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[SR-3865] A possible tuple-inference/parameter-resolution bug in Swift 3.0.1 #46450

swift-ci opened this issue Feb 5, 2017 · 3 comments


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@swift-ci swift-ci commented Feb 5, 2017

Previous ID SR-3865
Radar None
Original Reporter mvonballmo (JIRA User)
Type Bug

Attachment: Download


XCode Version 8.2.1 (8C1002), MacOS 10.12.3 (16D32)

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee None
Priority Medium

md5: f458cd138c0b5cd9ccfb13befee8169d

Issue Description:

The following code compiles and yields somewhat unexpected results.

func test<T>(_ a: T) -> String
  return String(describing: type(of: T.self))

var int = test(1)
var odd = test(1, 2)
var tuple = test((1, 2))
var labels = test(a: 1, b: 2)
var nestedTuple = test(((1, 2)))
var complexTuple = test((1, (2, 3)))

Here are the results in the Playground:

With argument label looks fine:

Without argument label seems strange:

  • odd: Swift interpreted the two parameters as a Tuple. Is that correct, though? Are the parentheses allowed to serve double-duty both as part of the function-call expression and as part of the tuple expression?

  • labels: The issue with double-duty parentheses isn’t limited to anonymous tuples. The compiler treats what looks like two labeled function-call parameters as a tuple with two Ints labeled {a:} and {b:}.

  • nestedTuple: This is not a tuple of a tuple, but just a two-element tuple

I wrote up more details on my blog before I discovered that this issue-tracker for Swift.

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@belkadan belkadan commented Feb 7, 2017

@slavapestov, I thought we managed to destroy these? I do see the errors in Swift 4 mode.

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@slavapestov slavapestov commented Feb 7, 2017

Do or don't see the errors in Swift 4 mode?

it behaves as expected for me:

tup.swift:7:19: error: extra argument in call
var odd = test(1, 2)
tup.swift:9:28: error: extra argument 'b' in call
var labels = test(a: 1, b: 2)

Marco, in Swift 3, the proposal to disambiguate argument lists from tuple arguments (SE-0110) was not fully implemented. On master (you can try the latest snapshot if you want), enabling Swift 4 mode will make the suspect cases into type errors.

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Collaborator Author

@swift-ci swift-ci commented Feb 7, 2017

Comment by Marco von Ballmoos (JIRA)

@slavapestov: Thanks for the tip about Swift 4 mode. I have a workaround for it, but wanted to report the bug in case you weren't aware of it. Thanks for the quick responses!

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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