boost::result_of uses decltype on compilers that implement N3276, merges [77905], [78195], [80352] from trunk

[SVN r80355]

include/boost/utility/detail/result_of_iterate.hpp

@@ -66,7 +66,7 @@ struct cpp0x_result_of_impl<F(BOOST_PP_ENUM_PARAMS(BOOST_PP_ITERATION(),T))>
 
 } // namespace detail 
 
-#else // defined(BOOST_NO_DECLTYPE)
+#else // !defined(BOOST_NO_DECLTYPE) && defined(BOOST_RESULT_OF_USE_DECLTYPE)
 
 #if !BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x551))
 template<typename F BOOST_PP_COMMA_IF(BOOST_PP_ITERATION())
@@ -75,7 +75,7 @@ struct result_of<F(BOOST_RESULT_OF_ARGS)>
     : tr1_result_of<F(BOOST_RESULT_OF_ARGS)> { };
 #endif
 
-#endif // defined(BOOST_NO_DECLTYPE)
+#endif // !defined(BOOST_NO_DECLTYPE) && defined(BOOST_RESULT_OF_USE_DECLTYPE)
 
 #undef BOOST_RESULT_OF_ARGS
 

include/boost/utility/result_of.hpp

@@ -30,6 +30,24 @@
 #  define BOOST_RESULT_OF_NUM_ARGS 10
 #endif
 
+// Use the decltype-based version of result_of by default if the compiler
+// supports N3276 <http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2011/n3276.pdf>.
+// The user can force the choice by defining either BOOST_RESULT_OF_USE_DECLTYPE or
+// BOOST_RESULT_OF_USE_TR1, but not both!
+#if defined(BOOST_RESULT_OF_USE_DECLTYPE) && defined(BOOST_RESULT_OF_USE_TR1)
+#  error Both BOOST_RESULT_OF_USE_DECLTYPE and BOOST_RESULT_OF_USE_TR1 cannot be defined at the same time.
+#endif
+
+#ifndef BOOST_RESULT_OF_USE_TR1
+#  ifndef BOOST_RESULT_OF_USE_DECLTYPE
+#    ifndef BOOST_NO_DECLTYPE_N3276 // this implies !defined(BOOST_NO_DECLTYPE)
+#      define BOOST_RESULT_OF_USE_DECLTYPE
+#    else
+#      define BOOST_RESULT_OF_USE_TR1
+#    endif
+#  endif
+#endif
+
 namespace boost {
 
 template<typename F> struct result_of;