Thic code (extract from Boost.Xpressive) fails to compile.
Comeau succeeds, as well as GCC 3.4.5.

----------------a.cpp----------------------
template<typename T>
struct is_op;

template<typename Node, bool IsOp = is_op<Node>::value> struct tag_type;

int main()
{
  return 0;
}
---------------------------------------

/Pavel