Annex F (informative) Core undefined behavior [ub]

F.2 [basic]: Basics [ub.basic]

F.2.18[ub:intro.execution.unsequenced.modification]
Specified in: [intro.execution]

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If a side effect on a memory location ([intro.memory]) is unsequenced relative to either another side effect on the same memory location or a value computation using the value of any object in the same memory location, and they are not potentially concurrent ([intro.multithread]), the behavior is undefined.
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[Example 1: void g(int i) { i = 7, i++, i++; // i becomes 9 i = i++ + 1; // the value of i is incremented i = i++ + i; // undefined behavior i = i + 1; // the value of i is incremented } — end example]