How can I force a compile error if a ref does not exist?

Question

I was curious if there was a way to force the compile to fail if a ref was not already defined. I do not want the ref to actually output anything into the document, but rather just check the existence condition. For example, I want to be able to do something like:

\begin{definition} \label{def C} \requires{def A} \requires{def B}
    A \underline{C} is an A and a B such that...  etc etc etc.
\end{definition} 

and I do not want the \requires{def A} to actually produce output but just enforce that there was somewhere above a \label{def A}. Ideally, you could use this for theorems and definitions. This way I can just add any dependencies in my theorems/definitions and not have to worry about whether or not I already defined everything above, the compiler will tell me. Does cleveref provide this? If not, how hard would it be to add it? I looked very briefly at the source code but I am not too familiar with the language so it would take me a while.

Answer 1

If you really want to enforce that the label is before the \requires, then you will have to redefine the \label command, as the current definition writes the label only to the aux-files and so its location relative to the \ref doesn't matter. E.g.

\documentclass{article}
\makeatletter
\let\Orilabel\label
\renewcommand\label[1]{\@namedef{mylabel@#1}{}\Orilabel{#1}}
\newcommand\require[1]{%
 \ifcsname mylabel@#1\endcsname
 \else
  \@latex@error{Reference #1 undefined}
 \fi}
\begin{document}
\section{A}
\label{A}

\require{A}\require{B}

\ref{A},\ref{B} \label{B}
\end{document}