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From: Alex Coplan <alex.coplan@arm.com>
To: Jason Merrill <jason@redhat.com>
Cc: gcc-patches@gcc.gnu.org, Iain Sandoe <iain@sandoe.co.uk>,
	Joseph Myers <joseph@codesourcery.com>,
	Nathan Sidwell <nathan@acm.org>,
	Jonathan Wakely <jwakely@redhat.com>
Subject: Re: [PATCH][RFC] c-family: Implement __has_feature and __has_extension [PR60512]
Date: Thu, 6 Jul 2023 14:58:05 +0100	[thread overview]
Message-ID: <ZKbIbQB1aq33HWMQ@arm.com> (raw)
In-Reply-To: <4d1f72c2-be3c-c548-00b8-d80216898c8a@redhat.com>

Hi Jason,

On 11/05/2023 16:25, Jason Merrill wrote:
> On 5/9/23 08:07, Alex Coplan wrote:
> > This patch implements clang's __has_feature and __has_extension in GCC.
> 
> Thanks!

Thanks a lot for the review, I posted a v2 patch incorporating your
feedback here:
https://gcc.gnu.org/pipermail/gcc-patches/2023-June/623057.html

do the C++ parts of the patch look OK?

Thanks,
Alex

> 
> > Currently the patch aims to implement all documented features (and some
> > undocumented ones) following the documentation at
> > https://clang.llvm.org/docs/LanguageExtensions.html with the following
> > omissions:
> >   - C++ type traits.
> >   - Objective-C-specific features.
> > 
> > C++ type traits aren't currently implemented since, as the clang
> > documentation notes, __has_builtin is the correct "modern" way to query
> > for these (which GCC already implements). Of course there's an argument
> > that we should recognize the legacy set of C++ type traits that can be
> > queried through __has_feature for backwards compatibility with older
> > code. I'm happy to do this if reviewers think that's a good idea.
> 
> That seems unnecessary unless there's a specific motivation.
> 
> > There are some comments in the patch marked with XXX, I'm looking for
> > review comments from C/C++ maintainers on those areas in particular.
> > 
> > Bootstrapped/regtested on aarch64-linux-gnu. Any comments?
> 
> All the has_*_feature_p functions need to check flag_pedantic_errors, for
> compatibility with the Clang documented behavior "If the -pedantic-errors
> option is given, __has_extension is equivalent to __has_feature."
> 
> > +static const cp_feature_info cp_feature_table[] =
> > +{
> > +  { "cxx_exceptions", &flag_exceptions },
> > +  { "cxx_rtti", &flag_rtti },
> > +  { "cxx_access_control_sfinae", { cxx11, cxx98 } },
> > +  { "cxx_alias_templates", cxx11 },
> > +  { "cxx_alignas", cxx11 },
> > +  { "cxx_alignof", cxx11 },
> > +  { "cxx_attributes", cxx11 },
> > +  { "cxx_constexpr", cxx11 },
> > +  { "cxx_constexpr_string_builtins", cxx11 },
> > +  { "cxx_decltype", cxx11 },
> > +  { "cxx_decltype_incomplete_return_types", cxx11 },
> > +  { "cxx_default_function_template_args", cxx11 },
> > +  { "cxx_defaulted_functions", cxx11 }, /* XXX: extension in c++98?  */
> 
> I'm not sure I see the benefit of advertising a lot of these as C++98
> extensions, even if we do accept them with a pedwarn by default.  The ones
> that indicate DRs like cxx_access_control_sfinae, yes, but I'm inclined to
> be conservative if it isn't an extension that libstdc++ relies on, like
> variadic templates or inline namespaces.  My concern is that important
> implementation is limited to C++11 mode even if we don't immediately give an
> error.  For instance,
> 
> struct A
> {
>   int i = 42;
>   A() = default;
> };
> 
> breaks in C++98 mode; even though we only warn for the two C++11 features,
> trying to actually combine them fails.
> 
> So if there's a question, let's say no.
> 
> > +  { "cxx_delegating_constructors", { cxx11, cxx98 } },
> > +  { "cxx_deleted_functions", cxx11 },
> > +  { "cxx_explicit_conversions", { cxx11, cxx98 } },
> > +  { "cxx_generalized_initializers", cxx11 },
> > +  { "cxx_implicit_moves", cxx11 },
> > +  { "cxx_inheriting_constructors", cxx11 }, /* XXX: extension in c++98?  */
> > +  { "cxx_inline_namespaces", { cxx11, cxx98 } },
> > +  { "cxx_lambdas", cxx11 }, /* XXX: extension in c++98?  */
> > +  { "cxx_local_type_template_args", cxx11 },
> > +  { "cxx_noexcept", cxx11 },
> > +  { "cxx_nonstatic_member_init", { cxx11, cxx98 } },
> > +  { "cxx_nullptr", cxx11 },
> > +  { "cxx_override_control", { cxx11, cxx98 } },
> > +  { "cxx_reference_qualified_functions", cxx11 },
> > +  { "cxx_range_for", cxx11 },
> > +  { "cxx_raw_string_literals", cxx11 },
> > +  { "cxx_rvalue_references", cxx11 },
> > +  { "cxx_static_assert", cxx11 },
> > +  { "cxx_thread_local", cxx11 },
> > +  { "cxx_auto_type", cxx11 },
> > +  { "cxx_strong_enums", cxx11 },
> > +  { "cxx_trailing_return", cxx11 },
> > +  { "cxx_unicode_literals", cxx11 },
> > +  { "cxx_unrestricted_unions", cxx11 },
> > +  { "cxx_user_literals", cxx11 },
> > +  { "cxx_variadic_templates", { cxx11, cxx98 } },
> > +  { "cxx_binary_literals", { cxx14, cxx98 } },
> > +  { "cxx_contextual_conversions", { cxx14, cxx98 } },
> > +  { "cxx_decltype_auto", cxx14 },
> > +  { "cxx_aggregate_nsdmi", cxx14 },
> > +  { "cxx_init_captures", { cxx14, cxx11 } },
> > +  { "cxx_generic_lambdas", cxx14 },
> > +  { "cxx_relaxed_constexpr", cxx14 },
> > +  { "cxx_return_type_deduction", cxx14 },
> > +  { "cxx_variable_templates", { cxx14, cxx98 } },
> > +  { "modules", &flag_modules },
> 
> 
> 

  parent reply	other threads:[~2023-07-06 13:58 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-05-09 12:07 Alex Coplan
2023-05-11 20:25 ` Jason Merrill
2023-05-11 21:26   ` Jonathan Wakely
2023-07-06 13:58   ` Alex Coplan [this message]
2023-05-14 16:05 ` Iain Sandoe
2023-06-20 12:30   ` Alex Coplan
2023-06-20 14:08     ` Iain Sandoe
2023-07-06 14:01       ` Alex Coplan
2023-07-06 15:23         ` Iain Sandoe

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