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#include <iostream>
#include <optional>
using namespace std;
 
class Foo {
	public:
	int f() { return 5; }
};
 
class Bar : public Foo {
	public:
 
	private:
	using Foo::f;
};
 
int main() {
	std::optional<Bar> bar;
	cout << bar.f() << "\n";
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:18:7: error: ‘optional’ is not a member of ‘std’
  std::optional<Bar> bar;
       ^~~~~~~~
prog.cpp:18:7: note: ‘std::optional’ is only available from C++17 onwards
prog.cpp:18:19: error: expected primary-expression before ‘>’ token
  std::optional<Bar> bar;
                   ^
prog.cpp:18:21: error: ‘bar’ was not declared in this scope
  std::optional<Bar> bar;
                     ^~~
prog.cpp:18:21: note: suggested alternative: ‘Bar’
  std::optional<Bar> bar;
                     ^~~
                     Bar

Standard output is empty