[swift-users] SubSequence Index bug in generic functions?

[Hooman Mehr]

The reason this is appearing here before filing a radar is getting a quick feedback as I have been assuming this is the expected behavior but Nate Cook told me otherwise. See this thread <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160509/017048.html> for the initial discussion that lead to this.

Run this code to see the issue: (Works with the latest Swift 3.0 snapshot)

    func subSequenceIndexBug
        
            <C: Collection
                where
                    C.Iterator.Element: Equatable,
                    C.SubSequence: Collection,
                    C.SubSequence.Index == C.Index,
                    C.SubSequence.Iterator.Element == C.Iterator.Element>
        
        (_ c: C, _ e: C.Iterator.Element)
    {
        let junction = c.index(c.startIndex, offsetBy: numericCast(5)) // Arbitrary split point.
        
        if let i = c.suffix(from: junction).index(of: e) {
        
            print("Searched for \"\(e)\" and found \"\(c[i])\". Result of testing them for equality is: \(e == c[i])")
        }
    }
    
    subSequenceIndexBug("ABCDEFGHIJKLMNOPQRSTUVWXYZ".characters, "Q")
    // Prints: "Searched for "Q" and found "L". Result of testing them for equality is: false"

Is this a generics design limitation, my bug, or a compiler bug?

Thanks for your,
Hooman

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