How does the compiler know that the comma in a function call is not a comma operator?

Consider the function call (calling int sum(int, int))

printf("%d", sum(a,b));

How does the compiler decide that the , used in the function call sum(int, int) is not a comma operator?

NOTE: I didn't want to actually use the comma operator in the function call. I just wanted to know how the compiler knows that it is not a comma operator.