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[SR-9787] Source code incompatibility in Swift compiler 5.0, language 4.2 #52212

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tonyarnold opened this issue Jan 29, 2019 · 4 comments
Closed

[SR-9787] Source code incompatibility in Swift compiler 5.0, language 4.2 #52212

tonyarnold opened this issue Jan 29, 2019 · 4 comments
Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. regression standard library Area: Standard library umbrella swift 5.0

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@tonyarnold
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Previous ID SR-9787
Radar None
Original Reporter @tonyarnold
Type Bug
Status Resolved
Resolution Won't Do
Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Bug, 5.0Regression
Assignee None
Priority Medium

md5: d0f3d0ff87f6655b49ad49acfb679737

Issue Description:

The following code compiles fine under Swift 4.2 with compiler version 4.2:

class Node {
    weak private(set) var parent: Node?

    init(parent: Node? = nil) {
        self.parent = parent
    }

    var parentNodes: AnySequence<Node> {
        return sequence(first: self, next: { $0.parent }).dropFirst()
    }
}

However under Swift 4.2 language mode with the 5.0 compiler, I get the following error:

Cannot convert return expression of type 'DropFirstSequence<UnfoldSequence<Node, (Node?, Bool)>>' to return type 'AnySequence<Node>'
@belkadan
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@airspeedswift, @lorentey, do we have a story for people who were using AnySequence in this way?

@belkadan
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belkadan commented Apr 2, 2019

Circling back: I think the core team decided this was necessary breakage. @airspeedswift?

@airspeedswift
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Yes, that's right. The Sequence.SubSequence removal wan't something that could easily be made source-compatibly but was important to get the ABI for Sequence right. Advice is either wrap the return value or change it to the new type.

@airspeedswift
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(as a apology-benefit, the old variant using AnySequence was epically slow compared to using DropFirstSequence if that's the fix you pick...

@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. regression standard library Area: Standard library umbrella swift 5.0
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@tonyarnold @belkadan @airspeedswift @AnthonyLatsis