foo.cc:41:5: error: no declaration matches 'int Date::operator-(const Date&)'
   41 | int Date::operator -(const Date &date2)
      |     ^~~~
foo.cc:11:8: note: candidate is: 'int& Date::operator-(const Date&)'
   11 |   int& operator -(const Date&);
      |        ^~~~~~~~
foo.cc:4:7: note: 'class Date' defined here
    4 | class Date
      |       ^~~~

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