[Patch] Inline Variables for the Standard Library (p0607r0)

[Daniel Krügler]

2017-03-11 23:14 GMT+01:00 Daniel Krügler <daniel.kruegler@gmail.com>:
> 2017-03-11 23:09 GMT+01:00 Tim Song <t.canens.cpp@gmail.com>:
>> On Sat, Mar 11, 2017 at 3:37 PM, Daniel Krügler
>> <daniel.kruegler@gmail.com> wrote:
>>> 2017-03-11 21:23 GMT+01:00 Tim Song <t.canens.cpp@gmail.com>:
>>>> On Sat, Mar 11, 2017 at 1:32 PM, Daniel Krügler
>>>> <daniel.kruegler@gmail.com> wrote:
>>>>> This patch applies inline to all namespace scope const variables
>>>>> according to options A and B2 voted in during the Kona meeting, see:
>>>>>
>>>>> http://wiki.edg.com/pub/Wg21kona2017/StrawPolls/p0607r0.html
>>>>>
>>>>> During that work it has been found that std::ignore was not declared
>>>>> constexpr (http://cplusplus.github.io/LWG/lwg-defects.html#2773),
>>>>> which was fixed as well.
>>>>
>>>> Just adding constexpr to std::ignore does ~nothing. The assignment
>>>> operator needs to be made constexpr too; there is really no other
>>>> reason to use std::ignore.
>>>
>>> There is nothing in the resolution of the issue (Nor in the discussion
>>> that argues for the change) that says so. Yes, I considered to make
>>> the assignment function template constexpr, but decided against it,
>>> because the wording of the issue doesn't have this requirement). I'm
>>> also aware of a test-case which would show the difference, but again:
>>> This was not what the issue said. In fact I'm in the process to open a
>>> new library issue that should impose that additional requirement.
>>>
>>> - Daniel
>>
>> Well, technically speaking, the issue resolution doesn't even
>> guarantee that the use case mentioned in the issue would compile since
>> the standard says nothing about whether the copy constructor of
>> decltype(std::ignore) is constexpr, or even whether it can be copied
>> at all. Yes, std::ignore is underspecified, but I'm not sure I see the
>> point in waiting; it's a completely conforming change and IMHO the
>> issue's intent is clearly to make std::ignore fully usable in
>> constexpr functions.
>>
>> Also, the only specification we have for std::ignore's semantics is
>> "when an argument in t is ignore, assigning any value to the
>> corresponding tuple element has no effect". I think one can argue that
>> "causes an expression to be non-constant" is an "effect".
>>
>> Re "new library issue", we already have issue 2933.
>
> Good point, it already exists ;-)

Revised patch attached.

> - Daniel
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ChangeLog_p0607r0.patch
Type: application/octet-stream
Size: 2049 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20170312/48823131/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: p0607r0.patch
Type: application/octet-stream
Size: 34051 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20170312/48823131/attachment-0001.obj>