jtemples

All decimal constants are signed.

In C90, an unsuffixed decimal constant can be "unsigned long".

That brings me to this question. In C99, is the unsuffixed decimal literal  18446744073709551615  an "unsigned long long" or it's out of range?
 

XC8 v2 in C99 mode gives this warning:
 
warning: integer literal is too large to be represented in a signed integer type, interpreting as unsigned [-Wimplicitly-unsigned-literal]

jtemples
I don't know if XC8 C99 has a "pedantic" mode like gcc does

 
Yes, there is: you have to set warning levels at the minimum level possible and enable the treat warning as errors option. Annoying because it throws all the (752) warnings
 
Annoying like you have to cast every single array index, let it be literal or variable
 
-mwarn=level (from -9 to 9)
-Wpedantic (which will warning that the c library may throw errors)
 
no option to treat warnings as errors though
 

Ric is of course correct, unfortunately you don't know which 'assumptions to adjust' until one of them jumps up and bites you.   ;-)