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[SR-13356] == Method on Protocol Overrides Synthesized == Method On Enum #55796

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SlaunchaMan opened this issue Aug 6, 2020 · 4 comments
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[SR-13356] == Method on Protocol Overrides Synthesized == Method On Enum #55796

SlaunchaMan opened this issue Aug 6, 2020 · 4 comments
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@SlaunchaMan
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior.

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@SlaunchaMan
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Previous ID SR-13356
Radar rdar://problem/67361230
Original Reporter @SlaunchaMan
Type Bug
Status Resolved
Resolution Won't Do
Environment

Xcode 11.6
macOS 10.15.6

Additional Detail from JIRA
Votes 0
Component/s
Labels Bug
Assignee @SlaunchaMan
Priority Medium

md5: 789acddb3405b4be56cd1dd9b9b62cef

Issue Description:

Given this code to switch between view edges using a trait collection:

import UIKit

public protocol ViewEdgeConvertible {

    func fixedViewEdge(
        using traitCollection: UITraitCollection?
    ) -> FixedViewEdge

    func directionalViewEdge(
        using traitCollection: UITraitCollection?
    ) -> DirectionalViewEdge

}

public func == (lhs: ViewEdgeConvertible, rhs: ViewEdgeConvertible) -> Bool {
    return lhs.fixedViewEdge(using: nil) == rhs.fixedViewEdge(using: nil)
}

public enum FixedViewEdge: Equatable {

    case top
    case left
    case right
    case bottom

}

extension FixedViewEdge: ViewEdgeConvertible {

    public func fixedViewEdge(
        using traitCollection: UITraitCollection?
    ) -> FixedViewEdge {
        self
    }

    public func directionalViewEdge(
        using traitCollection: UITraitCollection?
    ) -> DirectionalViewEdge {
        switch (self, traitCollection?.layoutDirection) {
        case (.top, _): return .top
        case (.left, .rightToLeft): return .trailing
        case (.left, _): return .leading
        case (.right, .rightToLeft): return .leading
        case (.right, _): return .trailing
        case (.bottom, _): return .bottom
        }
    }

}

public enum DirectionalViewEdge: Equatable {

    case top
    case leading
    case trailing
    case bottom

}

extension DirectionalViewEdge: ViewEdgeConvertible {

    public func fixedViewEdge(
        using traitCollection: UITraitCollection?
    ) -> FixedViewEdge {
        switch (self, traitCollection?.layoutDirection) {
        case (.top, _): return .top
        case (.leading, .rightToLeft): return .right
        case (.leading, _): return .left
        case (.trailing, .rightToLeft): return .left
        case (.trailing, _): return .right
        case (.bottom, _): return .bottom
        }
    }

    public func directionalViewEdge(
        using traitCollection: UITraitCollection?
    ) -> DirectionalViewEdge {
        self
    }

}

The == method produces this warning: “All paths through this function will call itself.” At runtime, this creates an infinite loop. I expected the == method to use the synthesized Equatable conformance of FixedViewEdge instead of the protocol method.

As a workaround, I can implement == directly on FixedViewEdge:

public func == (lhs: FixedViewEdge, rhs: FixedViewEdge) -> Bool {
    switch (lhs, rhs) {
    case (.top, .top), (.bottom, .bottom), (.left, .left), (.right, .right):
        return true
    default:
        return false
    }
}

This avoids the issue.
@SlaunchaMan
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Here is a Swift Forums post where some workarounds are discussed: https://forums.swift.org/t/all-paths-through-this-function-will-call-itself-on-protocol-equality-method/39131/15

@typesanitizer
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@swift-ci create

@CodaFi
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CodaFi commented Aug 24, 2020

This semantics seems intended, if unfortunate given the set of protocol conformances here. Equatable is not a more specific protocol than ViewEdgeConvertible from the perspective of the witness matching algorithm. Others have noted on the forums that this should be expressed as a defaulted requirement in a protocol extension for ViewEdgeConvertible instead.

@hborla
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hborla commented Aug 25, 2020

Yeah, unfortunately this is correct behavior. The reason is because == on Equatable is generic over Self, whereas the == that you've written here is not, and the compiler will always prefer non-generic overloads over generic ones. It works when you implement == for FixedViewEdge because that new overload is more specific than the one that takes in existential types, so lhs.fixedViewEdge(using: nil) == rhs.fixedViewEdge(using: nil) will choose that overload.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior.
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@SlaunchaMan @CodaFi @typesanitizer @hborla