Redefinition problem with \maketitle from KOMA class (with optional argument)

Question

I try to redefine the \maketitle command as shown below.

If I use an optional argument in the redefinition I get the TeX capacity exceeded error.

Any suggestion why?

MNWE:

\documentclass{scrreprt}

\title{My Title}
\author{My Name}

\let\oldmaketitle\maketitle

\renewcommand{\maketitle}[1][]{%
  \oldmaketitle
  \rule{\textwidth}{1pt}
}

\begin{document}

\maketitle

\end{document}

Answer 1

\let with commands with optional arguments (and in the KOMA classes \maketitle is defined with one optional argument) can produce undesired effects: you can use \LetLtxMacro from the letltxmacro package instead:

\documentclass{scrreprt}
\usepackage{letltxmacro}
\title{My Title}
\author{My Name}

\LetLtxMacro\oldmaketitle\maketitle

\renewcommand{\maketitle}[1][]{%
  \oldmaketitle
  \rule{\textwidth}{1pt}
}

\begin{document}

\maketitle

\end{document}

Your example code doesn't show the usage of the optional argument.

Answer 2

The \maketitle command in scrreprt has an optional argument, so redefining it with \let\oldmaketitle\maketitle and \renewcommand\maketitle exposes you to a very big problem, which is well described in the documentation of letltxmacro.

If all you need is to add something at the end of what \maketitle is doing, then a safe way is to use xpatch (or regexpatch):

\usepackage{xpatch} % or \usepackage{regexpatch}
\xapptocmd{\maketitle}{\noindent\rule{\textwidth}{1pt}}{}{}

However this won't add a rule below the date. The reason is that with the default options \maketitle prints a page by itself and the rule will go on the next page; if titlepage=false is specified in the options, then the \@maketitle internal command is used.

So, in order to get a rule below the date, a better trick is to patch the commands and add the rule exactly after the printing of the date, which is done by \@date:

\documentclass{scrreprt}

\usepackage{xpatch} % or \usepackage{regexpatch}

\makeatletter
%% the following patch is for the `titlepage=true` option
\xpatchcmd{\maketitle}{\@date}{\@date\par\rule{\textwidth}{1pt}}{}{}
%% the following patch is for the `titlepage=false` option
\xpatchcmd{\@maketitle}{\@date}{\@date\par\rule{\textwidth}{1pt}}{}{}
\makeatother

\begin{document}

\title{My Title}
\author{My Name}

\maketitle

\end{document}