C++模板与隐式转换

我有以下代码:

#include <iostream>
#include "boost/shared_ptr.hpp"

using boost::shared_ptr;

class Base {
 public:
  virtual ~Base() {}
  virtual void print() = 0;
};

template <typename T>
class Child : public Base {
 public:
  virtual void print() {
    std::cout << "in Child" << std::endl;
  }
};

class GrandChild : public Child<int> {
 public:
  virtual void print() {
    std::cout << "in GrandChild" << std::endl;
  }
};

template <typename T>
void call_print(shared_ptr<Child<T> > a) {
  a->print();
}

void call_base_print(shared_ptr<Base> a) {
  a->print();
}

int main() {
  shared_ptr<GrandChild> gchild(new GrandChild);
  call_print(shared_ptr<Child<int> >(gchild));
  // call_print(gchild);  // Cannot compile!
  call_base_print(gchild);  // This works.
  return 0;
}

我觉得很奇怪 call_base_print(gchild) call_print(gchild) 导致编译器错误。我知道C++不允许两个隐式转换,但我不认为这里有两个转换。有谁能启发我吗?