[swift-users] generic function called recursively not compiling

[Kevin Nattinger]

Looks like a bug in the type inference system to me, but I’ll let someone with a better knowledge of the compiler confirm. Meanwhile, you can work around by explicitly casting any of the pieces of the return statement to `[Element]` or using a temporary for one or more of them.

    let middle = [pivot]
    return quickSort(left) + middle + quickSort(right)

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

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


> On Aug 4, 2016, at 9:02 AM, Ray Fix via swift-users <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)
> //                 ^~~~
> 
> 
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