v/vlib/v/fmt/tests/sum_smartcast_keep.vv

struct S1 {
mut:
	i int
}

struct S2 {
}

type Sum = S1 | S2

fn f(sum Sum) {
	if mut sum is S1 {
		sum.i++
	}
	if sum is S1 {
	}
	a := [sum]
	if a[0] is S2 {
	}
}