To begin with, array-assignment appears Ok.
But if an array is a property of some other object then it is red-underlined with the tooltip "no viable alternative at input '='".
@a[i] = x
this.a[i] = x
any.nested.array[i] = x
this[i] = x
The last of those also causes an error even though it is valid, it is probably the same rule which legitimately highlights these bad statements.
null[i] = x
undefined[i] = x