foo.c: In function 'main':
foo.c:16:28: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
   16 |         if (year1 % 4 == 0 && year1 % 100 != 0 || year1 % 400 == 0) {
      |             ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
foo.c:21:21: warning: unused variable 'x' [-Wunused-variable]
   21 |                 int x;
      |                     ^
foo.c:32:28: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
   32 |             if (i % 4 == 0 && i % 100 != 0 || i % 400 == 0) {
      |                 ~~~~~~~~~~~^~~~~~~~~~~~~~~
foo.c:39:28: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
   39 |         if (year1 % 4 == 0 && year1 % 100 != 0 || year1 % 400 == 0) {
      |             ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
foo.c:40:20: error: too many arguments to function 'day'
   40 |             sum += day(year2, month2, day2) + 365 - day(year2, month2, day2);
      |                    ^~~
foo.c:2:5: note: declared here
    2 | int day(int year);
      |     ^~~
foo.c:40:53: error: too many arguments to function 'day'
   40 |             sum += day(year2, month2, day2) + 365 - day(year2, month2, day2);
      |                                                     ^~~
foo.c:2:5: note: declared here
    2 | int day(int year);
      |     ^~~
foo.c:43:20: error: too many arguments to function 'day'
   43 |             sum += day(year2, month2, day2) + 366 - day(year2, month2, day2);
      |                    ^~~
foo.c:2:5: note: declared here
    2 | int day(int year);
      |     ^~~
foo.c:43:53: error: too many arguments to function 'day'
   43 |             sum += day(year2, month2, day2) + 366 - day(year2, month2, day2);
      |                                                     ^~~
foo.c:2:5: note: declared here
    2 | int day(int year);
      |     ^~~
foo.c: At top level:
foo.c:48:5: error: conflicting types for 'day'; have 'int(int,  int,  int)'
   48 | int day(int year, int m, int d)
      |     ^~~
foo.c:2:5: note: previous declaration of 'day' with type 'int(int)'
    2 | int day(int year);
      |     ^~~
foo.c: In function 'day':
foo.c:53:23: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
   53 |     if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0) {
      |         ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~

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