\eappto and \aa doesn't work with pdflatex without fontenc

Question

I'm trying to use \aa in an \eappto command. The minimal code below works just fine in XeLaTeX and LuaLaTeX but fails with pdfLaTeX unless I include the fontenc package. This confuses me as \aa works perfectly outside of the \eappto command also without the fontenc package.

\documentclass{article}
% \usepackage[T1]{fontenc}
\usepackage{etoolbox}
\makeatletter
\newcommand{\zz}{%
  \def\@tmp{}%
  \eappto\@tmp{\aa}%
  \@tmp%
}
\makeatother
\begin{document}
  \aa tta% works just fine
  \zz% only works with `fontenc`
\end{document}

Answer 1

you should always use T1 enoding for Swedish in pdflatex, or hyphenation will be wrong, also you can not in general apply \edef to latex constructs. That said utf-8 characters are \protected so safe in an \edef so if you really want this, simply use å rather than \aa.

\documentclass{article}
% \usepackage[T1]{fontenc}
\usepackage{etoolbox}
\makeatletter
\newcommand{\zz}{%
  \def\@tmp{}%
  \eappto\@tmp{å}%
  \@tmp%
}
\makeatother
\begin{document}
  \aa tta % works just fine
  \zz tta    % only works with `fontenc`
\end{document}

If you have an old latex you can use the standard latex \protected@edef rather than rely on unsupported edef use.

\documentclass{article}
% \usepackage[T1]{fontenc}
\usepackage{etoolbox}
\makeatletter
\newcommand{\zz}{%
  \def\@tmp{}%
  \protected@edef\@tmp{\@tmp å}%
  \@tmp%
}
\makeatother
\begin{document}
  \aa tta % works just fine
  \zz tta    % only works with `fontenc`
\end{document}

Answer 2

The definition of \aa is

\r a

The definition of \r with XeLaTeX or LuaLaTeX is

\TU-cmd \r \TU\r

and we can see that the definition of \TU\r (which is a strange command with a backslash in its name) is

#1->\@text@composite \TU\r #1\@empty \@text@composite {\add@unicode@accent {"030A}{#1}}.

and \r a turns out to expand to the single Unicode character å.

With the default OT1 encoding in pdflatex, the \r command becomes a complicated construction aimed to add the ring accent above the a, which doesn't survive the \edef used by \eappto.

On the other hand, with T1 the situation is similar to the previous one, because T1 encoded fonts have a precomposed å character.

However, I'd not recommend using such encoding dependent commands in \edef under any circumstance.