Subscripts Much of this discussion also applies to subscripts, though in the language today subscripts are never technically stored. Accessing a component of a value type through a subscript is treated as accessing the entire value, and so is considered to overlap any other access to the value. The most important consequence of this is that two different array elements cannot be simultaneously accessed. This will interfere with certain common idioms for working with arrays, although some cases (like concurrently modifying different slices of an array) are already quite problematic in Swift. We believe that we can mitigate the majority of the impact here with targeted improvements to the collection APIs.