Let’s take the code
int a, b, c; ... if ((a + b) > C)
If we add the values of a and b and the sum exceeds the maximum value of an int, will the integrity of the comparison be compromised? I was thinking that there might be an implicit up cast or overflow bit check and that will be factored into the evaluation of this expression.
C will do no such thing. It will silently overflow and lead to a possibly incorrect comparison. You can up-cast yourself, but it will not be done automatically.