problems with hyperref when \thethm points to undefined reference

Question

Consider the following three minimal examples. I do not understand why the first doesn't work while the second and third is fine. Can anyone tell me? Hyperref seems to have problems when \thethm is pointing to an undefined reference. Please don't ask why pointing to an undefined reference makes sense. I do have a good reason for that but it has nothing to do with this question.

First example (produces strange error):

\documentclass{article}

\usepackage[thmmarks,hyperref]{ntheorem} 
\usepackage{hyperref}

\newtheorem{thm}{Theorem}                            
\newtheorem{prop}[thm]{Proposition}

\begin{document}
\begingroup
\def\thethm{\ref{a}'}
\addtocounter{thm}{-1}
\begin{prop} 
yyy
\label{test}
\end{prop}
\endgroup
\end{document}

Second example, hyperref removed, otherwise identical code (works fine):

\documentclass{article}

\usepackage[thmmarks]{ntheorem} 

\newtheorem{thm}{Theorem}                            
\newtheorem{prop}[thm]{Proposition}

\begin{document}
\begingroup
\def\thethm{\ref{a}'}
\addtocounter{thm}{-1}
\begin{prop} 
yyy
\label{test}
\end{prop}
\endgroup
\end{document}

Third example, code as in the first example but \thethm is changed (also works well):

\documentclass{article}

\usepackage[thmmarks,hyperref]{ntheorem} 
\usepackage{hyperref}

\newtheorem{thm}{Theorem}                            
\newtheorem{prop}[thm]{Proposition}

\begin{document}
\begingroup
\def\thethm{1'}
\addtocounter{thm}{-1}
\begin{prop} 
yyy
\label{test}
\end{prop}
\endgroup
\end{document}

Answer 1

I have no problem with this document:

\documentclass{article}

\usepackage{refcount}
\usepackage[thmmarks,hyperref]{ntheorem} 
\usepackage{hyperref}

\newtheorem{thm}{Theorem}
\newtheorem{prop}[thm]{Proposition}

\theoremstyle{nonumberplain}

\newcommand\specialref{}

\newtheorem{unnumbered}{\specialref}
\newenvironment{thmref}[2][$'$]
  {\edef\theunnumbered{\getrefnumber{#2}\unexpanded{#1}}%
   \renewcommand\specialref{Theorem \ref{#2}#1}\unnumbered}
  {\endunnumbered}

\newenvironment{propref}[2][$'$]
  {\edef\theunnumbered{\getrefnumber{#2}\unexpanded{#1}}%
   \renewcommand\specialref{Proposition \ref{#2}#1}\unnumbered}
  {\endunnumbered}

\begin{document}
\begin{prop}\label{a}
aaa
\end{prop}

\begin{propref}{a}\label{test}
yyy
\end{propref}

\ref{test}
\end{document}

Changing the meaning of \thethm with hyperref active is not recommendable.

Answer 2

Using unexpandable stuff apart from characters inside \the<counter> is a little risky. The number could be used in many contexts and might break in expandable context like here. The solution below uses package refcount to extract the reference data in an expandable way. Outside \refused notifies LaTeX about the reference usage and LaTeX can report undefined or changed references. The link is stripped.

The counter is decremented that means there are two theorems with the same number and therefore the same destination name. This can be resolved by modifying \theHthm that hyperref uses instead of \thethm for the destination name.

  \refused{a}%
  \edef\thethm{\getrefbykeydefault{a}{}{?}'}%
  \edef\theHthm{\theHthm'}%

The complete example:

\documentclass{article}

\usepackage[thmmarks,hyperref]{ntheorem} 
\usepackage{hyperref}
\usepackage{refcount}

\newtheorem{thm}{Theorem}                            
\newtheorem{prop}[thm]{Proposition}

\begin{document}
\begin{prop}\label{a}
Proposition with label a.
\end{prop}
\begingroup
  \refused{a}%
  \edef\thethm{\getrefbykeydefault{a}{}{?}'}%
  \edef\theHthm{\theHthm'}%
  \addtocounter{thm}{-1}%
  \begin{prop} 
  yyy
  \label{test}
  \end{prop}
\endgroup
\end{document}