SE-0024: Optional Value Setter `??=`

Introduction

Introduce a new operator an "Optional Value Setter". If the optional is set via this operator then the new value is only set if there isn't an already existing value.

Swift-evolution thread: link to the discussion thread for that proposal

Motivation

In certain cases the ?? operation doesn't help with lengthy variable names, i.e., really.long.lvalue[expression] = really.long.lvalue[expression] ?? "". In addtition to this other languages such as Ruby contain a pipe operator really.long.lvalue[expression] ||= "" which works the same way and which is very popular. This lowers the barrier of entry for programmers from that language.

In the interest in conciseness and clarity I feel this would be a great addition to swift and would bring the length of that previous statement from

really.long.lvalue[expression] = really.long.lvalue[expression] ?? ""

to

really.long.lvalue[expression] ??= ""

Proposed solution

In this solution an optonals value wouldn't be set if it already contains a value (i.e .Some), ideally willSet and didSet are only called if this operation occurs.

var itemsA:[Item]? = nil
var itemsB:[Item]? = [Item()]

itemsA ??= [] // itemsA is set since its value is .None
itemsB ??= [] // itemsB's value isn't changed since its value is .Some

Impact on existing code

Since this is a strictly additive and optional feature, it won't affect existing code and should make code more concise going forward.

Alternatives considered

Other syntaxes included ?= but we felt it didn't match the convention of the ?? in a = a ?? "".