C++ recursive template type deduction

I'm interested in learning a little about template meta-programming. In the code below I'm attempting to find an unsigned integral type large enough to hold N bits, specified at compile time, using some template recursion. ``` template <typename T> struct NextIntegralType { }; template <> struct NextIntegralType<void> { typedef unsigned char type; }; template <> struct NextIntegralType<unsigned char> { typedef unsigned short type; }; ...More type 'iteration' here... template<size_t BITS, typename T> struct FindIntegralType2 { typedef std::conditional<BITS <= sizeof(typename T::type)*8, T, FindIntegralType2<BITS, NextIntegralType<typename T::type>>> _type; typedef typename _type::type type; }; template<size_t BITS> struct FindIntegralType { typedef typename FindIntegralType2<BITS, NextIntegralType<void>>::type type; }; ``` When I declare a variable and assign an integral value to it... ``` FindIntegralType<15>::type test(4000); ``` I get the following: ``` error: no matching function for call to ‘FindIntegralType2<15u, NextIntegralType<unsigned char> >::FindIntegralType2(int)’ note: candidates are: note: constexpr FindIntegralType2<15u, NextIntegralType<unsigned char> >::FindIntegralType2() note: candidate expects 0 arguments, 1 provided note: constexpr FindIntegralType2<15u, NextIntegralType<unsigned char> >::FindIntegralType2(const FindIntegralType2<15u, NextIntegralType<unsigned char> >&) note: no known conversion for argument 1 from ‘int’ to ‘const FindIntegralType2<15u, NextIntegralType<unsigned char> >&’ ``` It seems like my recursion isn't 'unwinding'. Can anyone point me in the right direction? Note: I'm using GCC 4.6. EDIT: I found a post that I missed before: Automatically pick a variable type big enough to hold a specified number Which points to an answer in boost (where they always are): boost_integer This should solve both my practical need and intellectual curiosity.