Enforcing invariants in metaprogramming

I'd like to be able to check invariants for classes that are used in metaprograms. My first naive approach was

template <int N>
struct digit
{
  static_assert((N >= 0) && (N < 10), "bad invariant");
};

using boom = digit<99>;

However this compiles without any problems. The static assertion is triggered only when the illegal class is constructed.

It is possible when adding an additional template parameter:

#include <type_traits>

template <int N, 
          typename = typename std::enable_if<(N >= 0) && (N < 10)>::type>
struct digit;

using crash = digit<-7>;

When I wanted to apply this technique to a class that is used as a list of types:

#include <type_traits>

template <typename ...> struct are_integral;

template <typename T, typename ...Ts>
struct are_integral<T, Ts...>
{
  static const bool value = std::is_integral<T>::value &&
                            are_integral<Ts...>::value;
};

template <>
struct are_integral<> : std::true_type { };



template <typename ...Ts,
          typename = typename std::enable_if<are_integral<Ts...>::value>::type>
struct list;

using ok = list<int, long, char>;
using bad = list<double>;

It simply does not work since gcc complains that

error: parameter pack 'Ts' must be at the end of the template parameter list struct list;

Even if it would work, the class is useless as the template parameter pack does not reflect the typelist.

So I tried to use an "illegal" base class:

template <typename> struct check;

template <typename ...Ts>
struct list : check<typename std::enable_if<are_integral<Ts...>::value>::type>
{ };

using ok = list<int, long, char>;
using bad = list<double>;

This compiles without problems.

Is there any way to accomplish something like that in c++11 or do I have to wait for concepts?