Newtype without automatic protocol forwarding

[Douglas_Gregor]

@tomlokhorst

3. Add a Haskell-like deriving syntax

Similar to Haskell, we could add a new language feature where the user can explicitly opt-in to certain protocols by specifing them:

newtype X = CGFloat deriving (FloatingPoint)

In the future, this could also be extended to included the latest state-of-the-art in Haskell, deriving-via: https://www.kosmikus.org/DerivingVia/deriving-via-paper.pdf

Given the prevalence of protocols for expressing common functionality in Swift, I consider a newtype-like facility without some form of (opt-in) forwarding of protocol conformances to be too small to be useful. Indeed, I think the right feature for Swift is to add opt-in forwarding of protocol conformances without adding newtype, per my comments in Opaque result types - #177 by Douglas_Gregor.

Doug