Re: [PATCH] c++: Fix up __has_extension (cxx_init_captures)

[Alex Coplan <alex.coplan@arm.com>]

On 28/11/2023 09:22, Jakub Jelinek wrote:
> On Mon, Nov 27, 2023 at 10:58:04AM +0000, Alex Coplan wrote:
> > Many thanks both for the reviews, this is now pushed (with Jason's
> > above changes implemented) as g:06280a906cb3dc80cf5e07cf3335b758848d488d.
> 
> The new test FAILs everywhere with GXX_TESTSUITE_STDS=98,11,14,17,20,2b
> I'm normally using for testing.
> FAIL: g++.dg/ext/has-feature.C  -std=gnu++11 (test for excess errors)
> Excess errors:
> /home/jakub/src/gcc/gcc/testsuite/g++.dg/ext/has-feature.C:185:2: error: #error 
> 
> This is on
> #if __has_extension (cxx_init_captures) != CXX11
> #error
> #endif
> Comparing the values with clang++ on godbolt and with what is actually
> implemented:
> void foo () { auto a = [b = 3]() { return b; }; }
> both clang++ and GCC implement init captures as extension already in C++11
> (and obviously not in C++98 because lambdas aren't implemented there),
> unless -pedantic-errors/-Werror=pedantic, so I think we should change
> the FE to match the test rather than the other way around.
> 
> Tested on x86_64-linux with
> GXX_TESTSUITE_STDS=98,11,14,17,20,23,26 make check-g++ RUNTESTFLAGS="--target_board=unix\{-m32,-m64\} dg.exp='has-feature.C'"
> Ok for trunk?
> 
> Making __has_extension return __has_feature for -pedantic-errors and not
> for -Werror=pedantic is just weird, but as that is what clang++ implements
> and this is for compatibility with it, I can live with it (but perhaps
> we should mention it in the documentation).  Note, the warnings/errors
> can be changed using pragmas inside of the source, so whether one can
> use an extension or not depends on where in the code it is (__extension__
> to the rescue if it can be specified around it).
> I wonder if the has-feature.C test shouldn't be #included in other 2 tests,
> one where -pedantic-errors would be in dg-options and through some macro
> tell the file that __has_extension will behave like __has_feature, and
> another with -Werror=pedantic to document that the option doesn't change
> it.
> 
> 2023-11-28  Jakub Jelinek  <jakub@redhat.com>
> 
> 	* cp-objcp-common.cc (cp_feature_table): Evaluate
> 	__has_extension (cxx_init_captures) to 1 even for -std=c++11.
> 
> --- gcc/cp/cp-objcp-common.cc.jj	2023-11-27 17:34:25.000000000 +0100
> +++ gcc/cp/cp-objcp-common.cc	2023-11-28 08:55:18.868419864 +0100
> @@ -145,7 +145,7 @@ static constexpr cp_feature_info cp_feat
>    { "cxx_contextual_conversions", { cxx14, cxx98 } },
>    { "cxx_decltype_auto", cxx14 },
>    { "cxx_aggregate_nsdmi", cxx14 },
> -  { "cxx_init_captures", cxx14 },
> +  { "cxx_init_captures", { cxx14, cxx11 } },

FWIW it looks like this is what I had in the original RFC here:

https://gcc.gnu.org/pipermail/gcc-patches/2023-May/617878.html

but Jason suggested we be more conservative about what we advertise as
extensions in his review here:

https://gcc.gnu.org/pipermail/gcc-patches/2023-May/618232.html

so it looks like I just missed updating the test when making that change,
and I think it would be better to update the test.

Thanks,
Alex

>    { "cxx_generic_lambdas", cxx14 },
>    { "cxx_relaxed_constexpr", cxx14 },
>    { "cxx_return_type_deduction", cxx14 },
> 
> 
> 	Jakub
>