Suppose we have two template class and two specialized class

template< typename C >
class B {
 public: void inc( C & x ) { ++x; }
 };

template< template< typename > typename A , typename C >
 class One {
 C a{};
 void inc( void ) { A< C >::inc( a ); }
 };

// version one
template< template< typename > typename , typename C >
class One< B< C > , C >  {
void inc2( void ) { inc(); inc(); }
 };

// version two
template< typename C >
class One< B , C >  {
 void inc2( void ) { inc(); inc(); }
 };

int main(){

One< B<int>, int> _one;

One< B, int> _two;

}

If i compile commenting version two i obtain:

error: template argument for template template parameter must be a class     template or type alias template
class One< B< C > , C >  {

i.e One doesn’t know that B< C > is a template.

Otherwise, if i compile commenting version one i obtain:

error: use of undeclared identifier 'inc'; did you mean 'inc2'?
 void inc2( void ) { inc(); inc(); }
                    ^~~
                     inc2 

i.e One< B, C > isn’t a specialized version of One< B< C >, C >

Why this behaviour happens?