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#include <iostream>
using namespace std;
 
template<size_t>
class intWrapper
{
    int value;
public:
    explicit intWrapper(int value = 0) : value(value) {}
    explicit operator int() const { return value; }
    ...
};
 
using BookID = intWrapper<1>;
using CustomerID = intWrapper<2>;
 
int main()
{
	BookID bookId;
	CustomerID custId;
 
	bookId = custId; // <-- fails!
	return 0;
}

prog.cpp:11:5: error: expected unqualified-id before ‘...’ token
     ...
     ^~~
prog.cpp: In function ‘int main()’:
prog.cpp:22:11: error: no match for ‘operator=’ (operand types are ‘BookID’ {aka ‘intWrapper<1>’} and ‘CustomerID’ {aka ‘intWrapper<2>’})
  bookId = custId; // <-- fails!
           ^~~~~~
prog.cpp:5:7: note: candidate: ‘constexpr intWrapper<1>& intWrapper<1>::operator=(const intWrapper<1>&)’
 class intWrapper
       ^~~~~~~~~~
prog.cpp:5:7: note:   no known conversion for argument 1 from ‘CustomerID’ {aka ‘intWrapper<2>’} to ‘const intWrapper<1>&’
prog.cpp:5:7: note: candidate: ‘constexpr intWrapper<1>& intWrapper<1>::operator=(intWrapper<1>&&)’
prog.cpp:5:7: note:   no known conversion for argument 1 from ‘CustomerID’ {aka ‘intWrapper<2>’} to ‘intWrapper<1>&&’

Standard output is empty