Is it correct to use RequirePackage[utf8]{inputenc}?

Question

If I create a package, is it correct to use \RequirePackage[utf8]{inputenc} to support utf8. I guess not, but how do I state that this package must (should) be used with inputenc utf8?

Answer 1

This is what I do in newunicodechar that requires the utf8 option either to inputenc or inputenx and wouldn't work with other options and without either of those packages:

\def\nuc@stop{\PackageWarningNoLine{newunicodechar}
  {This package won't work without loading\MessageBreak
   `inputenc' or `inputenx' with the `utf8' option}%
  \let\newunicodechar\@gobbletwo\endinput}
\@ifpackageloaded{inputenx}
  {\def\nuc@tempa{inputenx}}
  {\@ifpackageloaded{inputenc}{\def\nuc@tempa{inputenc}}{\nuc@stop}}
\@ifpackagewith{\nuc@tempa}{utf8}{}{\nuc@stop}
\@ifpackagewith{\nuc@tempa}{utf8x}{\nuc@stop}{}

The \let\newunicodechar\@gobbletwo is just to enable processing the file nonetheless. If you want to require usage of utf8 you can modify it like this:

\ProvidesPackage{xyz}
% Define an error message
\def\xyz@stop{\PackageError{xyz}
  {`inputenc' or `inputenx' loaded with wrong option.\MessageBreak
   This is a fatal error}
  {This package won't work if either `inputenc' or `inputenx'\MessageBreak
   are loaded with the `utf8' option. The LaTeX run will be terminated}%
   \fi\@@end} % the `\fi` is to match `\if@tempswa`
% Check for inputenx or inputenc
\@tempswafalse
\@ifpackageloaded{inputenx}
  {\def\xyz@tempa{inputenx}\@tempswatrue}
  {\@ifpackageloaded{inputenc}
    {\def\xyz@tempa{inputenc}\@tempswatrue}
    {\RequirePackage[utf8]{inputenc}}%
  }
% Check for the right option
\if@tempswa
\@ifpackagewith{\xyz@tempa}{utf8}{}{\xyz@stop}
\@ifpackagewith{\xyz@tempa}{utf8x}{\xyz@stop}{}
\fi

So if the user types

\usepackage[utf8]{inputenc}
\usepackage{xyz}

nothing will be done. But if inputenc (or inputenx) is not loaded before it, xyz will load it. If a wrong option is passed to inputenc, the package will terminate the LaTeX run.