RFC: Default language standard mode policy

[Richard Smith via cfe-commits]

On Wed, Jun 29, 2016 at 12:30 PM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> On Wed, Jun 29, 2016 at 12:09 PM, Richard Smith via cfe-commits <
> cfe-commits at lists.llvm.org> wrote:
>
>> Hi all!
>>
>> I'd like to establish a policy for Clang's default language standard (if
>> none is specified with -std), as follows:
>>
>>   Clang defaults to the most recent published standard for the selected
>> language that it fully implements.
>>
>> The practical impact of this is that clang++ will default to C++14 for
>> C++ compilations (for version 3.9 onwards) and will default to C++17 once
>> our implementation support is complete and the standard is published
>> (whichever happens later).
>>
>> I'd suggest that we apply the same policy for clang-cl, but if it's
>> important that we enable a not-yet-fully-implemented standard for cl
>> compatibility, that seems reasonable.
>>
>> The question of whether the default mode for the GCC-compatible driver
>> should be -std=gnuXXX or -std=cXXX is separate, but also likely worth
>> discussing. Enabling GNU keywords by default is a very odd choice, and if
>> we believe we can change our defaults without breaking the world then this
>> seems like a good time to do so.
>>
>
> Using cXXX instead of gnuXXX would break the world on PS4. So for sure
> with `-std=c++latest` we would end up proposing a patch to change this for
> the PS4 target to gnu++latest (or gnu++11 if gnu++latest is not feasible).
>

OK. Out of interest, is it the GNU keywords that you need, or some other
effect of the difference between modes?


> (As I'm sure you're aware, we have pending to upstream a patch that makes
> -std=gnu++11 the default C++ language mode on PS4 (just blocked on fixing
> tests to not assume the existing std)).
>
> It may be feasible for us to use a "-std=gnu++latest" behavior and not
> have to have a PS4-specific default; CC'ing Warren and Paul.
>
> -- Sean Silva
>
>
>>
>> I also intend to make explicit in our documentation that our -std=XXX
>> flag enables the selected standard, *plus all relevant issues in Defect
>> Report status from the relevant language committee* (it doesn't make sense
>> to support a language without its bugfixes).
>>
>> Thoughts?
>>
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>>
>
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