Re: [PATCH v4 2/3] stdlib: Implement mbrtoc8(), c8rtomb(), and the char8_t typedef.

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 20/07/22 10:54, Florian Weimer wrote:
> * Adhemerval Zanella Netto via Libc-alpha:
> 
>> On 19/07/22 18:08, Joseph Myers wrote:
>>> This change appears to introduce a failure of 
>>> wcsmbs/check-installed-headers-cxx with GCC mainline, because uchar.h now 
>>> produces:
>>>
>>> ../wcsmbs/uchar.h:38:23: error: identifier 'char8_t' is a keyword in C++20 [-Werror=c++20-compat]
>>>    38 | typedef unsigned char char8_t;
>>>
>>> (recall we want our installed headers to avoid warnings *without* relying 
>>> on the default disabling of warnings in system headers).
>>>
>>> Unfortunately, GCC for C++ doesn't disable -Wc++20-compat inside 
>>> __extension__ (unlike what the C front end does), so simply adding 
>>> __extension__ to that declaration wouldn't help, but we could use 
>>> diagnostic disabling pragmas (as already done in some installed headers).
> 
>> My understanding is compiler will define __cpp_char8_t exactly to avoid
>> such redefinition.  But it seems from gcc documentation that it is only
>> actually enabled with -fchar8_t.  Do we a preprocessor flag to indicate
>> it? 
> 
> I think __cpp_char8_t is only defined if the language mode is active.
> The warning is independent of the language mode, though.

Maybe we also skip chat8_t definition for __cplusplus == 202002L ?