foo.cc:9:5: error: ‘optional’ does not name a type
    9 |     optional<Date> inc_month()
      |     ^~~~~~~~
foo.cc: In function ‘bool check(Date)’:
foo.cc:100:23: error: ‘class Date’ has no member named ‘inc_month’
  100 |     auto inc_m = date.inc_month();
      |                       ^~~~~~~~~