[swift-evolution] Refining SE-0185: Should providing a custom == suppress the default hashValue?

[Daniel Duan]

+1. The proposal wasn’t explicit enough to have either supported or be against this IMO. It’s a sensible thing to spell out.

Daniel Duan
Sent from my iPhone

> On Dec 15, 2017, at 9:58 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 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:
> 
> struct Foo: Hashable {
>  // This property is "part of the value"
>  var involvedInEquality: Int
>  // This property isn't; maybe it's a cache or something like that
>  var notInvolvedInEquality: Int
> 
>  static func ==(a: Foo, b: Foo) -> Bool {
>    return a.involvedInEquality == b.involvedInEquality
>  }
> }
> 
> 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?
> 
> -Joe
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