Swift "built-in" singleton

[hooman (Hooman Mehr)]

It needs this feature:

Allow `self = x` in class convenience initializers

In https://github.com/apple/swift/pull/19151, I'm changing code generation for class convenience initializers so that they only have "allocating" entry points, which are responsible for allocating and initializing the entire object. (See Changing class convenience initializers to perform whole object allocation and @objc interop for more background.) With this change, convenience initializers are nearly identical to value or protocol extension initializers at the implementation level, since the s…

I hope @Joe_Groff revisits it.

It has been proposed many times before in different forms. Here is the oldest I could find from 2015:

[Proposal] Factory Initializers

Recently, I proposed the idea of adding the ability to implement the "class cluster" pattern from Cocoa (Touch) in Swift. However, as we discussed it and came up with different approaches, it evolved into a functionality that I believe is far more beneficial to Swift, and subsequently should be the focus of its own proposal. So here is the improved (pre-)proposal: # Factory Initializers The "factory" pattern is common in many languages, including Objective-C. Essentially, instead of initializi…