C++ PATCH for __has_cpp_attribute and fallthrough

[Marek Polacek <polacek@redhat.com>]

This patch updates __has_cpp_attribute for fallthrough, which is now supported.

The system.h hunk should aid people building trunk GCC with older GCC 7 not
supporting this attribute yet.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2016-09-26  Marek Polacek  <polacek@redhat.com>

	* c-lex.c (c_common_has_attribute): Handle attribute fallthrough.

	* system.h: Use __has_attribute to check whether the fallthrough
	attribute is supported.

	* g++.dg/cpp1z/feat-cxx1z.C: Test attribute fallthrough.

diff --git gcc/c-family/c-lex.c gcc/c-family/c-lex.c
index 829c18b..5c6496e 100644
--- gcc/c-family/c-lex.c
+++ gcc/c-family/c-lex.c
@@ -350,7 +350,8 @@ c_common_has_attribute (cpp_reader *pfile)
 	      else if (is_attribute_p ("deprecated", attr_name))
 		result = 201309;
 	      else if (is_attribute_p ("maybe_unused", attr_name)
-		       || is_attribute_p ("nodiscard", attr_name))
+		       || is_attribute_p ("nodiscard", attr_name)
+		       || is_attribute_p ("fallthrough", attr_name))
 		result = 201603;
 	      if (result)
 		attr_name = NULL_TREE;
diff --git gcc/system.h gcc/system.h
index 8ca71cf..0952e4f 100644
--- gcc/system.h
+++ gcc/system.h
@@ -746,8 +746,12 @@ extern void fancy_abort (const char *, int, const char *) ATTRIBUTE_NORETURN;
 #define gcc_unreachable() (fancy_abort (__FILE__, __LINE__, __FUNCTION__))
 #endif
 
-#if GCC_VERSION >= 7000
-# define gcc_fallthrough() __attribute__((fallthrough))
+#if GCC_VERSION >= 7000 && defined(__has_attribute)
+# if __has_attribute(fallthrough)
+#  define gcc_fallthrough() __attribute__((fallthrough))
+# else
+#  define gcc_fallthrough()
+# endif
 #else
 # define gcc_fallthrough()
 #endif
diff --git gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
index 982572e..71c8c7d 100644
--- gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
+++ gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
@@ -370,6 +370,12 @@
 #    error "__has_cpp_attribute(nodiscard) != 201603"
 #  endif
 
+#  if ! __has_cpp_attribute(fallthrough)
+#    error "__has_cpp_attribute(fallthrough)"
+#  elif __has_cpp_attribute(fallthrough) != 201603
+#    error "__has_cpp_attribute(fallthrough) != 201603"
+#  endif
+
 #else
 #  error "__has_cpp_attribute"
 #endif

	Marek