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

[Florian Weimer <fweimer@redhat.com>]

* 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.

Thanks,
Florian