make_shared (and make_unique) for classes with private constructors

construct_tag

class Test
{
private:
	struct _constructor_tag { explicit _constructor_tag() = default; };

public:
	Test(_constructor_tag) {}

	static unique_ptr<Test> factory()
	{
		return make_unique<Test>(_constructor_tag{});
	}
};

void test()
{
  auto t1 = Test::factory(); // GOOD
  auto t2 = make_unique<Test>(); // ERROR
  auto t3 = make_unique<Test>()(Test::_constructor_tag); // ERROR
}