/in/foo.c:1:1: error: unknown type name 'def'
    1 | def days_in_month(month, year):
      | ^~~
/in/foo.c: In function 'days_in_month':
/in/foo.c:1:31: error: expected declaration specifiers before ':' token
    1 | def days_in_month(month, year):
      |                               ^
/in/foo.c:1:5: warning: type of 'month' defaults to 'int' [-Wimplicit-int]
    1 | def days_in_month(month, year):
      |     ^~~~~~~~~~~~~
/in/foo.c:1:5: warning: type of 'year' defaults to 'int' [-Wimplicit-int]
/in/foo.c:20: warning: control reaches end of non-void function [-Wreturn-type]