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 13:47, Tom Honermann wrote:
> Confirmed that this issue can be easily reproduced outside the testsuite.
> 
> $ cat t.cpp
> #include <uchar.h>
> 
> $ g++ --version
> g++ (GCC) 13.0.0 20220720 (experimental)
> ...
> 
> $ g++ -c -I/path/to/glibc-char8_t/include -std=c++17 -Werror=c++20-compat t.cpp
> In file included from t.cpp:1:
> /home/tom/products/glibc-char8_t/include/uchar.h:38:23: error: identifier ‘char8_t’ is a keyword in C++20 [-Werror=c++20-compat]
>    38 | typedef unsigned char char8_t;
>       |                       ^~~~~~~
> cc1plus: some warnings being treated as errors
> 
> The char8_t typedef is currently guarded by:
> 
> /* Declare the C2x char8_t typedef in C2x modes, but only if the C++
>   __cpp_char8_t feature test macro is not defined.  */
> #if __GLIBC_USE (ISOC2X) && !defined __cpp_char8_t
> /* Define the 8-bit character type.  */
> typedef unsigned char char8_t;
> #endif
> 
> __GLIBC_USE (ISOC2X) evaluates to true because gcc unconditionally defines _GNU_SOURCE. I believe otherwise, C++17 mode would only (or should only) imply __GLIBC_USE (ISOC11).
> 
> Regardless, it seems that directives should be added to suppress the diagnostic. I tried prototyping such a fix, but it doesn't seem to work for me. I don't understand why.

I have tried as well and I can't get to work either.  It would expect to work
as we have done bits/stdlib-bsearch.h, could it be a gcc issue?

> 
> $ diff -U3 uchar.h.old uchar.h
> --- uchar.h.old 2022-07-20 12:37:55.544301692 -0400
> +++ uchar.h     2022-07-20 12:43:21.124365563 -0400
> @@ -34,8 +34,17 @@
>  /* Declare the C2x char8_t typedef in C2x modes, but only if the C++
>    __cpp_char8_t feature test macro is not defined.  */
>  #if __GLIBC_USE (ISOC2X) && !defined __cpp_char8_t
> +/* Suppress the C++20 compatability diagnostic regarding char8_t being a
> +   keyword.  */
> +#if defined __GNUC__ && 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
> +# pragma GCC diagnostic push
> +# pragma GCC diagnostic ignored "-Wc++20-compat"
> +#endif
>  /* Define the 8-bit character type.  */
>  typedef unsigned char char8_t;
> +#if defined __GNUC__ && 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
> +# pragma GCC diagnostic pop
> +#endif
>  #endif
> 
>  #ifndef __USE_ISOCXX11
> 
> Tom.
> 
> On 7/19/22 5:08 PM, 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).
>>