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

[Jonathan Wakely]

N.B. please CC gcc-patches as well when sending patches to this list.

On Mon, 7 Jun 2021 at 03:16, 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() if provided by
> the C library.
>
> Autoconf changes determine if the C library declares c8rtomb and mbrtoc8
> at global scope when uchar.h is included and compiled with -fchar8_t;
> _GLIBCXX_USE_UCHAR_CHAR8_T is defined if so.  The <cuchar> header
> re-declares these functions in the std namespace only if available and
> the C++20 __cpp_char8_t feature test macro is defined.


Thanks, Tom. I'm surprised that there's no __cplusplus dependency
here. Some C libraries aren't going to declare these new functions
unconditionally, only for C2x and C++20 modes. Your new configure
check tests for them with -std=c++11 -fchar8_t (and the implicit
-D_GNU_SOURCE on GNU/Linux) which doesn't guarantee they'll be
available.

This isn't a problem for your glibc patches, because you do:

+#if defined _CHAR8_T_SOURCE || defined __cpp_char8_t
+# define __GLIBC_USE_CHAR8_T   1

But consider a hypothetical libc that only define the new functions
for C2x/C++20, ignoring the __cpp_char8_t set by -fchar8_t. For such a
libc the configure test using -std=c++11 -fchar8_t will fail, and
libstdc++ won't declare the functions in namespace std even though
they are in libc.

Shouldn't the configure test use -std=c++20 instead to check that
they're available for C++20 mode, and then in <cuchar> guard the using
declarations for ::c8rtomb and ::mbrtoc8 with #if __cplusplus >=
202002L ?

That would mean they're not declared in <cuchar> pre-C++20 when using
-fchar8_t, but that seems a lesser problem than having them not
declared in C++20 when they are actually present in libc. Maybe to
solve that we need two configure macros (or one macro with multiple
values), one that says the new functions are available for C++20, and
another that says they are also available pre-C++20 if __cpp_char8_t
is defined. Then <cuchar> can do something like:

#if (__cplusplus >= 202002 && _GLIBCXX_USE_UCHAR_CXX20) \
  || (__cpp_char8_t && _GLIBCXX_USE_UCHAR_CHAR8_T)

Messy. Hmm.