[Pitch #3] SE-0293: Extend Property Wrappers to Function and Closure Parameters

Evolution Pitches
60

Just thinking out load. This could still be allowed in the future right?

Some bikeshedding:

enum E {
  case foo(@W label: Int)
  // compiler generates some static initializer
  // .foo(label:)
  // .foo($label:)
  // baking storage of the associated value remains W
} 

let e = E.foo(label: 42)
// sugar for
let e = E.foo(_label: W(wrappedValue: 42))

switch e {
// only one of these can be used, not both at the same time
case foo(label: var intValue):
  ...
case foo($label: var projectedValue):
  ...
}

// this is basically sugar over
// case foo(_label: let wrapper):
//   var intValue = wrapper.wrappedValue
//   var projectedValue = wrapper.projectedValue

The baking storage remains private, like in the current proposal. The value will be wrapped and have some effects applied to it. I can only see benefits in terms of usage and consistency.

Thoughts?

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