g++ fails to compile a program that has a template parameter with a boolean expression involving a templated static constant

Issue

• g++ fails to compile a program that has a template parameter with a boolean expression involving a templated static constant

• This problem is seen when the boost library is used in the following manner:

#include <boost/mpl/if.hpp>
#include <boost/mpl/size.hpp>
#include <boost/mpl/vector.hpp>

template < class FieldTypes, class BaseType >
struct PlainTraits
{
  static const std::size_t fields_num = boost::mpl::size< FieldTypes >::value;
  static const std::size_t total_size = fields_num * sizeof( BaseType );

  //enum { can_be_viewed_as_plain = total_size % sizeof( BaseType ) == 0 };
 
  BOOST_MPL_ASSERT( total_size % sizeof( BaseType ) == 0 );
};

PlainTraits< boost::mpl::vector<int,int>, int > x;

• Uncommenting the line in bold above allows the code to compile.

• The compilation error for the above code:

test.cpp: In instantiation of 'PlainTraits<boost::mpl::vector<int, int, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, int>':
test.cpp:16:   instantiated from here
test.cpp:13: error: '((((long unsigned int)PlainTraits<boost::mpl::vector<int, int, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, int>::total_size) & 3ul) == 0ul)' is not a valid template argument for type 'bool' because it is a non-constant expression