}{Theorem}

\begin{document}

\begin{thm}\label{good}
    Cleveref is good. expl3 is awesome :D
\end{thm}

\section{Proof of \crtCref{good}}
    It is trivial.

\end{document}