c-family: Implement __has_feature and __has_extension [PR60512]

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-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.
	* c-objc-common.h (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.
	* cp-objcp-common.h (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.

Co-Authored-By: Iain Sandoe <iain@sandoe.co.uk>

1 files changed, 41 insertions, 0 deletions

diff --git a/gcc/doc/cpp.texi b/gcc/doc/cpp.texi
index 04f357c..7bfe72b 100644
--- a/gcc/doc/cpp.texi
+++ b/gcc/doc/cpp.texi
@@ -3199,6 +3199,8 @@ directive}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}.
 * @code{__has_cpp_attribute}::
 * @code{__has_c_attribute}::
 * @code{__has_builtin}::
+* @code{__has_feature}::
+* @code{__has_extension}::
 * @code{__has_include}::
 @end menu
 
@@ -3561,6 +3563,45 @@ the operator is as follows:
 #endif
 @end smallexample
 
+@node @code{__has_feature}
+@subsection @code{__has_feature}
+@cindex @code{__has_feature}
+
+The special operator @code{__has_feature (@var{operand})} may be used in
+constant integer contexts and in preprocessor @samp{#if} and @samp{#elif}
+expressions to test whether the identifier given in @var{operand} is recognized
+as a feature supported by GCC given the current options and, in the case of
+standard language features, whether the feature is available in the chosen
+version of the language standard.
+
+Note that @code{__has_feature} and @code{__has_extension} are not recommended
+for use in new code, and are only provided for compatibility with Clang.  For
+details of which identifiers are accepted by these function-like macros, see
+@w{@uref{https://clang.llvm.org/docs/LanguageExtensions.html#has-feature-and-has-extension,
+the Clang documentation}}.
+
+@node @code{__has_extension}
+@subsection @code{__has_extension}
+@cindex @code{__has_extension}
+
+The special operator @code{__has_extension (@var{operand})} may be used in
+constant integer contexts and in preprocessor @samp{#if} and @samp{#elif}
+expressions to test whether the identifier given in @var{operand} is recognized
+as an extension supported by GCC given the current options.  In any given
+context, the features accepted by @code{__has_extension} are a strict superset
+of those accepted by @code{__has_feature}.  Unlike @code{__has_feature},
+@code{__has_extension} tests whether a given feature is available regardless of
+strict language standards conformance.
+
+If the @option{-pedantic-errors} flag is given, @code{__has_extension} is
+equivalent to @code{__has_feature}.
+
+Note that @code{__has_feature} and @code{__has_extension} are not recommended
+for use in new code, and are only provided for compatibility with Clang.  For
+details of which identifiers are accepted by these function-like macros, see
+@w{@uref{https://clang.llvm.org/docs/LanguageExtensions.html#has-feature-and-has-extension,
+the Clang documentation}}.
+
 @node @code{__has_include}
 @subsection @code{__has_include}
 @cindex @code{__has_include}