Swift Regex Builders will crash if a named capture group is optional

[schwa]

macOS X 13.1 (22C65)
Xcode Version 14.1 (14B47b)

The following code that uses a Regex Builder will crash at runtime when the user tries to get the result of an optional capture group via the regex match subscript. The same regex using a regex literal successfully returns an optional (but nil) value. You can generate the result builder from the literal if needed.

import Foundation
import RegexBuilder

// Case 1 - using regex literals. Works

let regex1 = #/\s*(?<label>\w+:)?/#

guard let match1 = "NOP".firstMatch(of: regex1) else {
    fatalError()
}
print(match1.output.label) // Should be nil. Is nil!

// #######################

// Case 2 - using regex builder. Crashes.

let label = Reference(Substring.self)
let regex2 = Regex {
    ZeroOrMore(.whitespace)
    Optionally {
        Capture(as: label) {
            Regex {
                OneOrMore(.word)
                ":"
            }
        }
    }
}

guard let match2 = "NOP".firstMatch(of: regex2) else {
    fatalError()
}
print(match2[label])  // Should be nil. Crashes with "Could not cast value of type 'Swift.Optional<Swift.Substring>' (0x....) to 'Swift.Substring' (0x....)."

[hamishknight]

cc @milseman