[swift-evolution] [Pitch/Reality Check] Allow instance members as parameter default values

[T.J. Usiyan]

+1 if this doesn't have terrible implications.

The downside to making it IUO or Optional is that that changes the type
signature of  `index(i:offsetBy:)` for no reason other than a default
value. If you decide to change to or away from providing a default value it
probably shouldn't change the type accepted in that spot.

On Wed, Feb 22, 2017 at 7:32 PM, Ben Cohen via swift-evolution <
swift-evolution at swift.org> wrote:

>
> On Feb 22, 2017, at 10:42 AM, Nate Cook via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Oops, left out that there's this horrifying way of writing it right now:
>
> extension Collection {
>     func index(_ i: Index! = nil, offsetBy n: IndexDistance) -> Index {
>         let i = i ?? startIndex
>         // ...
>     }
> }
>
> Nobody wants that.
>
>
> Oh I don’t think it’s all that bad! It also doesn’t need to be an IUO,
> since you’re unwrapping it immediately into another variable no matter what.
>
> This also gives you the flexibility to write this:
>
> extension Collection {
>     func index(_ i: Index? = nil, offsetBy n: IndexDistance) -> Index {
>         let i = i ?? (n < 0 ? endIndex : startIndex)
>         // ...
>     }
> }
>
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