Returning a type from an Intent (basics)

Hello, I'm attempting to learn the basics of AppIntents.

My test Hello World intent takes a number and doubles it.

This page (https://developer.apple.com/documentation/appintents/providing-your-app-s-capabilities-to-system-services) seems to imply that you can return types from the perform() function.

My code compiles, but crashes at runtime with the error

perform() returned types not declared in method signature - Did not declare ReturnsValue but provided one

Code:

struct DoubleANumber: AppIntent {
	static var title: LocalizedStringResource = "Double a number"
	static var description =
		IntentDescription("Given a number, gives back twice that number.")
	
	@Parameter(title: "Start Number")
	var inputNumber: Double
	static var parameterSummary: some ParameterSummary {
		Summary("The number to double")
	}
	
	func perform() async throws -> some IntentResult & ReturnsValue {
		let outputNumber = inputNumber * 2
		return .result(value: outputNumber)
	}
}

The value returned in the value property of the IntentResult is a Double. I would have assumed that this would be a valid primitive type (as mentioned in the earlier section of that docs page, covering parameters) and that the actual type returned is the wrapper .result type would be covered in the type in the method signature some IntentResult & ReturnsValue

What am I missing? Thanks.