Cross-referencing: check that all cross-references are to earlier objects

Question

I have a huge LaTeX document with my lecture notes for a maths course; every now and then I change the order of the sections.

I want to make sure that all cross-references to, for instance, theorems are in "chronological order": referring in section 7 to stuff from section 5 is fine, but I don't want to use in a proof of section 5 references to results that won't be proved until section 7.

Is there a package that checks whether cross-references are to labels defined earlier or later in the document than that particular reference?

Answer 1

A proof of concept (it would require extensive work if hyperref is involved):

\documentclass{article}

\makeatletter
\let\latex@label\label
\let\latex@ref\ref
\def\label#1{\global\@namedef{LABEL@#1@USED}{}\latex@label{#1}}
\def\ref#1{%
  \ifcsname LABEL@#1@USED\endcsname\else
    \@latex@warning{Forward Reference '#1'}%
  \fi
  \latex@ref{#1}%
}
\makeatother

\begin{document}

\section{A}\label{a}

This refers to \ref{a} and to \ref{b}.

\section{B}\label{b}

OK.

\end{document}

The log file would contain

LaTeX Warning: Forward Reference 'b' on input line 19.

Answer 2

Here a solution that seems to work, though not within subequations. It uses zref for absolute page numbers, see How to find absolute page number as an integer? and Refer to the current page number, but not \thepage, but the real absolute You will find warnings in the log file for all not wanted cross-references.

\documentclass{article}

\usepackage{amsmath}
\usepackage[user,abspage]{zref}
\usepackage{xassoccnt}

\makeatletter
\let\oldref\ref
\let\oldeqref\eqref
\let\oldlabel\label
\newcommand\mylabel[1]{%
  \oldlabel{#1}%
  \zlabel{#1}%
}
\renewcommand\label[1]{%
  \mylabel{#1}%
}

\renewcommand{\ref}[1]{%
  \zref{#1}%
  \ifnum\zref@extract{#1}{abspage}<\value{realpage}
    \GenericWarning{}{Warning: Backwards refence '#1' on page \thepage}%
  \fi%
}
\renewcommand{\eqref}[1]{%
  (\zref{#1})%
  \ifnum\zref@extract{#1}{abspage}<\value{realpage}
    \GenericWarning{}{Warning: Backwards refence '#1' on page \thepage}%
  \fi%
}

\def\label@in@display#1{%
      \ifx\df@label\@empty\else
      \@amsmath@err{Multiple \string\label's:
          label '\df@label' will be lost}\@eha
      \fi
      \mylabel{#1}
      \typeout{XXX #1 XXX}
}
\makeatother

\newcounter{realpage}
\DeclareAssociatedCounters{page}{realpage}
\AtBeginDocument{%
  \stepcounter{realpage}
}

\begin{document}
\pagenumbering{roman}
\section{Sec~1}
\begin{align}\label{eqR1}
    x &= 1
\end{align}

Text~R1\label{textR1}

\clearpage
\pagenumbering{arabic}
\section{Sec~2}

\begin{align}\label{eq1}
     x &= 1
\end{align}

\begin{subequations}
  \begin{align}
      x &= 1\label{eqS1} \\
    y &= 1\label{eqS2}
      \\
    z&=1
    \\
    z&=10
  \end{align}
\end{subequations}

Text~1\label{text1}

\newpage

\begin{itemize}
  \item Equation~R1: \eqref{eqR1}
  \item Equation~1: \eqref{eq1}
  \item Equation~S1: \eqref{eqS1}
  \item Equation~2: \eqref{eq2}  
  \item Text~R1: \ref{textR1}  
  \item Text~1: \ref{text1}
  \item Text~2: \ref{text2}
\end{itemize}

\newpage 

\section{Sec~3}

\begin{align}\label{eq2}
    x &= 2
\end{align}

Text~2\label{text2}
\newpage

\end{document}