Swift generic cast to Protocol fails with swift_dynamicCastUnknownClass - swift

Question

The following example is taken from Apple Swift Reference guide. I only added the getHasAreaInstances() and getGenericHasAreaInstances()
import UIKit
#objc protocol HasArea {
var area: Double { get }
}
#objc protocol HasExtendedArea: HasArea {
var extendedArea: Double { get }
}
class Circle: HasArea {
let pi = 3.1415927
var radius: Double
var area: Double { return pi * radius * radius }
init(radius: Double) { self.radius = radius }
}
class Country: HasArea {
var area: Double
init(area: Double) { self.area = area }
}
class Continent: HasExtendedArea {
var area: Double { return 300 }
var extendedArea: Double { return 3000 }
}
let objects: [HasArea] = [
Circle(radius: 2.0),
Country(area: 243_610),
Continent()
]
for object in objects {
if let objectWithArea = object as? HasExtendedArea {
println("Extended Area is \(objectWithArea.area)")
} else {
println("Area is not extended")
}
}
// Extended Area is 300.0
// Area is not extended
// Area is not extended
The method below returns the correct array:
func getHasExtendedAreaInstances() -> [HasExtendedArea] {
var haveArea: [HasExtendedArea] = []
for object in objects {
if let objectWithArea = object as? HasExtendedArea {
haveArea.append(objectWithArea)
}
}
return haveArea
}
let areas = getHasExtendedAreaInstances()
//[Continent]
The method below returns the correct array:
func getGenericHasExtendedAreaInstances<T>() -> [T] {
var haveArea: [T] = []
for object in objects {
if let objectWithArea = object as? T {
haveArea.append(objectWithArea)
}
}
return haveArea
}
let areasGeneric: [HasExtendedArea] = getGenericHasExtendedAreaInstances()
//[Continent]
However, as soon as a constraint is imposed on the generic type, it no longer works
func getGenericConstraintHasExtendedAreaInstances<T: HasArea>() -> [T] {
var haveArea: [T] = []
for object in objects {
if let objectWithArea = object as? T {
// the line above fails with swift_dynamicCastUnknownClass
haveArea.append(objectWithArea)
}
}
return haveArea
}
let areasGenericConstraint: [HasExtendedArea] = getGenericConstraintHasExtendedAreaInstances()

Answer 1

Your generic function makes no sense. What would resolve it? What would satisfy it? Make a simpler example with the same basic declaration structure: it's an impossible function. For example, start with this nongeneric function:
class Thing : Printable {
var description : String {return "thing"}
}
func g() -> [Thing] {
return [Thing()]
}
let result : [Thing] = g()
Now modify g to be generic, exactly parallel to your function:
class Thing : Printable {
var description : String {return "thing"}
}
func g<T:Printable>() -> [T] {
return [Thing()]
}
let result : [Thing] = g()
It doesn't compile - because it makes no sense.

Answer 2

This is fixed in Swift 1.2, tested on Xcode 6.3 Beta 3
You can specify the type constraint without swift compiler failing on you