#include <iostream>
 
struct ostream {} cout; 
template<typename T> struct A{ T t; };
struct B{};
 
namespace {
template<typename T>
ostream& operator<< (ostream& out, const A<T>&v) 
  { return out << v.t; }
 
ostream& operator<< (ostream& out, const B&) 
  { std::cout << "This should not compile" << std::endl; return out; }
 
void foo (const B &b) { std::cout << "hello" << std::endl; }
 
template<typename T>
void foo (const A<T> &a) { foo(a.t); }
}
 
int main(){
  A<B> a;
  cout << a;
  foo(a);
}

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