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