“All paths through this function will call itself” on Protocol Equality Method

Using Swift
1

I have a protocol, ViewEdgeConvertible, that’s used to convert between directional and fixed view edges using a UITraitCollection:

public protocol ViewEdgeConvertible {

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

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

}

public enum FixedViewEdge: Equatable {

    case top
    case left
    case right
    case bottom

}

public enum DirectionalViewEdge: Equatable {

    case top
    case leading
    case trailing
    case bottom

}

Using this, I can convert freely between the two types given a trait collection, which lets me account for right-to-left layouts. But when I try to make an == implementation:

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

I get this warning: All paths through this function will call itself.

It looks like the compiler sees the == method between the two FixedViewEdge instances and defaults to this method I’ve implemented. Is there a way to write this that takes advantage of the synthesized Equatable conformance on the FixedViewEdge enum?

2

Perhaps make lhs and rhs generic types that are constrained to ViewEdgeConvertible and Equatable?

3

This has the same result (and results in an infinite loop at runtime):

public func == <T: ViewEdgeConvertible>(
    lhs: T,
    rhs: T
) -> Bool where T: Equatable {
    return lhs.fixedViewEdge(using: nil) == rhs.fixedViewEdge(using: nil)
}
4

This works but is ugly:

public func == (lhs: ViewEdgeConvertible, rhs: ViewEdgeConvertible) -> Bool {
    switch (lhs.fixedViewEdge(using: nil), rhs.fixedViewEdge(using: nil)) {
    case (.top, .top), (.left, .left), (.right, .right), (.bottom, .bottom):
        return true
    default:
        return false
    }
}
5

FWIW, I am not seeing the error in a playground running in Xcode 12 beta 4.

6

To reproduce the issue you need to add the implementation that makes the types conform to ViewEdgeConvertible:

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
        }
    }

}

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
    }

}
7

Thanks, I'm now able to reproduce! Here's a slightly condensed version of your workaround which also silences the error:

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

ETA: In any case, I would recommend filing a bug on bugs.swift.org

1 Like
8

Nice!

9

You can also implement FixedViewEdge.== to bump its priority:

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

You probably also want to do that on DirectionalViewEdge so that directionalViewEdge1 == directionalViewEdge2 use the more performant Equatable implementation. (or maybe it'll get constant-folded anyway).

Personally, I'm not sure it's advisable to implement == over heterogeneous types.

3 Likes
11

Filed as SR-13356.

12

I think you need to move the method into an extension on ViewEdgeConvertible, rather than keep it as a free function:

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

That should silence the warning for you.

2 Likes
13

Thank you, this does indeed silence the warning!

14

That's very spooky, that a free function == and a static function ViewEdgeConvertible.== have different priorities (one below synthesized == and one above). :scream:

15

Using the extension does satisfy the warning, but now I can’t seem to use the method. In a test, I have this line:

XCTAssertTrue(edge == edge)

And this fails to compile with the error Binary operator '==' cannot be applied to two 'ViewEdgeConvertible' operands. This is what I have for ==:

extension ViewEdgeConvertible {

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

}

Using the workaround from @Jumhyn, I can at least call == from a test. Very odd.