[swift-evolution] [Pre-pitch] Conditional default arguments

[Matthew Johnson]

As mentioned in my prior message, I currently have a PR open to update the generics manifesto (https://github.com/apple/swift/pull/13012 <https://github.com/apple/swift/pull/13012>).  I removed one topic from that update at Doug Gregor’s request that it be discussed on the list first.  

The idea is to add the ability to make default arguments conditional (i.e. depend on generic constraints).  It is currently possible to emulate conditional default arguments using an overload set.  This is verbose, especially when several arguments are involved.  Here is an example use case using the overload method to emulate this feature:

```swift
protocol Resource {
  associatedtype Configuration
  associatedtype Action
}
struct ResourceDescription<R: Resource> {
  func makeResource(with configuration: R.Configuration, actionHandler: @escaping (R.Action) -> Void) -> R {
    // create a resource using the provided configuration
    // connect the action handler
    // return the resource
  }
}

extension ResourceDescription where R.Configuration == Void {
  func makeResource(actionHandler: @escaping (R.Action) -> Void) -> R {
    return makeResource(with: (), actionHandler: actionHandler)
  }
}

extension ResourceDescription where R.Action == Never {
  func makeResource(with configuration: R.Configuration) -> R {
    return makeResource(with: configuration, actionHandler: { _ in })
  }
}

extension ResourceDescription where R.Configuration == Void, R.Action == Never {
  func makeResource() -> R {
    return makeResource(with: (), actionHandler: { _ in })
  }
}

```

Adding language support for defining these more directly would eliminate a lot of boilerplate and reduce the need for overloads.  Doug mentioned that it may also help simplify associated type inference (https://github.com/apple/swift/pull/13012#discussion_r152124535 <https://github.com/apple/swift/pull/13012#discussion_r152124535>).

The reason that I call this a pre-pitch and one reason Doug requested it be discussed on list is that I haven’t thought of a good way to express this syntactically.  I am interested in hearing general feedback on the idea.  I am also looking for syntax suggestions.

Matthew
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