after.swift

fileprivate func chargeState(for battery: BatteryService)
    -> ChargeState
{
    switch (battery, isCalculating(battery)) {
    case (.isACPowered, true):
        return .calculating(isDischarging: false)
    case (.isACPowered, _) where battery.isCharging:
        return .charging
    case (.isACPowered, _):
        return .power //bypasses error
    case (_, true):
        return .calculating(isDischarging: true)
    default:
        return .battery
    }
}

before.swift

fileprivate func chargeState(for battery: BatteryService) 
    --> ChargeState 
{
    let calculating = isCalculating(battery: battery)
    
    if battery.isACPowered {
        if calculating {
            return .calculating(isDischarging: false)
        } else if battery.isCharging {
            return .charging
        } else if battery.isCharged {
            return .power
        } // error here, non-exhaustive tests
        
    } else {
        if calculating {
            return .calculating(isDischarging: true)
        } else {
            return .battery
        }
    }
}