<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">I think it’s something to do with subclasses overriding the method and potentially returning a type that’s different that what the compiler is expecting. Or something like that. <div class=""><br class=""></div><div class="">I hope it gets addressed at some point, though, because it drives me nuts, too.<div class=""><br class=""><div class=""><div class=""><div><blockquote type="cite" class=""><div class="">On Feb 16, 2016, at 1:47 PM, Diego Sánchez via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">Hi all,<div class=""><br class=""></div><div class="">Covariance works properly with classes:</div><div class=""><p class=""><span class="">class</span><span class=""> M</span>yType {}<br class="">class MySubtype: MyType {}<br class=""></p><div class="">class MyClass {</div><div class=""> func createMyType() -> MyType {</div><div class=""> return MyType()</div><div class=""> }</div><div class="">}</div><div class=""><br class=""></div><div class="">class MySubClass: MyClass {</div><div class=""> override func createMyType() -> MySubtype {</div><div class=""> return MySubtype()</div><div class=""> }</div><div class="">}</div><div class=""><br class=""></div><div class="">However it doesn't work for protocol conformance:</div><div class=""><br class=""></div><div class=""><div class="">protocol MyProtocol {</div><div class=""> func createMyType() -> MyType</div><div class="">}</div><div class=""><br class=""></div><div class="">class MyConformingClass: MyProtocol {</div><div class=""> func createMyType() -> MySubtype {</div><div class=""> return MySubtype()</div><div class=""> }</div><div class="">}</div></div><div class=""><br class=""></div><div class="">Compiler error for this case:</div><div class=""><div class="">error: type 'MyConformingClass' does not conform to protocol 'MyProtocol'</div><div class="">note: protocol requires function 'createMyType()' with type '() -> MyType'</div><div class="">note: candidate has non-matching type '() -> MySubtype'</div></div><div class=""><br class=""></div><div class="">I know I can fix it by using associated objects in MyProtocol definition, but then if I want to declare a property of type MyProtocol I have to go dancing with generics everywhere.</div><div class=""><br class=""></div><div class="">Is this a known limitation/bug? Any plans to improve this?</div><div class=""><br class=""></div><div class="">Cheers,</div><div class="">Diego</div><div class=""><span class=""></span><br class="webkit-block-placeholder"></div></div></div>
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