[swift-users] Swift 4 protocol with associatedtype conforming to itself

[Slava Pestov]

This is a rather complex feature that is not actually implemented in Swift 4 (or Swift 3 for that matter). Work is underway to support this, though. The fact that the second example does not produce a diagnostic is a bug (probably you will not be able to define a type that conforms to P2 anyway).

The proposal is here if you’d like to read more about it: https://github.com/apple/swift-evolution/blob/master/proposals/0157-recursive-protocol-constraints.md

Slava

> On Jun 6, 2017, at 10:37 PM, Jens Persson via swift-users <swift-users at swift.org> wrote:
> 
> In Swift 4:
> 
> protocol P1 {
>     associatedtype A: P1 // Error: Type may not reference itself as a requirement
> }
> protocol P2 {
>     associatedtype A where A: P2 // OK
> }
> 
> What is the rationale behind this?
> 
> /Jens
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