Deprecating arithmetic on std::atomic<void*>

[Florian Weimer]

On 04/20/2017 11:22 AM, Jonathan Wakely wrote:
> On 20/04/17 10:18 +0100, Jonathan Wakely wrote:
>> On 20/04/17 08:19 +0200, Florian Weimer wrote:
>>> On 04/19/2017 07:07 PM, Jonathan Wakely wrote:
>>>> I know it's a bit late, but I'd like to propose deprecating the
>>>> libstdc++ extension that allows arithmetic on std::atomic<void*>.
>>>> Currently we make it behave like arithmetic on void*, which is also a
>>>> GNU extension (https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html).
>>>> We also allow arithmetic on types such as std::atomic<void(*)()> which
>>>> is probably not useful (PR 69769).
>>>
>>> Why is it acceptable to have the extension for built-in types, but 
>>> not for library types wrapping them?  Why be inconsistent about this?
>>
>> C++17 [atomic.types.pointer] paragraph 4 says:
>>
>> Requires: T shall be an object type, otherwise the program is
>> ill-formed. [Note: Pointer arithmetic on void* or function pointers
>> is ill-formed. — end note]
>>
>> That doesn't give us any leeway to support it.

If we can ignore the Note, we can also ignore the requirement …

> I suppose we could support it without -Wpedantic, although there's no
> way for the library to know if the front-end is being pedantic or not.

Could you use SFINAE to detect whether the compiler supports arithmetic 
on void *?

Thanks,
Florian