class OperatorTest {
public:
  OperatorTest() = default;
 
  OperatorTest(int a) : a(a) {}
 
  int a = 0;
};
 
class OperatorInside : public OperatorTest {
public:
  using OperatorTest::OperatorTest;
 
  OperatorInside operator+(const OperatorInside &rhs) {
    const OperatorInside sum(a + rhs.a);
    return sum;
  }
};
 
class OperatorOutside : public OperatorTest {
public:
  using OperatorTest::OperatorTest;
};
 
OperatorOutside operator+(const OperatorOutside &lhs,
                          const OperatorOutside &rhs) {
 
  const OperatorOutside sum(lhs.a + rhs.a);
  return sum;                  	
}
 
int main() {
  OperatorInside ai(1);
  OperatorInside bi(2);
 
  OperatorInside si = ai + bi;
 
  OperatorOutside ao(1);
  OperatorOutside bo(2);
 
  OperatorOutside so = ao + bo;
}

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