先看定义在头文件中的语句：

template <bool x> struct STATIC_ASSERTION_FAILURE;

template <> struct STATIC_ASSERTION_FAILURE<true> { enum { value = 1 }; };

template<int x> struct static_assert_test{};

…

使用的是显示转换

// Note that the argument to the assert is explicitly cast to bool using old-
// style casts: too many compilers currently have problems with static_cast
// when used inside integral constant expressions.

…

#elif defined(BOOST_MSVC)
#define BOOST_STATIC_ASSERT( B ) \
   typedef ::boost::static_assert_test<\
      sizeof(::boost::STATIC_ASSERTION_FAILURE< (bool)( B ) >)>\ // ***********
         BOOST_JOIN(boost_static_assert_typedef_, __COUNTER__)

关键的地方在于 sizeof() 中的句子。当 (bool)( B ) 为 false 时，因为没有这个特化版本，编译器将报错，如

《Beyond the C++ Standard Library: An Introduction to Boost》中所述：

Error: use of undefined type
  'boost::STATIC_ASSERTION_FAILURE<false>'

BOOST_JOIN(X, Y) 最终解释为 X##Y。__COUNTER__ 在 MSDN 中的描述：

Expands to an integer starting with 0 and incrementing by 1 every time it is used in a compiland.

每使用一次它就增1。