Is possible to make the compile check what is used inside of == and then provide the hash value? And if compile cannot check (for whatever reason) what’s properties is used inside == then raise a compile error?<br><div class="gmail_quote"><div dir="ltr">Em sex, 15 de dez de 2017 às 15:58, Joe Groff via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> escreveu:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">SE-0185 is awesome, and brings the long-awaited ability for the compiler to provide a default implementation of `==` and `hashValue` when you don't provide one yourself. Doug and I were talking the other day and thought of a potential pitfall: what should happen if you provide a manual implementation of `==` without also manually writing your own `hashValue`? It's highly likely that the default implementation of `hashValue` will be inconsistent with `==` and therefore invalid in a situation like this:<br>
<br>
struct Foo: Hashable {<br>
// This property is "part of the value"<br>
var involvedInEquality: Int<br>
// This property isn't; maybe it's a cache or something like that<br>
var notInvolvedInEquality: Int<br>
<br>
static func ==(a: Foo, b: Foo) -> Bool {<br>
return a.involvedInEquality == b.involvedInEquality<br>
}<br>
}<br>
<br>
As currently implemented, the compiler will still give `Foo` the default hashValue implementation, which will use both of `Foo`'s properties to compute the hash, even though `==` only tests one. This could be potentially dangerous. Should we suppress the default hashValue derivation when an explicit == implementation is provided?<br>
<br>
-Joe<br>
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</blockquote></div>