Create an alias to a label

Question

This is probably an unusual way of using labels, but I would be happy if there was a way for me to create an alias to a label without just writing another \label at the target environment.

I imagine something like this:

% File A 
\documentclass{article}
\begin{docuement}
\begin{proposition}
    \label{original_label}
    ...
\end{proposition}
\end{docuement}

% File B
\documentclass{article}
\usepackage{xr-hyper}
\usepackage{hyperref}

\externaldocument[A-]{a.tex}
\labelalias{new_label}[A-original_label}
\begin{document}
    See Proposition \hyperref{new_label}...
\end{docuement}

Thanks!

Answer 1

\ref{foo} uses (if defined) the control sequence \r@foo.

You can abstract the definition. I choose to do it at begin document, so when the .aux files have been read in.

\documentclass{article}
\usepackage{xr}
\usepackage{hyperref}

\externaldocument[A-]{external}

\ExplSyntaxOn
\NewDocumentCommand{\definelabelalias}{mm}
 {
  \AtBeginDocument
   {
    \cs_if_exist:cT {r@#2} { \cs_set_eq:cc {r@#1} {r@#2} }
   }
 }
\ExplSyntaxOff

\definelabelalias{foo}{A-test-external}
\definelabelalias{baz}{test-internal}

\begin{document}

\section{Test}\label{test-internal}

\ref{A-test-external} = \ref{foo}

\ref{baz} = \ref{test-internal}

\end{document}

The external file I used is

\documentclass{article}
\begin{document}
\section{Title}\label{test-external}
\end{document}

Note that you don't need xr-hyper. Everything produces 1 as expected and the links to the external labels have different colors than the internal.

If no .aux file is available (or isn't up-to-date) you get

LaTeX Warning: Reference `A-test-external' on page 1 undefined on input line 24.
LaTeX Warning: Reference `foo' on page 1 undefined on input line 24.
LaTeX Warning: Reference `baz' on page 1 undefined on input line 26.
LaTeX Warning: Reference `test-internal' on page 1 undefined on input line 26.

If the external .aux file is available, you just get

LaTeX Warning: Reference `baz' on page 1 undefined on input line 26.
LaTeX Warning: Reference `test-internal' on page 1 undefined on input line 26.