[P-Nuts]

To get 2, there are (at least) a couple of ways it can happen, we can do i=0,i++ and get LHS=0, now i=1,++i and get RHS=2. Or we can do i=0,++i and get RHS=1, then i=1,i++ and get LHS=1.

However we’re also allowed to do something like this: i=0, a=i, b=i, b=b+1, RHS=b (RHS=1), LHS=a (LHS=0), a=a+1, i=a, i=b.

Probably quite a lot of other things are allowed to happen. Usual disclaimer that a standard-compliant compiler is allowed to vaporise your cat etc as part of UB.

The thing to Google is “sequence points”.