Concrete fulfillment of existential requirement

Using Swift
1 
protocol P {}

struct P1: P {}

protocol R {
    var anyPs: [any P] { get }
}

struct R1: R { // Error: Type 'R1' does not conform to protocol 'R'
    let anyPs: [P1]
}

Is this intentional, oversighted or a known temporary limitation?

2

My understanding is that you can't change the types of properties required by protocols. If you remove the brackets and just make the properties typed any P and P1, it should fail the same way.

3

Oh I just realized that arrays cannot be coerced anyways ([P1] as [any P]) due to variance so yeah...
Nevermind.

4

Wait. It indeed can be coerced.

let _ = [P1()] as [any P] // ok

And the compiler can do also implicit coercion.

let p1s = [P1()]
let _: [any P] = p1s // ok

So I guess the question is relevant. Does the type requirement really need to be so strict in this case?

5

Without the brackets the question remains the same, even if with get only requirement you want to use associatedtype rather than existential in 99% of cases.

6

Can't help but note this topic would make a great name for a film.

3 Likes
7

Not saying it's an ideal solution, but you could always do something like this:

protocol P {}

struct P1: P {}

protocol R {
    var anyPs: [any P] { get }
}

struct R1: R {
    let concretePs: [P1]
    var anyPs: [any P] { concretePs as [any P] }
}

This gets you both the benefits of the protocol conformance and you can still interact with the struct's stored property using concrete types.

8

It's intentional, the type of a witness must match the protocol requirement exactly. Perhaps you want to use an associated type instead?

protocol R {
    associatedtype MyP: P
    var anyPs: [MyP] { get }
}
9

Ok thank you!

No I meant to allow conformers to have heterogenous array.

10

In that case conformers must declare the type of their array as any P also, sorry.

2 Likes