Tricky C++ question

main.cpp

template<class T, int ... X>
T pi(T(X...));

#include <iostream>

void foo() { std::cout << __LINE__ << '\n'; }
void foo(int) { std::cout << __LINE__ << '\n'; }
template<typename T> void foo(T) { std::cout << __LINE__ << '\n'; }
template<> void foo(int) { std::cout << __LINE__ << '\n'; }
template<typename T>void foo(T *) { std::cout << __LINE__ << '\n'; }
struct S {};
void foo(S) { std::cout << __LINE__ << '\n'; }
struct ConvertibleToInt { ConvertibleToInt(int); };
void foo(ConvertibleToInt) { std::cout << __LINE__ << '\n'; }
namespace N {
namespace M { void foo(char) { std::cout << __LINE__ << '\n'; }}
void foo(double) { std::cout << __LINE__ << '\n'; }
}

int main() {
  foo(1);

  using namespace N::M;
  foo(1);

  return pi<int, 42>;
}