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

[Tom Honermann <tom@honermann.net>]

On 7/21/22 4:56 PM, Adhemerval Zanella Netto wrote:
>
> On 21/07/22 17:51, Tom Honermann wrote:
>> On 7/21/22 3:22 PM, Adhemerval Zanella Netto wrote:
>>> 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?
>> Yes, this appears to be a gcc issue. I spent some time looking at gcc source code, but didn't find anything obvious. I verified the same technique does work to suppress the similar warning issued for use of, e.g., constexpr, as an identifier when -Wc++11-compat is enabled. I found tests that exercise #pragma GCC diagnostic ignored "-Wc++-compat", but none for -Wc++20-compat (or -Wc++11-compat).
>>
>> Tom.
>>
> In any case I think the fix below is the correct way (in fact I don't see
> another way so I am assuming a compiler issue here).
I agree. I debugged gcc tonight and discovered what the problem was. 
I'll submit a patch to gcc.
> We also need to avoid
> declare the typedef for __cplusplus >= 202002L.
The typedef is already avoided if the __cpp_char8_t feature test macro 
is defined (builtin char8_t support can be enabled in previous C++ 
standard modes via the -fchar8_t option).
>
>>>> $ 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__)
> Use __GNUC_PREREQ.

Yes, thank you. I also determined that the -Wc++20-compat option was 
added to gcc 10 and is only recognized in C++ modes, so the diagnostic 
suppression guards will be __GNUC_PREREQ (10, 0) && defined __cplusplus.

I'll follow up with a patch soon.

Tom.

>
>>>> +# 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).
>>>>>