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#include <iostream>
using namespace std;
 
class BookID
{
    int id;
public:
    explicit BookID(int id = 0) : id(id) {}
    explicit operator int() const { return id; }
};
 
class CustomerID
{
    int id;
public:
    explicit CustomerID(int id = 0) : id(id) {}
    explicit operator int() const { return id; }
};
 
int main()
{
	BookID bookId;
	CustomerID custId;
 
	bookId = custId; // <-- fails!
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:25:11: error: no match for ‘operator=’ (operand types are ‘BookID’ and ‘CustomerID’)
  bookId = custId; // <-- fails!
           ^~~~~~
prog.cpp:4:7: note: candidate: ‘constexpr BookID& BookID::operator=(const BookID&)’
 class BookID
       ^~~~~~
prog.cpp:4:7: note:   no known conversion for argument 1 from ‘CustomerID’ to ‘const BookID&’
prog.cpp:4:7: note: candidate: ‘constexpr BookID& BookID::operator=(BookID&&)’
prog.cpp:4:7: note:   no known conversion for argument 1 from ‘CustomerID’ to ‘BookID&&’

Standard output is empty