[PATCH] C++ P0482R6 char8_t: declare std::c8rtomb and std::mbrtoc8 if provided by the C library

[Tom Honermann]

On 1/10/22 8:23 AM, Jonathan Wakely wrote:
>
>
> On Sat, 8 Jan 2022 at 00:42, Tom Honermann via Libstdc++ 
> <libstdc++@gcc.gnu.org <mailto:libstdc%2B%2B@gcc.gnu.org>> wrote:
>
>     This patch completes implementation of the C++20 proposal P0482R6
>     [1] by
>     adding declarations of std::c8rtomb() and std::mbrtoc8() in
>     <cuchar> if
>     provided by the C library in <uchar.h>.
>
>     This patch addresses feedback provided in response to a previous
>     patch
>     submission [2].
>
>     Autoconf changes determine if the C library declares c8rtomb and
>     mbrtoc8
>     at global scope when uchar.h is included and compiled with either
>     -fchar8_t or -std=c++20. New
>     _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_FCHAR8_T
>     and _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20 configuration macros
>     reflect the probe results. The <cuchar> header declares these
>     functions
>     in the std namespace only if available and the _GLIBCXX_USE_CHAR8_T
>     configuration macro is defined (by default it is defined if the C++20
>     __cpp_char8_t feature test macro is defined)
>
>     Patches to glibc to implement c8rtomb and mbrtoc8 have been
>     submitted [3].
>
>     New tests validate the presence of these declarations. The tests pass
>     trivially if the C library does not provide these functions.
>     Otherwise
>     they ensure that the functions are declared when <cuchar> is included
>     and either -fchar8_t or -std=c++20 is enabled.
>
>     Tested on Linux x86_64.
>
>     libstdc++-v3/ChangeLog:
>
>     2022-01-07  Tom Honermann  <tom@honermann.net
>     <mailto:tom@honermann.net>>
>
>             * acinclude.m4 Define config macros if uchar.h provides
>             c8rtomb() and mbrtoc8().
>             * config.h.in <http://config.h.in>: Re-generate.
>             * configure: Re-generate.
>             * include/c_compatibility/uchar.h: Declare ::c8rtomb and
>             ::mbrtoc8.
>             * include/c_global/cuchar: Declare std::c8rtomb and
>             std::mbrtoc8.
>             * include/c_std/cuchar: Declare std::c8rtomb and std::mbrtoc8.
>             * testsuite/21_strings/headers/cuchar/functions_std_cxx20.cc:
>             New test.
>             *
>     testsuite/21_strings/headers/cuchar/functions_std_fchar8_t.cc:
>             New test.
>
>
>
> Thanks, Tom, this looks good and I'll get it committed for GCC 12.
Thank you!
>
> My only concern is that the new tests depend on an internal macro:
>
> +#if _GLIBCXX_USE_UCHAR_C8RTOMB_MBRTOC8_CXX20
> +  using std::mbrtoc8;
> +  using std::c8rtomb;
>
> I prefer if tests are written as "user code" when possible, and not 
> using our internal macros. That isn't always possible, and in this 
> case would require adding new effective-target keyword to 
> testsuite/lib/libstdc++.exp just for use in these two tests. I don't 
> think we should bother with that.
I went with this approach solely due to my unfamiliarity with the test 
system. I knew there should be a way to conditionally make the test 
"pass" as unsupported or as an expected failure, but didn't know how to 
go about implementing that. I don't mind following up with an additional 
patch if such a change is desirable. I took a look at 
testsuite/lib/libstdc++.exp and it looks like it may be pretty straight 
forward to add effective-target support. It would probably be a good 
learning experience for me. I'll prototype and report back.
>
> I suppose strictly speaking we should not define __cpp_lib_char8_t 
> unless these two functions are present in libc. But I'm not sure we 
> want to change that now either.

All of libstdc++, libc++, and MS STL have been defining 
__cpp_lib_char8_t despite the absence of these functions, so yeah, I 
don't think we want to change that.

Tom.