Moving the kernel to modern C

Agreed that the implicit promotions and convert rules in C is a bit complex (and actually it would be nice if there where a tool one could use to highlight an expression and have the tool explain exactly what happens in this regard) but they are at least specified in the standard.

For your example at hand with "a + b" where we have "int a" and "unsigned int b" and a have a negative value then since both are of the same rank (integers) but different types (signed vs unsigned) the signed integer is implicitly converted to an unsigned integer. Overflow is only undefined for the signed integer case while defined to wrap around for the unsigned integer which is why most of the "a + b" code works as intended.

Aka if a is -1 and b is 2 then -1 is "converted" to 0xFFFFFFFF, add +2 and we wrap around to 1. In practice the compiler simply moves both a register and does the x86 ADD instruction since converting signed to unsigned of the same size is a no op and a binary add in particular is signedness agnostic.