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[SR-6915] Array += is stricter when adding arrays of conformers to arrays of protocols #49464

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ZevEisenberg opened this issue Feb 3, 2018 · 6 comments
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[SR-6915] Array += is stricter when adding arrays of conformers to arrays of protocols #49464

ZevEisenberg opened this issue Feb 3, 2018 · 6 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself regression swift 4.1 type checker Area → compiler: Semantic analysis

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@ZevEisenberg
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Previous ID SR-6915
Radar rdar://problem/37290898
Original Reporter @ZevEisenberg
Type Bug
Status Resolved
Resolution Done
Environment 
$ swift --version
Apple Swift version 4.1 (swiftlang-902.0.34 clang-902.0.30)
Target: x86_64-apple-darwin17.4.0
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, 4.1Regression, TypeChecker
Assignee None
Priority Medium

md5: 25774d7409bb46b73b10c3ab18e06880

Issue Description:

The following code has one expected error in Xcode 9.2, but it has a new error in Xcode 9.3. It also occurs if you use the newly renamed compactMap instead of flatMap. Both errors can be worked around with `as [MyProtocol]`, but I wasn't sure if it was expected that the second case would be an error in Swift 4.1.

This was reported as a bug on my open source repo here: Rightpoint/BonMot#320

protocol MyProtocol {}

struct ConformingStruct: MyProtocol {}

var arrayOfMyProtocol: [MyProtocol] = []

arrayOfMyProtocol += [ConformingStruct()] // no error

let arrayOfConformingStruct = [ConformingStruct()]
arrayOfMyProtocol += arrayOfConformingStruct // EXPECTED Xcode 9.2 and 9.3 error: Binary operator '+=(_:_:)' cannot be applied to operands of type '[MyProtocol]' and '[ConformingStruct]'

var optionalStruct: ConformingStruct?

arrayOfMyProtocol += [optionalStruct].flatMap { $0 } // NEW Xcode 9.3 ONLY error: Binary operator '+=(_:_:)' cannot be applied to operands of type '[MyProtocol]' and '[ConformingStruct]'
@belkadan
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belkadan commented Feb 5, 2018

Hm. What you really want there is just map, not compactMap (previously flatMap). But since it's a regression it might indicate a bigger problem.

cc @rudkx

@rudkx
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rudkx commented Feb 5, 2018

I see the same error for the second line when compiling with Xcode 9.2. Are you sure there is a change in behavior from that version?

Actually, I used DEVELOPER_DIR to point to 9.2, but then explicitly used my locally built compiler, so yes, I see a different behavior here as well.

@rudkx
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rudkx commented Feb 5, 2018

@swift-ci create

@ZevEisenberg
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@belkadan I don't think I do want map there. I'm trying to flatten an a [T?] to a [T] with the nils removed. Good point about compactMap, but I left it as flatMap because, as you say, it is a regression, so I wanted to keep the reduced test case as close to real-world usage as possible.

I wanted to add this library to the compatibility repo, but I had a little trouble getting Swift PM set up. Maybe I'll take another crack at it soon, because I'm guessing it would have caught this sooner.

@belkadan
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belkadan commented Feb 5, 2018

Oh, I'm sorry, I did in fact miss the optional. Yes, it seems like there could be enough type context here to make things work.

@ZevEisenberg
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Looks like this is fixed in 9.4.1

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself regression swift 4.1 type checker Area → compiler: Semantic analysis
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@rudkx @ZevEisenberg @belkadan @AnthonyLatsis