<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=""><br class=""><div><blockquote type="cite" class=""><div class="">On 31 Jan 2017, at 09:07, Alex Hoppen via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="">This was a deliberate change between Swift 3 beta 1 and beta 2 after a friend of mine pointed the following inconsistency out to me:<br class=""><br class="">struct Foo {<br class=""> func bar() {}<br class="">}<br class="">let foo: Foo? = Foo()<br class="">foo?.bar() // Does not create a warning<br class="">true ? foo?.bar() : foo?.bar() // expression of type '()?' is unused<br class=""><br class="">After some offline discussion at WWDC with the Swift team we decided to move to a consistent model where ()?, ()??, … is always discardable since we didn't want to take the convenience of foo?.bar() away (something that regularly occurs with weak variables, e.g. captures in closures).<br class=""><br class="">So much for the history of this feature.</div></div></blockquote><br class=""></div><div>Thanks for clarifying, but this is an interesting case actually; it seems the problem here is the ternary, as presumably the following would have worked just fine:</div><div><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">if</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">true</span><span style="font-variant-ligatures: no-common-ligatures" class=""> { foo?.bar() }</span></div></div><div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">else</span><span style="font-variant-ligatures: no-common-ligatures" class=""> { foo?.bar() }</span></div></div></blockquote><div><br class=""></div><div>So your example's problem seems to stem then from the fact that the ternary's type isn't inheriting the discardable nature of the two branches. I wonder then if an alternative solution might to have discardable be an inheritable property, while keeping optional chaining implicitly discardable?</div><div><br class=""></div><div>For example:</div><div><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(187, 44, 162);" class=""><div style="margin: 0px; line-height: normal;" class=""><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">@discardableResult</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">func</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> foo() -> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> { </span><span style="font-variant-ligatures: no-common-ligatures" class="">return</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> }</span></div><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class="">func</span><span style="font-variant-ligatures: no-common-ligatures" class=""> bar() -> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures" class=""> { </span><span style="font-variant-ligatures: no-common-ligatures;" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">2</span><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class="">struct</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Baz { </span><span style="font-variant-ligatures: no-common-ligatures;" class="">func</span><span style="font-variant-ligatures: no-common-ligatures" class=""> baz() {}}</span></div><div style="margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> a:</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">Baz</span><span style="font-variant-ligatures: no-common-ligatures" class="">? = </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">Baz</span><span style="font-variant-ligatures: no-common-ligatures" class="">()</span></div><div style="margin: 0px; line-height: normal; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">true</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> ? </span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">foo</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">() : </span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">bar</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">() </span><span style="font-variant-ligatures: no-common-ligatures" class="">// type is Int, should produce a warning</span></div><div style="margin: 0px; line-height: normal; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">true</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> ? </span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">foo</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">() : </span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">foo</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">() </span><span style="font-variant-ligatures: no-common-ligatures" class="">// type is discardable Int, no warning necessary</span></div><div style="margin: 0px; line-height: normal; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">true</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> ? </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">a</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">?.</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">baz</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">() : </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">a</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">?.</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">baz</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">() </span><span style="font-variant-ligatures: no-common-ligatures" class="">// type is discardable Void, no warning necessary</span></div></div></div></div></blockquote><div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(187, 44, 162);" class=""><div style="margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span></div></div></div><div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div>The idea basically being that @discardableResult becomes a property of return types that is passed for as long as it is in common, but does not prevent two types (one discardable, one not) from being equal.</div><div><br class=""></div><div>This might give a best of both? As method chaining producing implicitly discardable results would allow your example to behave as expected, but other cases can still be configured as desired with @discardableResult (or not).</div></body></html>