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#include <iostream>
 
class A
{
public:
	class Key
	{
		Key() {}
		Key(Key const &) {}
	};
 
	A(Key key, int a = 5) {}
};
 
int main() {
	A a(A::Key()); // this compiles !!!
	A a2(A::Key(), 5); // this doesn't
	// somehow defaulting the argument causes the private constructor
	// to be OK - no idea why
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:17:14: error: ‘A::Key::Key()’ is private within this context
  A a2(A::Key(), 5); // this doesn't
              ^
prog.cpp:8:3: note: declared private here
   Key() {}
   ^~~
prog.cpp:17:18: error: ‘A::Key::Key(const A::Key&)’ is private within this context
  A a2(A::Key(), 5); // this doesn't
                  ^
prog.cpp:9:3: note: declared private here
   Key(Key const &) {}
   ^~~
prog.cpp:12:2: note:   initializing argument 1 of ‘A::A(A::Key, int)’
  A(Key key, int a = 5) {}
  ^

Standard output is empty