From a17884f425f14fbaba88a14410a8332f4ed04e57 Mon Sep 17 00:00:00 2001
From: jzmaddock <john@johnmaddock.co.uk>
Date: Wed, 16 Sep 2015 18:27:48 +0100
Subject: [PATCH] Add support for __unreachable_return on clang and GCC. See
 https://svn.boost.org/trac/boost/ticket/11661.

---
 include/boost/config/compiler/clang.hpp |  5 +++++
 include/boost/config/compiler/gcc.hpp   |  5 +++++
 include/boost/config/suffix.hpp         | 10 ++++++----
 3 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/include/boost/config/compiler/clang.hpp b/include/boost/config/compiler/clang.hpp
index 47ea65b0..897f9f07 100644
--- a/include/boost/config/compiler/clang.hpp
+++ b/include/boost/config/compiler/clang.hpp
@@ -255,6 +255,11 @@
 // All versions with __cplusplus above this value seem to support this:
 #  define BOOST_NO_CXX14_DIGIT_SEPARATORS
 #endif
+//
+// __builtin_unreachable:
+#if defined(__has_builtin) && __has_builtin(__builtin_unreachable)
+#define BOOST_UNREACHABLE_RETURN(x) __builtin_unreachable();
+#endif
 
 
 // Unused attribute:
diff --git a/include/boost/config/compiler/gcc.hpp b/include/boost/config/compiler/gcc.hpp
index 07147ee0..6d32b5bb 100644
--- a/include/boost/config/compiler/gcc.hpp
+++ b/include/boost/config/compiler/gcc.hpp
@@ -278,6 +278,11 @@
 #if __GNUC__ >= 4
 #  define BOOST_ATTRIBUTE_UNUSED __attribute__((unused))
 #endif
+//
+// __builtin_unreachable:
+#if BOOST_GCC_VERSION >= 40800
+#define BOOST_UNREACHABLE_RETURN(x) __builtin_unreachable();
+#endif
 
 #ifndef BOOST_COMPILER
 #  define BOOST_COMPILER "GNU C++ version " __VERSION__
diff --git a/include/boost/config/suffix.hpp b/include/boost/config/suffix.hpp
index 91a62acb..17bf1020 100644
--- a/include/boost/config/suffix.hpp
+++ b/include/boost/config/suffix.hpp
@@ -444,10 +444,12 @@ namespace std {
 // is defined, in which case it evaluates to return x; Use when you have a return
 // statement that can never be reached.
 
-#ifdef BOOST_NO_UNREACHABLE_RETURN_DETECTION
-#  define BOOST_UNREACHABLE_RETURN(x) return x;
-#else
-#  define BOOST_UNREACHABLE_RETURN(x)
+#ifndef BOOST_UNREACHABLE_RETURN
+#  ifdef BOOST_NO_UNREACHABLE_RETURN_DETECTION
+#     define BOOST_UNREACHABLE_RETURN(x) return x;
+#  else
+#     define BOOST_UNREACHABLE_RETURN(x)
+#  endif
 #endif
 
 // BOOST_DEDUCED_TYPENAME workaround ------------------------------------------//