On Thu, Sep 17, 2015 at 11:15:37AM +0100, Joseph Wright wrote:
>Hello all,
>
>Some of you may have noticed that the team have not been happy for a
>while with \tl_to_(lower|upper)case:n. There are a few issues:
>
> - The names are wrong as "_to_" is in all other cases used
>   for expandable 1-to-1 type conversions
> - Using the \lowercase/\uppercase primitives means
>   these functions fail for Unicode case mapping
> - Using a case changing function for generating
>   odd catcodes is not ideal
>
>We've been working on alternative interfaces that allow the required
>outcomes, and now are just about there. Over the past few weeks, we've added
>
> - \char_(g)set_active_eq:NN
> - \char_(g)set_active_eq:nN
>
>for active characters and
>
> - \char_generate:nn
>
>for creating arbitrary character tokens. The latter covers (almost) all
>sensible use cases: catcodes 1, 2, 3, 4, 6, 7, 8, 11 and 12. Currently
>catcode 10 and 13 are not available, but we expect to address at least
>the catcode 10 case shortly.
>
>We have done a sweep over current uses in TeX Live of
>\tl_to_(lower|upper)case:n and \tex_(lower|upper)case:D and believe we
>now can deprecate use of \tl_to_(lower|upper)case:n in favour of the
>above (for odd chars), using x-type expansion and \tl_to_str:n (for
>catcode 12 chars) or the expandable case changing functions. I've
>contacted various package authors to ask for adjustments in this regard
>and will shortly contact the remaining people. We'll also adjust the
>expl3 internals themselves to use the higher-level functions as far as
>possible (internally we do need the primitives to set up the interfaces).
>
>There is one proviso in the above. On XeTeX, \char_generate:nn uses
>\Ucharcat if available, and is otherwise limited to the 8-bit range.
>This means that at present there are a small number of use cases that
>would break with older XeTeX's if modified to use \char_generate:nn. The
>team will in time alter the minimum XeTeX version to reflect that but at
>present the small number of people generating Unicode chars on XeTeX
>will need to continue to use \tex_lowercase:D.
>
>Assuming the above transition can be carried out smoothly, we will
>formally deprecate \tl_to_(lower|upper)case:n shortly for removal after
>the end of next year.
>
>Regards,
>
>Joseph

I am puzzled. What exactly do I have to do in order to adjust existing
code which uses the lowercase trick

\group_begin:
\char_set_lccode:nn{...}{...}
\tl_to_lowercase:n{
  \group_end:
  ...
}

?

Alexander