Open Radar

 
Summary:
In the following code, Xcode complains that there are four candidate, while it’s not ambiguous at all:

import Foundation

enum DrillingError: ErrorType {
    case BadKey
    case NotAContainer
    case KeyMissing
}

extension Dictionary {

    func drill(keys: [Key]) throws -> AnyObject {
        guard !keys.isEmpty else {
            throw DrillingError.KeyMissing
        }
        if let childDictionary = self[keys.first!] as? Dictionary {
            do {
                try childDictionary.drill(Array(keys.dropFirst()))
            } catch let error {
                throw error
            }
        } else {
            if let childArray = self[keys.first!] as? Array { // <— Xcode complains that this subscript is ambiguous

            }
        }
    }

    func drill(keys: Key ...) throws -> AnyObject {
        do {
            return try drill(keys)
        } catch let error {
            throw error
        }
    }
}


Steps to Reproduce:
Try compile the code above in Xcode project or Playground

Expected Results:
The compiler should proceed to find more legit errors.

Actual Results:
The complier reports an error that’s bogus.