NSRegularExpression customises the isEqual function but does not customise hashValue and so is unsound

[Lukasa]

The title really explains everything. This program will sometimes crash, and frequently produce silly outcomes:

import Foundation

func main() {
    var s = [NSRegularExpression: Int]()

    let (one, two) = try! (NSRegularExpression(pattern: "foo"), NSRegularExpression(pattern: "foo"))

    print(one.hash)
    print(two.hash)
    print(one == two)

    for _ in 0..<1_000_000 {
        try! s[NSRegularExpression(pattern: "foo")] = 5
    }

    print(s.count)
}

main()