Semiautovivification with slices is broken

arr = []
x = arr{}[0, 1] #=> expected [{}, {}], got {}
arr             #=> expected [{}, {}], got [null, null]

The above compiles to

var arr, x, ref$;
arr = [];
x = (ref$ = {}, arr[0] || (arr[0] = ref$[0]), arr[1] || (arr[1] = ref$[1]), ref$);
arr;

but I should get something more like

var arr, x, ref$;
arr = [];
x = [(ref$ = arr)[0] || (ref$[0] = {}), ref$[1] || (ref$[1] = {})];
arr;

right?

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