Not calling right method when wrapped into a function with no specific generic type

Question

Sample code which I am experimenting in playground

protocol Shape {
    init()
}

class Circle: Shape {
    required init() {
    }
}

class Square: Shape {
    required init() {
    }
}

class ShapeMapping<T> {
    func map() -> T? {
        print("Any shape called")
        return nil
    }
}

extension ShapeMapping where T: Shape {
    func map() -> T? {
        print("Shape type called")
        return T.self()
    }
}

extension ShapeMapping where T: Square {
    func map() -> T? {
        print("Square type called")
        return Square() as? T
    }
}

class ShapeWrapper<T> {
    func determineShape() -> T?  {
        return ShapeMapping<T>().map()
    }

}

Code works correctly and calls expected method for type Square when I call

let square: Square? = ShapeMapping<Square>().map()

Output: Square type called

But, when I call a wrapper method and pass Square type output is different

let mappingSquareShape: Square? = ShapeWrapper<Square>().determineShape()

Actual: Any shape called
Expected: Square type called

Is type information does not get passed from ShapeWrapper to ShapeMapping? Any idea what's going on here?

As per apple docs nothing wrong when explicitly specifying type with extension https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Generics.html#//apple_ref/doc/uid/TP40014097-CH26-ID553 — Gopal, May 27 '17 at 01:56
No there's nothing wrong with it, it just won't have the polymorphic behaviour youre looking for — Alexander, May 27 '17 at 02:30
Compare [Wrong specialized generic function gets called in Swift 3 from an indirect call](https://stackoverflow.com/q/41980001/2976878). Inside `determineShape()`, the compiler doesn't know anything about the type of `T` – it could be an arbitrary type. Therefore the only overload of `map()` it can dispatch to is the unconstrained one on `ShapeMapping`. — Hamish, May 27 '17 at 09:13

score 0 · Answer 1 · answered May 27 '17 at 06:28

In your code I found "func map()" will be ambiguous when i define the class ShapeMapping(), The compiler will not know which one to use, maybe you can change the method name in the extension

extension ShapeMapping where T: Shape{
  func test() -> T? {
    print("Shape type called")
    return nil
  }
}

Not calling right method when wrapped into a function with no specific generic type

1 Answers1