PSA: Default C++ dialect is now C++17

[Jeff Law]

On 9/14/20 7:17 PM, Marek Polacek via Gcc-patches wrote:
> On Mon, Sep 14, 2020 at 11:13:18AM -0400, Jason Merrill via Gcc-patches wrote:
>> On Mon, Jun 29, 2020 at 1:25 PM Martin Liška <mliska@suse.cz> wrote:
>>> On 6/29/20 4:57 PM, Marek Polacek wrote:
>>>> On Mon, Jun 29, 2020 at 09:51:57AM +0200, Martin Liška wrote:
>>>>> On 6/26/20 9:34 PM, Marek Polacek via Gcc-patches wrote:
>>>>>> As discussed last month:
>>>>>> <https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545879.html>
>>>>>> it's time to change the C++ default to gnu++17.  I've committed the patch after
>>>>>> testing x86_64-pc-linux-gnu and powerpc64le-unknown-linux-gnu.  Brace yourselves!
>>>>>>
>>>>>> Marek
>>>>>>
>>>>> Just a small note that 510.parest_r SPEC 2017 benchmark can't be built now
>>>>> with default changed to -std=c++17. The spec config needs to be adjusted.
>>>> Interesting, do you know why?  Does it use the register keyword?
>>> Apparently it needs -fno-new-ttp-matching for successful compilation.
>>> There's a reduced test-case I made:
>>>
>>> cat fe.ii
>>> template <int, int> class FiniteElement;
>>> template <int, int> class DoFHandler;
>>> class FETools {
>>>    template <int dim, class InVector, class OutVector, int spacedim>
>>>    void back_interpolate(const DoFHandler<dim, spacedim> &, const InVector &,
>>>                          const FiniteElement<dim, spacedim> &, OutVector &);
>>>    template <int dim, template <int> class DH, class InVector, class OutVector,
>>>              int spacedim>
>>>    void back_interpolate(const DH<dim> &, InVector,
>>>                          const FiniteElement<dim, spacedim> &, OutVector);
>>> };
>>> template <int dim, int = dim> class DoFHandler;
>>> template <int dim, int = dim> class FiniteElement;
>>> template <int dim, class InVector, class OutVector, int spacedim>
>>> void FETools::back_interpolate(const DoFHandler<dim, spacedim> &,
>>>                                 const InVector &,
>>>                                 const FiniteElement<dim, spacedim> &,
>>>                                 OutVector &) {}
>>> template void FETools::back_interpolate(const DoFHandler<3> &, const float &,
>>>                                          const FiniteElement<3> &, float &);
>> Hmm, looks like I never sent this.
>>
>> Further reduced:
>>
>> template <int N, int M = N> class A;
>> template <int N, int M> void fn(A<N, M> &) {}
>> template <int N, template <int> class TT>  void fn(TT<N> &);
>> template void fn(A<3> &);
>>
>> This breaks due to the C++17 changes to template template parameters
>> causing A to now be considered a valid argument for TT; with that
>> change both function templates are valid candidates, and neither is
>> more specialized than the other, so it's ambiguous.
>>
>> There are still some open core issues around these changes.
> Thanks.  I just pushed a patch to introduce GCC 11 porting_to:
> <https://gcc.gnu.org/gcc-11/porting_to.html> and documented this change.
>
> Let me know if you have any comments.

And I just pushed a placeholder into that file for a few other things
I've seen during Fedora testing.  I thought I'd pushed my initial gcc-11
porting_to a month or so ago.  Apparently not :(


jeff

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