Environment prop undefined and also already defined LNCS template

Question

Trying to put my paper into LNCS format and I'm having a weird error if I comment out the line \newtheorem{prop}[theorem]{Proposition} it tells me prop is undefined but if I uncomment the line it tells me it is already defined.

\documentclass[runningheads]{llncs}
\usepackage{thm-restate}

%\newtheorem{prop}[theorem]{Proposition}
\begin{document}
\title{Contribution Title\thanks{Supported by organization x.}}
\maketitle              
\begin{abstract}
The abstract 
\end{abstract}
\section{First Section}
\subsection{A Subsection Sample}
Please note 
\begin{prop}
    dsafdf
\end{prop}
\end{document}

LNCS file available here https://www.overleaf.com/read/ygfkzwkxjhbw#d20610

Answer 1

LLNCS already defines a Proposition theorem environment, you should use \begin{proposition} ... \end{proposition} instead of defining your own. It has the advantage of properly localizing to various languages.

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That said, the issue with prop is not a conflict with LLNCS, but with thm-restate. And it is not really that the environment prop has been defined, but that there is a bug in how thmtools is trying to latch onto the LLNCS macros and provide its own interpretation of stuff.

I've been able to isolate the bug to the file thm-llncs.sty; the specific error is coming from the call to \thmt@newtheorem@predefinition in the definition of \thmt@spnewtheoremiv. (This part I am sure about.)

The problem, I think (the following is a guess, which may be incorrect), is that as part of the ...predefinition, it calls ...autorefsetup, which creates a counter-alias when sibling counters are used. But then the original \spnewtheorem from the Springer LLNCS file is called, which tries to create the counter again.

One can check that without specifying a sibling counter, commands such as \newtheorem{prop}{Proposition}, or \declaretheorem[name=Proposition]{prop}, or \spnewtheorem{prop}{Proposition}{\bfseries}{\itshape} all work as expected, but as soon as you specify a sibling counter everything breaks.

You should file a bug with the thmtools maintainers linking to this post and ask them to fix it.