Comparing two `any Error`s for equality

[taylorswift]

i have an error type that wraps another existential any Error:

@frozen public
struct RecursiveError<Location>:Error
{
    /// The location (key or index) where the error occurred.
    public
    let location:Location
    /// The underlying error that occurred.
    public
    let error:any Error

    @inlinable public
    init(_ error:any Error, in location:Location)
    {
        self.location = location
        self.error = error
    }
}

now i want to test if my API is throwing the correct error with a unit test like:

$0.test(name: "int32-to-uint8", decoding: bson,
    failure: RecursiveError<String>.init(
        IntegerOverflowError<UInt8>.int32(.max),
        in: "int32"))
{
    try $0["int32"].decode(to: UInt8.self)
}

but that would require RecursiveError to be Equatable. and making RecursiveError Equatable requires comparing two any Error existentials. is there a way to do this?

[taylorswift]

update: got this to work

extension Error where Self:Equatable
{
    fileprivate
    func equals(_ other:any Error) -> Bool
    {
        (other as? Self).map { $0 == self } ?? false
    }
}

[tera]

I didn't try it but it should work:

extension Error where Self: Equatable {
    func isEqualTo(other: Error) -> Bool {
        if let o = other as? Self { return self == o }
        return false
    }
}

let a: any Error & Equatable = ...
let b: any Error & Equatable = ...
let equal = a.isEqual(b) && b.isEqual(a)

Inspired by the "bridge building" part of the famous Crusty video.

PS. corrected the code above.