[swift-evolution] [Proposal] Explicit Synthetic Behaviour

Tony Allevato tony.allevato at gmail.com
Thu Sep 7 13:36:08 CDT 2017

On Thu, Sep 7, 2017 at 11:18 AM Haravikk via swift-evolution <
swift-evolution at swift.org> wrote:

> On 7 Sep 2017, at 18:53, Tony Allevato via swift-evolution <
> swift-evolution at swift.org> wrote:
> On Thu, Sep 7, 2017 at 10:39 AM Gwendal Roué <gwendal.roue at gmail.com>
> wrote:
>> Le 7 sept. 2017 à 14:45, Tony Allevato <tony.allevato at gmail.com> a écrit
>> :
>> Right, let's make sure we're talking about the right thing here. Gwendal,
>> your issue isn't with synthesis in the form of Codable or the new additions
>> to Equatable/Hashable which are opt-in-by-conformance, it's with the
>> specific case of raw value enums or enums without associated values where
>> the synthesis is implicit with no way to opt-out. That's a big difference.
>> Yes.
>> I can definitely see the latter being an issue if it were more
>> widespread, and I'd be supportive of those enums being required to declare
>> their conformance for consistency (though it would be source breaking).
>> Yes, unfortunately.
>> However, I still haven't seen a real issue that has come up because of
>> the distinction being drawn here between default implementations vs.
>> implementations that can access other parts of the concrete type. It sounds
>> like this discussion is trying to protect against a hypothetical problem
>> that hasn't happened yet and may not happen; it would be helpful to show
>> some motivating real-world cases where this is indeed a severe problem.
>> Yes. I'm not talking about implementation itself. I know this has been
>> the main topic until I have tried to bring in the topic of the consequences
>> of non-avoidable synthesis (extra methods that may conflict with userland
>> methods).
>> If you ask me for a real-world case, then I think I gave one. Let me
>> rephrase it:
>> it's impossible to define a value-backed enum without getting free
>> Equatable conformance. This free conformance is sometimes unwanted, and I
>> gave the example of DSLs. Now this problem is not *severe*. It's more a
>> blind spot in the language, and finally just an unwanted side-effect of a
>> compiler convenience,
> Again, this is not the issue that Haravikk is describing in this thread.
> I'll clarify—your issue is specifically with the fact that enums with raw
> values and enums without associated values receive Equatable even without
> explicitly conforming to it, and therefore users have no way of opting out
> of it. This predates SE-0185, and I didn't propose making any changes to
> the conformance of those enums for source compatibility reasons, though I
> wouldn't be opposed to it because it makes them consistent across the board.
> Haravikk's argument is about synthesized conformances like Codable and
> Equatable/Hashable in SE-0185, where the user must explicitly conform the
> type to those protocols. His claim is that that act of opting in is not
> sufficient and that it is still dangerous if those synthesized conformances
> can access members that are not also declared in the protocol. That's a
> completely separate issue to yours, and one that I hope he'll present more
> evidence of. Right now, requiring that you not only explicitly conform to
> the protocol but also explicitly request the synthesis feels like a
> solution without an actual problem, and is a situation we already have
> today with default method implementations.
> The simplest real-world case is easy:
> struct Foo { var data:String }
> extension Foo : Equatable {} // This currently produces an error, in
> future it will not
Why is this a problem? It's no different than if someone extended Foo to
conform to a protocol with a default implementation that was written in

> I argued this point on the specific topic for Equatable/Hashable, but it
> was, both during and after review, essentially ignored and the decision to
> synthesise implicitly never sufficiently justified. The closest that I got
> was "but Codable does it" which is about as weak a justification as you
> could possibly get as I don't really agree with it in the case of Codable
> either. In the case of Equatable/Hashable specifically this is arguably a
> breaking change that has IMO been totally ignored by the core team who
> haven't given any reasonable response.

That's not a fair characterization. Just because your concerns were
disagreed with does not mean they were ignored; my understanding is that
the core team views these synthesized conformances as a different kind of
default method (and one which could be hoisted out of the compiler once
sufficient metaprogramming facilities are available).

The way to handle synthesized conformances was discussed during the review
period for Codable, during the earlier pitch a few months ago for what
became SE-0185, and again during its formal review. It's not accurate to
reduce the argument you disagree with to "but Codable does it" when what
you're referring to is established precedent based on those prior

> In the broader case however I still feel that synthesised behaviour should
> require explicit rather than implicit opt-in, as it allows us to
> distinguish between a developer who wants to implement requirements
> themselves, versus one who is happy to have one derived from their concrete
> type automatically. The current setup does not allow this at all.

I feel like we keep going back to this, but this statement applies equally
to non-synthesized default implementations. Are you suggesting that users
should have to opt-in specifically to all default implementations provided
by a protocol in some way beyond merely conforming to that protocol? If
not, what specifically makes synthesized conformances a special case?

> In future if the role keywords are adopted we will always be able to
> distinguish one who wants to explicitly implicit more than just the minimum
> requirements.

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