Why is this Undefined Behavior?

Question

Why does the following given expression invoke undefined behavior?

int i = 5;
i = (i,i++,i) + 1 

My question is influenced by Als\' questio

Answer 1

It is undefined in C++ to assign an incremented value to itself:

i = i++

What should i be after this? Should it be the previous value or one plus the previous value? The order of execution is left to the compiler, so different platforms will have different results.