Am Tue, 14 Jul 2015 09:42:13 +0100 schrieb Joseph Wright:

> I was thinking of
> 
>     \keys_define:nn {test}
>       {
>         testa .code:n = {\tl_show:n {#1}},
>         testa .value_required:,
>         testb .code:n = {\tl_show:n {#1}},
>         testb .value_required:,
>         testb .default:n = {}
>     }
> 
>     \keys_set:nn {test} {testa,testb}
> 
> where one might expect to get an error only in the first case. That's
> not currently the situation!

Hm. My first reaction was that it works as expected. If I explictly
require that the *user* gives a value, a *package* default doesn't
make sense and shouldn't replace the user action.

It is also consistent with the other way round:

\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn
  \keys_define:nn {test}
      {
        testb .code:n = {#1},
        testb .value_forbidden:,
        testb .default:n = {xxx}
    }

   \keys_set:nn {test} {testb}

\end{document} 




-- 
Ulrike Fischer 
http://www.troubleshooting-tex.de/