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

[Jonathan Wakely <jwakely@redhat.com>]

On Sat, 8 Jan 2022 at 00:42, Tom Honermann via Libstdc++ <
libstdc++@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>
>
>         * acinclude.m4 Define config macros if uchar.h provides
>         c8rtomb() and mbrtoc8().
>         * 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.

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 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.