Apply patch from #8408.

Fixes #8408.

[SVN r83958]

include/boost/config/compiler/clang.hpp

@@ -38,6 +38,16 @@
 #  define BOOST_SYMBOL_VISIBLE __attribute__((__visibility__("default")))
 #endif
 
+// 
+// The BOOST_FALLTHROUGH macro can be used to annotate implicit fall-through 
+// between switch labels. 
+// 
+#if __cplusplus >= 201103L && defined(__has_warning) 
+#  if __has_feature(cxx_attributes) && __has_warning("-Wimplicit-fallthrough") 
+#    define BOOST_FALLTHROUGH [[clang::fallthrough]] 
+#  endif 
+#endif 
+
 #if !__has_feature(cxx_auto_type)
 #  define BOOST_NO_CXX11_AUTO_DECLARATIONS
 #  define BOOST_NO_CXX11_AUTO_MULTIDECLARATIONS

include/boost/config/suffix.hpp

@@ -883,6 +883,16 @@ namespace std{ using ::type_info; }
 #  define BOOST_NOEXCEPT_IF(Predicate) noexcept((Predicate))
 #  define BOOST_NOEXCEPT_EXPR(Expression) noexcept((Expression))
 #endif
+//
+// Helper macro BOOST_FALLTHROUGH 
+// Fallback definition of BOOST_FALLTHROUGH macro used to mark intended 
+// fall-through between case labels in a switch statement. We use a definition 
+// that requires a semicolon after it to avoid at least one type of misuse even 
+// on unsupported compilers. 
+// 
+#ifndef BOOST_FALLTHROUGH 
+#  define BOOST_FALLTHROUGH ((void)0) 
+#endif 
 
 //
 // constexpr workarounds