Why is unsigned integer overflow defined behavior but signed integer overflow isn't?

Why is unsigned integer overflow defined behavior but signed integer

overflow isn't?



Unsigned integer overflow is well defined by both the C and C++ standards.

For example, the C99 standard (§6.2.5/9) states

A computation involving unsigned operands can never overflow,

because a result that cannot be represented by the resulting unsigned

integer type is reduced modulo the number that is one greater than the

largest value that can be represented by the resulting type.

However, both standards state that signed integer overflow is undefined

behavior. Again, from the C99 standard (§3.4.3/1)

An example of undefined behavior is the behavior on integer

overflow

Is there an historical or (even better!) a technical reason for this

discrepancy?