Existe uma maneira de incorporar um assertionFailure em '?' expressão

Tente escrever extensionpara Optionalonde você pode lidar com o seu caso, por exemplo:

extension Optional {
    func rescue(
        _ assertion: String, 
        _ fallback: Wrapped
    ) -> Wrapped {
        guard case let .some(value) = self else {
            assertionFailure(assertion)
            return fallback
        }
        
        return value
    }
}

TimeZone(abbreviation: "GMT+1").rescue("what weird runtime have you've got????", .current) // GMT+0100
TimeZone(abbreviation: "GMT+1lalala").rescue("what weird runtime have you've got????", .current) // what weird runtime have you've got????

Eu tenho uma solução ligeiramente diferente usando uma função genérica.

import Foundation

func fail<T>(_ message: @autoclosure () -> String, fallback: T) -> T
{
    assertionFailure(message())
    return fallback
}

let x = TimeZone(abbreviation: "GMT+1") ?? fail("x is not a valid time zone", fallback: TimeZone.current)

let y = TimeZone(abbreviation: "GMT+1fdsgfds") ?? fail("y is not a valid time zone", fallback: TimeZone.current)
// On evaluating the last statement, aborts with Fatal error: y is not a valid time zone
// in a debug or playground context