Pitch: Allow Protocols to be Nested in Non-Generic Contexts

[anon9791410]

@Karl

func doSomething() {  
   func impl<T: Abstraction>(_ input: T) -> T.ResultType {
     // ...
   }
}

This pitch is good, but should at least come with a caveat about this spelling. Outside of a function, it would be a protocol extension.

fileprivate extension Abstraction {
  var impl: ResultType { // …
}

Without nested protocol extensions, switching to the function form will be necessary, trading good spelling for good scoping. So I don't think this pitch should be implemented without being paired with nested extensions.

[Pre-pitch] Allow extension of nested members within extension of parent

This is a tiny syntactic sugar pitch. Swift's ability to group code by use of extension is probably one of the best features of Swift and something Apple themselves use a lot, combined with Swift's ability to nest types these two language features make up the backbone of what we love about Swift. Today extensions are limited to global scope, I cannot extend a nested type within an extension of the enclosing parent type. This makes code hard to read, I would argue. Simple case study (sorry for…