<div dir="ltr">Ok, I thought it was part of SE-0142 (which has status "implemented (Swift 4)".<div>/Jens</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Jun 7, 2017 at 9:39 AM, Slava Pestov <span dir="ltr"><<a href="mailto:spestov@apple.com" target="_blank">spestov@apple.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">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).<br>
<br>
The proposal is here if you’d like to read more about it: <a href="https://github.com/apple/swift-evolution/blob/master/proposals/0157-recursive-protocol-constraints.md" rel="noreferrer" target="_blank">https://github.com/apple/<wbr>swift-evolution/blob/master/<wbr>proposals/0157-recursive-<wbr>protocol-constraints.md</a><br>
<br>
Slava<br>
<div><div class="h5"><br>
> On Jun 6, 2017, at 10:37 PM, Jens Persson via swift-users <<a href="mailto:swift-users@swift.org">swift-users@swift.org</a>> wrote:<br>
><br>
> In Swift 4:<br>
><br>
> protocol P1 {<br>
> associatedtype A: P1 // Error: Type may not reference itself as a requirement<br>
> }<br>
> protocol P2 {<br>
> associatedtype A where A: P2 // OK<br>
> }<br>
><br>
> What is the rationale behind this?<br>
><br>
> /Jens<br>
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</blockquote></div><br></div>