[SR-115] Support generic constraints in subscripts

[swift-ci]

Previous ID	SR-115
Radar	rdar://problem/21461357
Original Reporter	radex (JIRA User)
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	11
Component/s	Compiler
Labels	Improvement, LanguageFeatureRequest
Assignee	@slavapestov
Priority	Medium

md5: ea5cb771b99703784bb1dd3c9e7cf4ed

blocks:

• SR-3460 Replace _AnyHashable protocol with a generic subscript

relates to:

• SR-238 Support throwing subscripts

Issue Description:

It would really help me and my library (https://github.com/radex/swiftyuserdefaults) to be able to use generic constraints on subscripts, e.g.

subscript<T: SomeProtocol>(key: DefaultsKey<T>) -> T { ... }

@lattner already indicated on the mailing list that this is considered a current language limitation, not a design choice, so I thought it might be useful to have a JIRA issue for tracking progress on this.

[stephencelis]

If anyone is curious or wants to help, I've started a naïve branch here: https://github.com/stephencelis/swift/tree/generic-subscripts

I've gotten the parser to stop complaining and am working on getting the resolver to play nicely with subscript access.

Once I have things working, I'll submit a pull request and elicit feedback from people with more compiler experience.

[swift-ci]

Comment by Johannes Lund (JIRA)

Don't know if this should be its own issue, but if the generic subscripts also could be throwing it would be really interesting for all of the Swift JSON deserializers/mappers, including mine https://github.com/Anviking/Decodable

[swift-ci]

Comment by Radek Pietruszewski (JIRA)

@johannes Related, but deserves its own issue, IMO

[stephencelis]

anviking (JIRA User) Yep. Feel free to post another issue and/or start a swift-evolution thread around possible syntax, etc.

[benrimmington]

@slavapestov has implemented:

• apple/swift#7597

• apple/swift#7608

• apple/swift#7628

• apple/swift#7683

• apple/swift#7697

• apple/swift#7752

[benrimmington]

@@slavapestov: Does your implementation support default arguments for subscript parameters?

[slavapestov]

Not yet, but I'm waiting for some SILGen work to settle down before looking at that. There's a separate SR tracking default arguments for subscripts, IIRC.

[benrimmington]

I could only find SR-4104.

[swift-ci]

Comment by Roberto Neira (JIRA)

Is there a way to tell in which release of Swift this improvement is included?

[benrimmington]

@neirar (JIRA User): Swift 4.0

[swift-ci]

Comment by Roberto Neira (JIRA)

@benrimmington Thanks for the quick reply.

Is there a way to see which release a fix or improvement will be included in, or a list that shows the fixes/improvements planned for a certain release?

[ole]

neirar (JIRA User) The changelog at https://github.com/apple/swift/blob/master/CHANGELOG.md gives a pretty good overview of the high-level changes in each release (including the upcoming release). It also includes this one (search for "Subscript declarations can now be defined to have generic parameter lists"). It doesn't mention every single bugfix, of course.