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#include <iostream>
using namespace std;
 
struct Widget {
	Widget(int) { cout << "ctor" << endl; }
	Widget(const int&) { cout << "ctor" << endl; }
};
 
struct Gadget {
	Gadget(const int&) { cout << "ctor" << endl; }
	Gadget(int&&) { cout << "ctor" << endl; }
};
 
int main() {
	int x = 5;
	Widget{x};
	Widget{10};
	Gadget{x};
	Gadget{10};
	return 0;
}

prog.cpp: In function 'int main()':
prog.cpp:16:10: error: call of overloaded 'Widget(<brace-enclosed initializer list>)' is ambiguous
  Widget{x};
          ^
prog.cpp:6:2: note: candidate: Widget::Widget(const int&)
  Widget(const int&) { cout << "ctor" << endl; }
  ^
prog.cpp:5:2: note: candidate: Widget::Widget(int)
  Widget(int) { cout << "ctor" << endl; }
  ^
prog.cpp:4:8: note: candidate: constexpr Widget::Widget(const Widget&)
 struct Widget {
        ^
prog.cpp:4:8: note: candidate: constexpr Widget::Widget(Widget&&)
prog.cpp:17:11: error: call of overloaded 'Widget(<brace-enclosed initializer list>)' is ambiguous
  Widget{10};
           ^
prog.cpp:6:2: note: candidate: Widget::Widget(const int&)
  Widget(const int&) { cout << "ctor" << endl; }
  ^
prog.cpp:5:2: note: candidate: Widget::Widget(int)
  Widget(int) { cout << "ctor" << endl; }
  ^
prog.cpp:4:8: note: candidate: constexpr Widget::Widget(const Widget&)
 struct Widget {
        ^
prog.cpp:4:8: note: candidate: constexpr Widget::Widget(Widget&&)

Standard output is empty