> On Mar 22, 2017, at 7:12 AM, Karl Wagner <[email protected]> wrote:
>
> Do we support where clauses on these existentials? For example, can I write:
>
> func doSomething<T>(_: NSObject & Collection where Iterator.Element == T)
>
> The proposal says nothing about where clauses.
No, this proposal does not enable where clauses on existentials. That’s gone
under the name “generalized existentials” or “enhanced existentials” as a
separate (post-Swift-4) feature.
- Doug
>
> - Karl
>
>
>> On Mar 20, 2017 at 7:00 pm, <Douglas Gregor via swift-evolution
>> <mailto:[email protected]>> wrote:
>>
>> Proposal link:
>>
>> https://github.com/apple/swift-evolution/blob/master/proposals/0156-subclass-existentials.md
>>
>> <https://github.com/apple/swift-evolution/blob/master/proposals/0156-subclass-existentials.md>
>>
>> Hello Swift community,
>>
>> The review of SE-0156 “Class and Subtype Existentials” ran from February
>> 28...March 7, 2017. The proposal was very well-received and is accepted with
>> one modification: the ordering rules for existential types that involve
>> AnyObject or a class type will be removed.
>>
>> The ordering rules were intended to improve code clarity by requiring that
>> the class (or AnyObject) constraint come first—“AnyObject & P” would be
>> well-formed but “P & AnyObject” would be an error—enforcing more uniformity
>> for Swift code and echoing a similar restriction that already exists for
>> class definitions, where the superclass must come first. However, the
>> ability to compose typealiases complicated the ordering rules considerably,
>> and—as noted by Matthew Johnson
>> <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170227/033365.html>—don’t
>> provide the guarantee that the class constraint will always be first.
>> Therefore, the core team felt that the resulting ordering rules introduced
>> more complexity than they provided clarity, and therefore do not belong in
>> the language.
>>
>> Thanks,
>> Doug Gregor, Review Manager
>>
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