foo.cc:4:1: error: ISO C++ forbids declaration of 'yearflag' with no type [-fpermissive]
    4 | yearflag(int year){
      | ^~~~~~~~
foo.cc: In function 'int yearflag(int)':
foo.cc:6:22: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
    6 |         if(year%4==0 && year%100!=0 || year%400==0)
      |            ~~~~~~~~~~^~~~~~~~~~~~~~

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