what do you actually mean by overflow? For what happen with a+b, the value stored in a and b cannot be greater than 2^32-1, so it will just store the modulo of 2^32. read this

for your updated question, it will be 4294967294 which is 2^32-2.as 2^32-1+2^32-1 = 2*(2^32) — 2 = -2 (mod 2^32)

Note that some languages (C and C++ are the most notable examples) include the notion of "undefined behavior"

Not for overflowing unsigned integers, though. If he knew about undefined behavior (I doubt it) but didn't know whether unsigned integer overflow was considered such, then google is a friend of his.