> Doing this is perfectly defined by the standard if X is an unsigned integer, but not if it's signed. That's what doesn't make sense, since they could've made the unsigned case undefined as well.

It does make sense if you understand the historical context: There was a time when some processors still used ones-complement representation for signed integers. That's why the highest performance choice of instructions for signed integer addition would give different results in the overflow case on different processors. Which is why the standard leaves the behaviour undefined, so that compiler writers can choose the instructions that make for the fastest signed addition on the respective target, while programmers can not rely on any specific behaviour.

None of that applies for unsigned integers, and also, the common overflow behaviour of unsigned integers is commonly used intentionally for many types of computations (such as in cryptography), so it is both useful and it doesn't cost anything in terms of performance to define the semantics of unsigned overflow.