Constrained associated type on protocol

Using Swift
1

Why associated type requirement persists when a constrain is defined on a protocol?
For instance:

protocol P1 {
    var id: String { get }
}
var p1Array: [P1]  // OK

protocol P2: Identifiable where Self.ID == String {
}
var p2Array: [P2]  // Protocol 'P2' can only be used as a generic constraint because it has Self or associated type requirements

Thank you

2

P2 has no associated types of its own, but it inherits from Identifiable, which does. (ID is the associated type.)

3

Thank you but I know that and this is not my question.

IMHO when a constrain is defined by equality like here, the associated type should be requalified as a typealias and therefore [P2] declaration accepted.

I should not create a AnyP2 type eraser since in scope of P2, all types are resolved: ID can't be anything else than a String.

4

Ah, I see what you are getting at: Why doesn’t the compiler check whether the constraints narrow it to a single type? I don’t know if there is a technical reason or if it is just an oversight. Someone more familiar with the compiler implementation will have to answer.

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5
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6

Thank you for you replies.