[Accepted] SE-0353: Constrained Existential Types

[Joe_Groff]

Hi everyone. The review for SE-0353: Constrained Existential Types completed on May 18, 2022. The core team has decided to accept this proposal. Reviewers supported the proposed functionality, although many also had concern about the back-deployability of it. John McCall gave a response detailing how code targeting older runtimes could still make limited use of the feature:

@John_McCall

We’ve realized that, if we can allow constrained existential types to be stored directly in structs without back-deployment constraints, you can effectively do almost anything you want with them, just with some minor inconvenience. You can’t back-deploy an Array<any Collection<Int>> , but you can back-deploy an Array<MyAnyCollection<Int>> , where that’s simply a struct wrapping any Collection<T> . And you can dynamically cast from an any Collection<T> by converting it to Any first. The only hard restrictions will be that reflection won’t work and dynamic casts to these types won’t work.

We still cannot promise that you’ll be able to do that, however.

Doug Gregor also noted that the new functionality allows for a limitation in SE-0309 to be lifted:

@Douglas_Gregor

I do have one request. SE-0309 specifies that we cannot call a member of an existential value if the result type of that member uses any of the associated types in an invariant position. That prevents code like this from working:

extension Collection {
  func doSomething() -> some Collection<Element> { ... }
}

func test(strings: any Collection<String>) {
  let otherStrings = strings.doSomething() // error: result type uses Element in an invariant position
}

We don't have to prevent this, and indeed we want this call to succeed and produce an any Collection<String> . I believe the change is straightforward, allowing same-type requirements on primary associated types and inferring an existential where those primary associated types are bound appropriately.

The authors subsequently revised the proposal to include this change, and the core team agrees to accept the proposal with it. Thank you to everyone who participated in the review!

[jjatie]

Is this slated for Swift 5.7? I see the Evolution Status Page only marks it as accepted with no version assigned to it.

[Joe_Groff]

Yeah, it should be available in recent Swift 5.7 beta compilers.

[hborla (Holly Borla)]

I changed the status of SE-0353 and SE-0361 to implemented in Swift 5.7, thanks!