Add helper macro - BOOST_ATTRIBUTE_UNUSED - to suppress unused type/variable warnings.

doc/macro_reference.qbk

@@ -1152,6 +1152,8 @@ Usage example:
     handle_error("ptr is NULL");
 ``
 ]]
+[[`BOOST_ATTRIBUTE_UNUSED`][Expands to `__attribute__((unused))` when this is available - 
+can be used to disable compiler warnings relating to unused types or variables.]]
 ]
 
 [endsect]

include/boost/config/compiler/clang.hpp

@@ -205,8 +205,10 @@
 #  define BOOST_NO_CXX11_FINAL
 #endif
 
-// Clang always supports variadic macros
+// Unused attribute:
-// Clang always supports extern templates
+#if defined(__GNUC__) && (__GNUC__ >= 4)
+#  define BOOST_ATTRIBUTE_UNUSED __attribute__((unused))
+#endif
 
 #ifndef BOOST_COMPILER
 #  define BOOST_COMPILER "Clang version " __clang_version__

include/boost/config/compiler/gcc.hpp

@@ -235,6 +235,12 @@
 #  define BOOST_NO_CXX11_REF_QUALIFIERS
 #endif
 
+//
+// Unused attribute:
+#if __GNUC__ >= 4
+#  define BOOST_ATTRIBUTE_UNUSED __attribute__((unused))
+#endif
+
 #ifndef BOOST_COMPILER
 #  define BOOST_COMPILER "GNU C++ version " __VERSION__
 #endif

include/boost/config/suffix.hpp

@@ -940,6 +940,13 @@ namespace std{ using ::type_info; }
 #define BOOST_CONSTEXPR_OR_CONST constexpr
 #endif
 
+//
+// Unused variable/typedef workarounds:
+//
+#ifndef BOOST_ATTRIBUTE_UNUSED
+#  define BOOST_ATTRIBUTE_UNUSED
+#endif
+
 #define BOOST_STATIC_CONSTEXPR  static BOOST_CONSTEXPR_OR_CONST
 
 //

test/Jamfile.v2

@@ -89,6 +89,7 @@ test-suite config
     [ compile-fail threads/test_thread_fail1.cpp ]
     [ compile-fail threads/test_thread_fail2.cpp ]
     [ compile boost_fallthrough_test.cpp : [ check-target-builds has_clang_implicit_fallthrough "Clang implicit fallthrough" : <toolset>clang:<cxxflags>"-std=c++11 -Wimplicit-fallthrough" <warnings-as-errors>on <warnings>all ] ]
+    [ compile helper_macro_test.cpp ]
     [ run cstdint_test.cpp : : : <warnings>all <toolset>gcc:<cxxflags>"-Wno-long-long -Wextra" <toolset>darwin:<cxxflags>-Wno-long-long ]
     [ run cstdint_test2.cpp : : : <warnings>all <toolset>gcc:<cxxflags>"-Wno-long-long -Wextra" <toolset>darwin:<cxxflags>-Wno-long-long ]
     [ compile cstdint_include_test.cpp : <warnings>all <toolset>gcc:<cxxflags>-Wextra ]

test/helper_macro_test.cpp

@@ -0,0 +1,60 @@
+//  Use, modification and distribution are subject to the  
+//  Boost Software License, Version 1.0. (See accompanying file  
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 
+
+#include <boost/config.hpp>
+
+int test_fallthrough(int n) 
+{ 
+   switch (n) 
+   { 
+   case 0: 
+      n++; 
+      BOOST_FALLTHROUGH; 
+   case 1: 
+      n++; 
+      break; 
+   } 
+   return n; 
+}
+
+int test_unreachable(int i)
+{
+   if(BOOST_LIKELY(i)) return i;
+
+   throw i;
+   BOOST_UNREACHABLE_RETURN(0);
+}
+
+BOOST_FORCEINLINE int always_inline(int i){ return ++i; }
+BOOST_NOINLINE int never_inline(int i){ return ++i; }
+
+BOOST_NORETURN void always_throw()
+{
+   throw 0;
+}
+
+
+#define test_fallthrough(x) foobar(x)
+
+
+int main()
+{
+   typedef int unused_type BOOST_ATTRIBUTE_UNUSED;
+   try
+   {
+      int result = test_fallthrough BOOST_PREVENT_MACRO_SUBSTITUTION(0);
+      BOOST_STATIC_CONSTANT(bool, value = 0);
+      result += test_unreachable(1);
+      result += always_inline(2);
+      result += never_inline(3);
+      if(BOOST_UNLIKELY(!result))
+         always_throw();
+   }
+   catch(int)
+   {
+      return 1;
+   }
+   return 0;
+}
+