+1
I think the simple solution of if you provide either == or hashValue you have
to provide both is the best approach. Good catch of this bug.
-- Howard.
> On 16 Dec 2017, at 6:24 am, Daniel Duan via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> +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@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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