Match.subscript fails to properly cast non-matched Capture

This code should produce an Optional<Int>. Instead it produces an Optional<Substring> and crashes.

import RegexBuilder

let ref = Reference(Int.self)

let regex = Regex {
	"test"
	Optionally {
		TryCapture(as: ref) {
			OneOrMore(.digit)
		} transform: {
			Int($0)
		}
	}
	"a"
}

let match = "testa".wholeMatch(of: regex)

print("Int \(match![ref])")

This aborts with:

Could not cast value of type 'Swift.Optional<Swift.Substring>' (0x1f2db92c0) to 'Swift.Int' (0x1f3da0bf0).

Start by reproducing the Swift snippet with RegexBuilder, focusing on Reference, TryCapture, Optionally, and Match.subscript when the capture is absent. Trace how the non-matched capture is represented and verify that the completed behavior returns an Optional without a cast crash.