xparse's \DeclareDocumentEnvironment gives error on already-defined environment

Question

I'm trying to redefine an existing environment subfigure defined by the subcaption package. To do this, I'm using xparse's DeclareDocumentEnvironment since that's suppose to declare a new environment no matter if it already exists or not. But for some reason I still get a compilation warning that the \subfigure command already exists.

Here's a MWE:

\documentclass{article}

\usepackage{subcaption}
\usepackage{xparse}

\DeclareDocumentEnvironment{subfigure}{m}{%
  % Do nothing
}{%
  \subcaption{#1}%
}

\begin{document}
\begin{figure}
  \begin{subfigure}{The caption}
    The figure
  \end{subfigure}
\end{figure}
\end{document}

Why doesn't this work as intended?

A workaround is of course to simply use another name for my custom environment, but I'm curious why this doesn't work as I want it to.

Answer 1

The subcaption package delays the definition of subfigure and subtable when executing the \AtBeginDocument hook. So the error is because \newenvironment{subfigure} wants to define an already existing environment. Indeed, you get exactly the same error if you use \NewDocumentEnvironment.

If you do

\AtBeginDocument{%
  \DeclareDocumentEnvironment{subfigure}{m}{%
    % Do nothing
  }{%
    \subcaption{#1}%
  }%
}

you get no error.