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#include <iostream>
 
template <class T, typename U = void> class A;
 
template <class T>
class C
{
public:
	typedef T Var_t;
};
 
template <class T>
class B : public C<T>
{
};
 
template <class T>
class A<B<T>>
{
public:
	A() { std::cout << "Here." << std::endl; }
};
 
template <class T>
class A<T, typename std::enable_if<
                             std::is_base_of<C<typename T::Var_t>, T>::value>
                             //true>
                             ::type>
{
public:
	A() { std::cout << "There." << std::endl;}
};
 
int main() 
{
	A<B<int>> a;
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:36:12: error: ambiguous template instantiation for ‘class A<B<int> >’
  A<B<int>> a;
            ^
prog.cpp:18:7: note: candidates are: template<class T> class A<B<T> > [with T = int]
 class A<B<T>>
       ^~~~~~~
prog.cpp:25:7: note:                 template<class T> class A<T, typename std::enable_if<std::is_base_of<C<typename T::Var_t>, T>::value>::type> [with T = B<int>]
 class A<T, typename std::enable_if<
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                              std::is_base_of<C<typename T::Var_t>, T>::value>
                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                              //true>
                              ~~~~~~~
                              ::type>
                              ~~~~~~~
prog.cpp:36:12: error: aggregate ‘A<B<int> > a’ has incomplete type and cannot be defined
  A<B<int>> a;
            ^

Standard output is empty