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

[Joe Groff]

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