Why is a struct with const members assignable?

Question

I needed to use the std::is_assignable type traits, which happens not to be available in the oldest compiler version that I want to support. Since I had to implement this myself (okay, I admit it, there was some interwebs peeking), I now wonder if this is a bug in my implementation or a general problem of std::is_assignable.

First of all, this is my test struct:

struct Bar {
  bool const cb; // this should kill the default assignment operator
  int i;
};

and here is my stdreplace::is_assignable

template <typename T> struct is_assignable : private std::__sfinae_types {
  private:
    template <typename T1> 
      static decltype(std::declval<T1>() = std::declval<T1>(),__one()) test(int);
    template <typename T1> 
      static __two test(...);
  public:
    static bool const value = sizeof(test<T>(0)) == sizeof(__one);
};

However, it tells me, Bar is assignable:

int: 1
Bar: 1
int[2]: 0

Where is the problem?