#include <iostream>
#include <string>

template <typename T>
class IObserver {
 public:
  virtual void Next(const T& value) noexcept = 0;
};

template <typename T1, typename T2>
class BinaryObserver : public IObserver<T1>, public IObserver<T2> {
 public:
  void Next(const T1& value) noexcept override{};

  void Next(const T2& value) noexcept override{};
};

int main() {
  // This is OK
  BinaryObserver<int, float> mc1;
  mc1.Next(0);
  mc1.Next(0.0f);

  // This fails to compile with "multiple overloads"
  BinaryObserver<int, int> mc2;
  mc2.Next(0);
  mc2.Next(0);
}

template <typename T>
class BinaryObserver<T, T> : public IObserver<T> {
 public:
  void Next(const T & value) noexcept override{};
};

BinaryObserver<int, int> mc2;
mc2.Next(0);
mc2.Next(0);

class BinaryObserver : public IObserver<T1>, public IObserver1<T2>
...
mc2.Next(0);
mc2.Next1(0);

template <typename T1, typename T2>
class BinaryObserver : public IObserver<T1>, public IObserver<Shell<T2>> {
 public:
  void Next(const T1& value) noexcept override{T1 t = value;};
  void Next(const Shell<T2>& value) noexcept override{T2 t = value;};
};

...

  BinaryObserver<int, float> mc1;
  mc1.Next(0);
  mc1.Next(0.0f);

  BinaryObserver<int, int> mc2;
  mc2.Next(0);
  mc2.Next(Shell<int>(0));