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#include <iostream>
 
struct T1
{
	int x = 0;
};
 
bool operator == (T1 lhs, int rhs)
{
	return lhs.x == rhs;
}
 
bool operator == (int lhs, T1 rhs)
{
	return lhs == rhs.x;
}
 
struct T2
{
	int x;
 
	bool operator == (int rhs)
	{
		return this->x == rhs;
	}
 
	bool operator == (T2 rhs)
	{
		return this->x == rhs.x;
	}
};
 
int main()
{
	std::cout << (T1() == 0) << std::endl;
	std::cout << (0 == T1()) << std::endl;
 
	std::cout << (T2() == 0) << std::endl;
	std::cout << (0 == T2()) << std::endl;
}

prog.cpp: In function ‘int main()’:
prog.cpp:39:18: error: no match for ‘operator==’ (operand types are ‘int’ and ‘T2’)
  std::cout << (0 == T2()) << std::endl;
                  ^
prog.cpp:39:18: note: candidates are:
prog.cpp:8:6: note: bool operator==(T1, int)
 bool operator == (T1 lhs, int rhs)
      ^
prog.cpp:8:6: note:   no known conversion for argument 1 from ‘int’ to ‘T1’
prog.cpp:13:6: note: bool operator==(int, T1)
 bool operator == (int lhs, T1 rhs)
      ^
prog.cpp:13:6: note:   no known conversion for argument 2 from ‘T2’ to ‘T1’

Standard output is empty