Hi,

This is a v5 patch to address Marek's feedback here:
https://gcc.gnu.org/pipermail/gcc-patches/2023-November/635157.html

I also implemented Jason's suggestion to use constexpr for the tables
from this review:
https://gcc.gnu.org/pipermail/gcc-patches/2023-October/634484.html

I'll attach the incremental change in reply to Marek's review to make
things easier to compare.

Bootstrapped/regtested on aarch64-linux-gnu.  Bootstrap/regtest on
x86_64-apple-darwin in progress (on top of this libsanitizer fix:
https://github.com/llvm/llvm-project/issues/72639).

OK for trunk if testing passes?

Thanks,
Alex

-- >8 --

This patch implements clang's __has_feature and __has_extension in GCC.
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 exception
of the legacy features for C++ type traits.  These are omitted, since as
the clang documentation notes, __has_builtin is the correct "modern" way
to query for these (which GCC already implements).

gcc/c-family/ChangeLog:

        PR c++/60512
        * c-common.cc (struct hf_feature_info): New.
        (c_common_register_feature): New.
        (init_has_feature): New.
        (has_feature_p): New.
        * c-common.h (c_common_has_feature): New.
        (c_family_register_lang_features): New.
        (c_common_register_feature): New.
        (has_feature_p): New.
        (c_register_features): New.
        (cp_register_features): New.
        * c-lex.cc (init_c_lex): Plumb through has_feature callback.
        (c_common_has_builtin): Generalize and move common part ...
        (c_common_lex_availability_macro): ... here.
        (c_common_has_feature): New.
        * c-ppoutput.cc (init_pp_output): Plumb through has_feature.

gcc/c/ChangeLog:

        PR c++/60512
        * c-lang.cc (c_family_register_lang_features): New.
        * c-objc-common.cc (struct c_feature_info): New.
        (c_register_features): New.

gcc/cp/ChangeLog:

        PR c++/60512
        * cp-lang.cc (c_family_register_lang_features): New.
        * cp-objcp-common.cc (struct cp_feature_selector): New.
        (cp_feature_selector::has_feature): New.
        (struct cp_feature_info): New.
        (cp_register_features): New.

gcc/ChangeLog:

        PR c++/60512
        * doc/cpp.texi: Document __has_{feature,extension}.

gcc/objc/ChangeLog:

        PR c++/60512
        * objc-act.cc (struct objc_feature_info): New.
        (objc_nonfragile_abi_p): New.
        (objc_common_register_features): New.
        * objc-act.h (objc_common_register_features): New.
        * objc-lang.cc (c_family_register_lang_features): New.

gcc/objcp/ChangeLog:

        PR c++/60512
        * objcp-lang.cc (c_family_register_lang_features): New.

libcpp/ChangeLog:

        PR c++/60512
        * include/cpplib.h (struct cpp_callbacks): Add has_feature.
        (enum cpp_builtin_type): Add BT_HAS_{FEATURE,EXTENSION}.
        * init.cc: Add __has_{feature,extension}.
        * macro.cc (_cpp_builtin_macro_text): Handle
        BT_HAS_{FEATURE,EXTENSION}.

gcc/testsuite/ChangeLog:

        PR c++/60512
        * c-c++-common/has-feature-common.c: New test.
        * c-c++-common/has-feature-pedantic.c: New test.
        * g++.dg/ext/has-feature.C: New test.
        * gcc.dg/asan/has-feature-asan.c: New test.
        * gcc.dg/has-feature.c: New test.
        * gcc.dg/ubsan/has-feature-ubsan.c: New test.
        * obj-c++.dg/has-feature.mm: New test.
        * objc.dg/has-feature.m: New test.