Looking at the definition of BOOST_LIKELY as #define BOOST_LIKELY(x) __builtin_expect(x, 1) in boost/config/compiler/{clang,intel,gcc}.hpp, and given that at lest the GCC and Clang __builtin_expect has a first parameter of type long, I wonder (1) whether it would be better to define it as #define BOOST_LIKELY(x) __builtin_expect(bool(x), 1) so that

$ cat test.cc #include "boost/config.hpp" #include "boost/logic/tribool.hpp" int f(boost::tribool b1, boost::tribool b2) { if (b1) return 1; else if (BOOST_LIKELY(b2)) return 2; else return 3; }

$ g++ -c test.cc In file included from .../boost_1_56_0/boost/config.hpp:39:0, from test.cc:1: test.cc: In function ‘int f(boost::logic::tribool, boost::logic::tribool)’: .../boost_1_56_0/boost/config/compiler/gcc.hpp:90:46: error: cannot convert ‘boost::logic::tribool’ to ‘long int’ for argument ‘1’ to ‘long int __builtin_expect(long int, long int)’ #define BOOST_LIKELY(x) __builtin_expect(x, 1) ^ test.cc:5:14: note: in expansion of macro ‘BOOST_LIKELY’ else if (BOOST_LIKELY(b2)) return 2; ^

would not fail (effectively changing the requirement for the argument to BOOST_LIKELY from "[the] expression should result in a boolean value" to "it should be contextually convertible to bool"). (2) if not that, whether to at least wrap the expansion of x in parentheses, #define BOOST_LIKELY(x) __builtin_expect((x), 1) Stephan