Deprecating arithmetic on std::atomic<void*>

[Florian Weimer]

On 04/20/2017 11:52 AM, Jonathan Wakely wrote:
> On 20/04/17 11:43 +0200, Florian Weimer wrote:
>> On 04/20/2017 11:25 AM, Jonathan Wakely wrote:
>>
>>>> I mean, with -pedantic-errors we already error on void * arighmetics
>>>> or function pointer arithmetics.  If std::atomic<void*> would use
>>>> the void * arithmetics, it would also reject it.  Or does it use 
>>>> integer
>>>> arithmetics instead?
>>>
>>> No, it does it on void*, but the __atomic built-ins still perform that
>>> arithmetic even with -pedantic-errors.
>>
>> sizeof is not defined for the relevant types, either, and already 
>> triggers a warning by default:
>>
>> t.cc:25:58: warning: invalid application of ‘sizeof’ to a void type 
>> [-Wpointer-arith]
>>   _M_type_size(ptrdiff_t __d) const { return __d * sizeof(_PTp); }
> 
> That could be easily avoided if we care about supporting arithmetic on
> std::atomic<void*>.
> 
>> I think the actual problem here is the suppression of warnings from 
>> system headers.
> 
> The atomic built-ins still don't warn even with -Wsystem-headers

Sorry, this is what I meant: -pedantic-errors does not catch this case 
due to the system header warning suppression.  Otherwise, with the 
current std::atomic implementation, -pedantic-errors would already 
disable the extension on std::atomic alongside with the extension on 
built-in pointer types, due to the use of sizeof in the definition of 
_M_type_size.

Thanks,
Florian