template<int i>
struct place_holder{};
place_holder<1> _1;
place_holder<2> _2;

template<typename T>
struct is_placeholder:public integral_constant<int,0>{};
template<int i>
struct is_placeholder<place_holder<i> >:public integral_constant<int,i>{};

error: explicit specialization of non-template struct 'is_placeholder'
struct is_placeholder<place_holder<i> >:public integral_constant<int,i>{};
   ^             ~~~~~~~~~~~~~~~~~~
 1 error generated.