[swift-users] generic function called recursively not compiling

[Dave Abrahams]

on Thu Aug 04 2016, Ray Fix <swift-users-AT-swift.org> wrote:

> I filed rdar://27700622 <rdar://27700622> and attached a playground.
> Any workaround magic I can do here?
>
> func quickSort<Element: Comparable>(_ input: [Element]) -> [Element] {
>     if input.count < 2 {
>         return input
>     }
>     let pivot = input.first!
>     let left = input.dropFirst().filter { $0 <= pivot }
>     let right = input.dropFirst().filter { $0 > pivot }
>
>     // Does not compile with (Swift 3pre) Xcode 8 b1,2,3,4
>     // Does compile with (Swift 2.2) Xcode 7.3
>     return quickSort(left) + [pivot] + quickSort(right)
> }
>
> quickSort([3,5,1,2])
>
> Error:
> //Playground execution failed: error: quicksort.playground:11:22: error: cannot convert value of type '[Element]' to expected argument type '[_]'
> //return quickSort(left) + [pivot] + quickSort(right)
> //                 ^~~~
>
Hi Ray,

I don't know why, but the type checker is obviously falling over here.
This works around it:

  return quickSort(left) as [Element] + [pivot] + quickSort(right) 
                         ^^^^^^^^^^^^

HTH,
-- 
-Dave