Are you sure?

>unsequenced side effects on the same scalar object are UB

>6.5.3.3.8 tells us that the evaluations of function arguments are indeterminately sequenced w.r.t. each other.

Read 5.1.2.4.3:

"If A is not sequenced before or after B, then A and B are unsequenced."

"Evaluations A and B are indeterminately sequenced when A is sequenced either before or after B, but it is unspecified which."

With a footnote saying this:

"9)The executions of unsequenced evaluations can interleave. Indeterminately sequenced evaluations cannot interleave, but can be executed in any order."

I.e the standard makes a distinction between "unsequenced" and "indeterminately sequenced". And with no mention of side effects on "indeterminately sequenced" being UB it leads me to conclude that your example is not UB.