Re: [PATCH][RFC] c-family: Implement __has_feature and __has_extension [PR60512]

[Jonathan Wakely <jwakely@redhat.com>]

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On Thu, 11 May 2023 at 21:25, Jason Merrill <jason@redhat.com> wrote:

> On 5/9/23 08:07, Alex Coplan wrote:
> > This patch implements clang's __has_feature and __has_extension in GCC.
>
> Thanks!
>
> > 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.


FWIW, I think the only other C++11 feature that libstdc++ assumes is
unconditionally available in C++98 mode is 'long long' (which is
technically not defined until C99 and C++11).



> 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 },
>
>
>
>