@available(*, unavailable) API unexpected behavior

Using Swift
1

I'm trying to understand why this snippet type checks successfully but when building for tvOS I get an Undefined symbol: _OBJC_CLASS_$_ASWebAuthenticationSession error?

@available(iOS 12.4, *)
@available(tvOS, unavailable)
class TestClass {

    private var authenticationSession: ASWebAuthenticationSession?

    func test() {
        self.authenticationSession = ASWebAuthenticationSession(
            url: URL(string: "https://test.com")!,
            callbackURLScheme: nil,
            completionHandler: { _, _ in }
        )
    }
}

I was under the impression that the @available(tvOS, unavailable) would take care of excluding this entire class from being built for the specific platform? I'm trying to avoid using #if os(tvOS) if I can help it.

Thanks!
Rog