Hi,

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 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.

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?

Thanks,
Alex

gcc/c-family/ChangeLog:

	PR c++/60512
	* c-common.cc (struct hf_feature_info): New.
	(has_generic_feature_p): New.
	* c-common.h (c_common_has_feature): New.
	(has_generic_feature_p): New.
	(has_lang_feature_p): New.
	* c-lex.cc (init_c_lex): Plumb through has_feature callback.
	(c_common_has_builtin): Adapt into more generic helper function.
	Rename to ...
	(c_common_lex_availability_macro): ... this.
	(c_common_has_feature): New.
	* c-ppoutput.cc (init_pp_output): Plumb through has_feature callback.

gcc/c/ChangeLog:

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

gcc/cp/ChangeLog:

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

gcc/ChangeLog:

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

libcpp/ChangeLog:

	PR c++/60512
	* include/cpplib.h (struct cpp_callbacks): Add has_feature callback.
	(enum cpp_builtin_type): Add types for __has_{feature,extension}.
	* init.cc (builtin_array): Add entries for __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.
	* g++.dg/ext/has-feature.C: New test.
	* gcc.dg/asan/has-feature-asan.c: New test.
	* gcc.dg/has-feature.c: New test.