Deprecating arithmetic on std::atomic<void*>

[Jonathan Wakely]

On 20/04/17 11:24 +0200, Florian Weimer wrote:
>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 *?

Probably. We'd have to move the relevant member functions into a new
base class in order to make them conditionally disabled.