Deprecating arithmetic on std::atomic<void*>

[Jonathan Wakely]

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.

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.