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

[Jason Merrill <jason@redhat.com>]

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.  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 },