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@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 > _______________________________________________ > swift-users mailing list > swift-users@swift.org > https://lists.swift.org/mailman/listinfo/swift-users _______________________________________________ swift-users mailing list swift-users@swift.org https://lists.swift.org/mailman/listinfo/swift-users
