[libcxx] r345870 - Use C++11 fallthrough attribute syntax when available and add a break

[Reid Kleckner]

Author: rnk
Date: Thu Nov  1 11:24:03 2018
New Revision: 345870

URL: http://llvm.org/viewvc/llvm-project?rev=345870&view=rev
Log:
Use C++11 fallthrough attribute syntax when available and add a break

Summary:
This silences the two -Wimplicit-fallthrough warnings clang finds in
ItaniumDemangle.h in libc++abi.

Clang does not have a GNU attribute spelling for this attribute, so this
is necessary.

I will commit the same change to the LLVM demangler soon.

Reviewers: EricWF, ldionne

Subscribers: christof, erik.pilkington, cfe-commits

Differential Revision: https://reviews.llvm.org/D53985

Modified:
    libcxx/trunk/include/__config

Modified: libcxx/trunk/include/__config
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/__config?rev=345870&r1=345869&r2=345870&view=diff
==============================================================================
--- libcxx/trunk/include/__config (original)
+++ libcxx/trunk/include/__config Thu Nov  1 11:24:03 2018
@@ -1248,8 +1248,12 @@ _LIBCPP_FUNC_VIS extern "C" void __sanit
 #  define _LIBCPP_DIAGNOSE_ERROR(...)
 #endif
 
-#if __has_attribute(fallthough) || _GNUC_VER >= 700
 // Use a function like macro to imply that it must be followed by a semicolon
+#if __cplusplus > 201402L && __has_cpp_attribute(fallthrough)
+#  define _LIBCPP_FALLTHROUGH() [[fallthrough]]
+#elif __has_cpp_attribute(clang::fallthrough)
+#  define _LIBCPP_FALLTHROUGH() [[clang::fallthrough]]
+#elif __has_attribute(fallthough) || _GNUC_VER >= 700
 #  define _LIBCPP_FALLTHROUGH() __attribute__((__fallthrough__))
 #else
 #  define _LIBCPP_FALLTHROUGH() ((void)0)