[SR-11885] Allow operator functions to have extra arguments with default values

[CodaFi]

Previous ID	SR-11885
Radar	rdar://problem/57711758
Original Reporter	@CodaFi
Type	New Feature

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	New Feature, LanguageFeatureRequest, StarterBug, StarterProposal
Assignee	mininny (JIRA)
Priority	Medium

md5: 210d53f6b32e8240f74a2e7677a3c698

Issue Description:

It might make sense to allow operators to have default arguments, and for those defaults to not count towards the overall arity of the operator. For example, a custom operator `<-` could have `#line` and `#file` defaults in a debug configuration. It could then check some extra invariants in a particular configuration, or do some error reporting, etc.

infix operator <-
// Still infix because it has 2 formal arguments and 1 default
public func <- (lhs: T, rhs :T, file: StaticString = #file) -> U { /**/ }

This change requires an evolution proposal.

[beccadax]

@swift-ci create

[mininny]

This issue is fairly old and I'm not sure if it's worth bringing back, but it doesn't seem complex so I've come up with a POC implementation. #69051

If this requires a swift evolution proposal, I want to go through one as it's my first SE proposal.

Any guidance on how to get this through would be very appreciated! 😄

[AnthonyLatsis]

Yeah, this must go through the Swift evolution process.

[mininny]

Thanks for weighing in always! @AnthonyLatsis

I see, I've drafted up a proposal document here: https://github.com/mininny/swift-evolution/blob/default-arguments-in-operator-functions/proposals/NNNN-default-arguments-in-operator-functions.md.

Should I get this reviewed by anybody, or is it fine to post it in the forums as a pitch?

[AnthonyLatsis]

or is it fine to post it in the forums as a pitch

Yes, I recommend initiating a draft pull request and referring to it in the pitch so that you can refine the proposal in the open without having to continuously update the root post throughout the discussion. The guide to proposing features is here.

[joshua-arnold]

Hey! This would be super useful to have for testing-related libraries. Are you still working on this @mininny? Thanks!

[mininny]

Hey! This would be super useful to have for testing-related libraries. Are you still working on this @mininny? Thanks!

Hi @joshua-arnold , thanks for the nudge! I did complete the POC implementation and evolution proposal last year, but haven't had the time to put this up as a pitch and forgot about it. I'll go ahead and create one 😄 (created here: https://forums.swift.org/t/pitch-allow-operator-functions-to-have-extra-arguments-with-default-values/73236)