libstdc++: Simplify detection of built-in functions

This fixes a regression affecting the Intel compiler. Because that
compiler defines __GNUC__ to match whatever version of GCC it finds on
the host system, it might claim to be a brand new GCC despite not
actually supporting all the built-ins that the latest GCC supports. This
means the config checks for __GNUC__ don't work. Most recently this
broke when r11-3569-g73ae6eb572515ad627b575a7fbdfdd47a4368e1c switched
us from using __is_same_as to __is_same when __GNUC__ >= 11.

Because __has_builtin is supported by all of GCC, Clang, and Intel we can
use that to reliably detect whether a given built-in is supported,
instead of hardcoding anything based on __GNUC__. The big caveat is
that for versions of Clang <= 9.0.0 and for (as far as I can tell) all
released versions of Intel icc, __has_builtin only evaluates to true for
built-ins with a name starting "__builtin_". For __is_aggregate,
__is_same, and __has_unique_object_representations it's necessary to use
__is_identifier to check if it's a valid identifeir token instead.

The solution used in this patch is to define _GLIBCXX_HAS_BUILTIN and
use that instead of using __has_builtin directly. For compilers that
define __is_identifier as well as __has_builtin we use both, so that if
__has_builtin evaluates to false we try again using !__is_identifier.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX_HAS_BUILTIN): Define macro to
	work around different implementations of __has_builtin.
	(_GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP)
	(_GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE)
	(_GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED)
	(_GLIBCXX_HAVE_BUILTIN_IS_SAME, _GLIBCXX_HAVE_BUILTIN_LAUNDER):
	Define using _GLIBCXX_HAS_BUILTIN.

1 files changed, 26 insertions, 25 deletions

diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index 2e6c880..27302ed 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -653,35 +653,36 @@ namespace std
 #define _GLIBCXX_USE_FLOAT128
 #endif
 
-#if __GNUC__ >= 7
-// Assume these are available if the compiler claims to be a recent GCC:
+#ifdef __has_builtin
+# ifdef __is_identifier
+// Intel and older Clang require !__is_identifier for some built-ins:
+#  define _GLIBCXX_HAS_BUILTIN(B) __has_builtin(B) || ! __is_identifier(B)
+# else
+#  define _GLIBCXX_HAS_BUILTIN(B) __has_builtin(B)
+# endif
+#endif
+
+#if _GLIBCXX_HAS_BUILTIN(__has_unique_object_representations)
 # define _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP 1
+#endif
+
+#if _GLIBCXX_HAS_BUILTIN(__is_aggregate)
 # define _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE 1
-# define _GLIBCXX_HAVE_BUILTIN_LAUNDER 1
-# if __GNUC__ >= 9
-#  define _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED 1
-# endif
-# if __GNUC__ >= 11
-#  define _GLIBCXX_HAVE_BUILTIN_IS_SAME 1
-# endif
-#elif defined(__is_identifier) && defined(__has_builtin)
-// For non-GNU compilers:
-# if ! __is_identifier(__has_unique_object_representations)
-#  define _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP 1
-# endif
-# if ! __is_identifier(__is_aggregate)
-#  define _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE 1
-# endif
-# if __has_builtin(__builtin_launder)
-#  define _GLIBCXX_HAVE_BUILTIN_LAUNDER 1
-# endif
-# if __has_builtin(__builtin_is_constant_evaluated)
+#endif
+
+#if _GLIBCXX_HAS_BUILTIN(__builtin_is_constant_evaluated)
 #  define _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED 1
-# endif
-# if ! __is_identifier(__is_same)
+#endif
+
+#if _GLIBCXX_HAS_BUILTIN(__is_same)
 #  define _GLIBCXX_HAVE_BUILTIN_IS_SAME 1
-# endif
-#endif // GCC
+#endif
+
+#if _GLIBCXX_HAS_BUILTIN(__builtin_launder)
+# define _GLIBCXX_HAVE_BUILTIN_LAUNDER 1
+#endif
+
+#undef _GLIBCXX_HAS_BUILTIN
 
 #if _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
 # define __glibcxx_assert_1(_Condition)		\