Subsection throws LaTeX Error: There's no line here to end

Question

I have section and subsection commands in my LaTeX code described as follows. These are included in the style file, foo.cls:

\newenvironment{aside}{%
  \let\oldsection\section
  \renewcommand{\section}[1]{
    \par\vspace{\baselineskip}{\Large\headingfont\color{headercolor} ##1}
  }
  \begin{textblock}{3.6}(1.5, 4.33)
  \begin{flushright}
  \obeycr
}{%
  \restorecr
  \end{flushright}
  \end{textblock}
  \let\section\oldsection
}

\renewcommand{\section}[1]{
  \par\vspace{\parskip}
  {%
    \LARGE\headingfont\color{headercolor}%
    \@sectioncolor #1%
  }
  \par\vspace{\parskip}
}

\renewcommand{\subsection}[1]{
  \par\vspace{.5\parskip}%
  {\Large\headingfont\color{headercolor} #1}
  \par\vspace{.25\parskip}%
}

When I try and use these commands in the following setting, I ran into an error:

\documentclass{foo}

\begin{document}
\begin{aside}
\section{foo}
bar
~
\section{foo}
\subsection{bar}
\end{aside}
\end{document}

Specifically the error I am encountering is:

LaTeX Error: There's no line here to end.

l.55 \subsection{foo}

What's going wrong here?

Answer 1

The problem you see is because you use \obeycr. The effect of \obeycr is (roughtly) the same as adding \\ at the end of every line of input, so inside your aside environment it is like if you had typed:

\section{foo}\\
bar\\
~\\
\section{foo}\\
\subsection{bar}\\

In the body of the text, under usual circumstances, the command \\ means a line break. However this command does some sanity checking and raises an error when there is no line to end, which is precisely the error you see. More specifically, when you use the command \\ in vertical mode (after a \par), the error is raised because you can't make a line break in vertical mode. A very quick way to make that error is:

\documentclass{article}
\begin{document}
\\
\end{document}

because when the document begins TeX is in vertical mode, and then you use \\. To fix the error, just type something before the \\.

You see that error with your definition because your \subsection command does \par after writing the subsection title (and the \vspace that follows doesn't change TeX's mode), and right after that there is a line end (thus a \\), and we already know what happens in this case. To work around this issue you can simply add a comment character after \subsection{bar}, then TeX will not insert the line end character.

A handier way to work around that is to add this \removecr command at the end of the definition of \subsection:

{\makeatletter\obeycr%
 \gdef\removecr{\@ifnextchar^^M{\@gobblecr}{}}%
 \def\@gobblecr^^M{}}

then it will look for one (only one!) line end, and remove it, if found.

Here's a compilable example:

\documentclass{article}
{\makeatletter\obeycr%
 \gdef\removecr{\@ifnextchar^^M{\@gobblecr}{}}%
 \def\@gobblecr^^M{}}
\newenvironment{aside}{%
  \renewcommand{\section}[1]{%
    \par \vspace{\baselineskip}{\LARGE ##1}}%
  \renewcommand{\subsection}[1]{%
    \par \vspace{.5\parskip}%
      {\Large #1}%
    \par \vspace{.25\parskip}\removecr}
  \begin{flushright}%
    \obeycr
}{\end{flushright}}
\begin{document}
\begin{aside}
\section{foo}
bar
~
\section{foo}
\subsection{bar}
\end{aside}
\end{document}

Note also that you had several unprotected line ends which were generating spurious spaces (add % at the end of the line to get rid of those). You also don't need to save the definition of \section because it is automatically restored at the end of the environment. The same goes for the effect of \obeycr.

I highly recommend you don't use \obeycr. It will make you more problems than it will solve. Also, the way you are redefining section breaks a lot of LaTeX's sectioning functionalities...