Optional setters for non-optional values

[Nevin]

Possibly related:

Set-only subscripts

Introduction Currently, subscripts can be read-only or they can be read-write, but there is no way to declare a subscript as write-only. Write-only subscripts have a number of important uses, and this proposal aims to bring them into the language. Motivation The MutableCollection protocol overrides the Range subscript from Collection to allow assignment. However, because the return type of that subscript is SubSequence, it follows that the only thing which can be assigned through that subscri…