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#include <iostream>
#include <string>
 
namespace other {
	enum class Type {
		Type1,
		Type2
	};
 
	std::string to_string(const Type& type) {
		switch(type) {
			case Type::Type1:
				return "Type1";
				break;
			case Type::Type2:
				return "Type2";
				break;
			default:
			{}
		}
 
		return "Unknown";
	}
 
	void run() {
		using namespace std;
		cout << string("Type: ") + to_string(Type::Type1) << endl;
		cout << string("int: " )  + to_string(42) << endl;  // this one generates compile-time errors
	}
}
 
int main() {
	other::run();
 
	using namespace std;
	cout << string("int: " )  + to_string(42) << endl;  // This one is ok
 
	return 0;
}

prog.cpp: In function 'void other::run()':
prog.cpp:28:43: error: invalid initialization of reference of type 'const other::Type&' from expression of type 'int'
   cout << string("int: " )  + to_string(42) << endl;  // this one generates compile-time errors
                                           ^
prog.cpp:10:14: note: in passing argument 1 of 'std::string other::to_string(const other::Type&)'
  std::string to_string(const Type& type) {
              ^

Standard output is empty