Range subscript is ambiguous

Using Swift
#1

This function seems simple enough:

    func foo(a: [Int], n: Int) {
        var x : [Int] = a[0..<n]
    }

But it doesn’t compile.

    error: ambiguous subscript with base type '[Int]' and index type 'Range<Int>'
    var x : [Int] = a[0..<n]
        ~^~~~~~~
        Swift.Array:100:12: note: found this candidate
    public subscript (subRange: Range<Int>) -> ArraySlice<Element> { get set }
    ^
    Swift.MutableCollectionType:3:12: note: found this candidate
    public subscript (bounds: Range<Self.Index>) -> MutableSlice<Self> { get set }
    ^
    Swift.CollectionType:2:12: note: found this candidate
    public subscript (bounds: Range<Self.Index>) -> Slice<Self> { get }
    ^

The oddity is that if I change the assignment to this

    var y : [Int] = Array(a[0..<n])

then the compiler is happy.

Shouldn’t it be able to do any necessary type inference from the fact that the expression is in a context where an array is required?

  Neil Faiman

#2

The error message is misleading (if you have time, we'd appreciate a bug report!). What's really going on is that a[0..<n] produces an ArraySlice<T>, not an Array<T>, in order to share memory with the underlying array. The type doesn't match in your assignment. If `x` is just temporary, I'd recommend leaving the type annotation out, since `var x = a[0..<n]` should just work.

-Joe

···

On May 15, 2016, at 5:31 AM, Neil Faiman via swift-users <swift-users@swift.org> wrote:

This function seems simple enough:

   func foo(a: [Int], n: Int) {
       var x : [Int] = a[0..<n]
   }

But it doesn’t compile.

   error: ambiguous subscript with base type '[Int]' and index type 'Range<Int>'
   var x : [Int] = a[0..<n]
       ~^~~~~~~
       Swift.Array:100:12: note: found this candidate
   public subscript (subRange: Range<Int>) -> ArraySlice<Element> { get set }
   ^
   Swift.MutableCollectionType:3:12: note: found this candidate
   public subscript (bounds: Range<Self.Index>) -> MutableSlice<Self> { get set }
   ^
   Swift.CollectionType:2:12: note: found this candidate
   public subscript (bounds: Range<Self.Index>) -> Slice<Self> { get }
   ^

The oddity is that if I change the assignment to this

   var y : [Int] = Array(a[0..<n])

then the compiler is happy.

Shouldn’t it be able to do any necessary type inference from the fact that the expression is in a context where an array is required?

#3

FWIW, master produces the error:

test.swift:2:27: error: cannot subscript a value of type '[Int]' with an index of type 'CountableRange<Int>'
         var x : [Int] = a[0..<n]
                          ^
test.swift:2:27: note: overloads for 'subscript' exist with these partially matching parameter lists: (Int), (Range<Int>), (Range<Self.Index>), (ClosedRange<Self.Index>), (CountableClosedRange<Self.Index>)
         var x : [Int] = a[0..<n]
                          ^

-Chris

···

On May 16, 2016, at 10:09 AM, Joe Groff via swift-users <swift-users@swift.org> wrote:

The oddity is that if I change the assignment to this

  var y : [Int] = Array(a[0..<n])

then the compiler is happy.

Shouldn’t it be able to do any necessary type inference from the fact that the expression is in a context where an array is required?

The error message is misleading (if you have time, we'd appreciate a bug report!). What's really going on is that a[0..<n] produces an ArraySlice<T>, not an Array<T>, in order to share memory with the underlying array. The type doesn't match in your assignment. If `x` is just temporary, I'd recommend leaving the type annotation out, since `var x = a[0..<n]` should just work.

#4

I think I just got bitten by this:

struct Foo<T:BinaryInteger> {
  let bar : [T]
  
  func getrange(start: Array<T>.Index, end: Array<T>.Index) -> Range<Array<T>.Index> { return start..<end }

  func getind(v:Range<Array<T>.Index>) -> [T] { return self.bar[v] }

  func doit() -> [T] {
    let r = getrange(start:1, end:3)
    return getind(r)
  }
}

let a = [UInt(2),UInt(4),UInt(6),UInt(8),UInt(10)] 

let f = Foo(bar: a)

print(f.doit())

yields

err.swift:9:20: error: ambiguous subscript with base type '[T]' and index type 'Range<Array.Index>' (aka 'Range<Int>')
    return self.bar[v]
           ~~~~~~~~^~~

Changing Foo.getind() to return Array(self.bar[v]) fixes things.

Is this the same error?

#5

Yeah, that looks similar. The real problem is that you have to either return ArraySlice<T> or write Array(self.bar[v]) to make a new array.

#6

I take it this is a bug, though? Has it been filed?

#7

Yeah, the bad diagnostic is a bug. I’m not sure whether it’s been filed yet. As Chris noted, it may be fixed in Swift 4.2 already.

#8

Ah, gotcha. The problem is in the error message text, but the behavior is appropriate. Thanks.

1 Like
#9

Is there a reason you’re calling UInt(#) when creating the a array, if not you might consider writing it in one of the following ways:

let a: [UInt] = [1, 2, 3]
let a = [1 as UInt, 2, 3]

My personal preference is the first option:)

2 Likes
#10

Only my inexperience with the language. :wink: Thanks for the alternate suggestions.