So i've been trying to append an element to an array and am getting the error i mentioned in the title. Everything i've read says its from a mismatch of types but I can not figure out where this mismatch is coming from.

Here is where the warning is coming from. newValue is a BTDevice? and i'm unwrapping to BTDevice so that it may be added to $connectedDevices which is an array of BTDevice

.onChange(of: listener.connectedPublished) { newValue in
                                        guard let device = newValue else {return }
                                        appState.$connectedDevices.append(device) // warning on this line

                                        doneLoading.toggle()
                                        show.toggle()
                                        let bool = isDoneSpeaker
                                        self.isDoneOnboarding = bool
                                    }

Here is the AppState

class AppState: ObservableObject {
    @Published var selectedTab: ContentViewTab = .home
    @Published var connectedDevices = [BTDevice]()
    var devices: [BTDeviceHolderInfo]? {
        get {
            var newArray = [BTDeviceHolderInfo]()
            if let data = UserDefaults.standard.data(forKey: UserDefaults.Key.identifierArray.rawValue) {
                do {
                    let decoder = JSONDecoder()
                    let array = try decoder.decode([BTDeviceHolderInfo].self, from: data)
                    newArray = array
                } catch {
                    return [BTDeviceHolderInfo]()
                }
            }
            return newArray
        }
    }
    @Published var newlyConnectedBass50: BASS50? 

}

Here is where listener.connectedPublished is set

func didConnect(to device: BTDevice) {
        self.canConnect = true 
        connectedPublished = device // RIGHT HERE
        let holderInfo = BTDeviceHolderInfo(deviceId: device.uuid, rssi: device.rssi)
        let haveSaved = self.onboardedDevices.contains { info in
            return info == holderInfo
        }
        if !haveSaved {
            do {
                // Create JSON Encoder
                let encoder = JSONEncoder()

                // Encode Note
                let data = try encoder.encode(holderInfo)

                // Write/Set Data
                UserDefaults.standard.set(data, forKey: UserDefaults.Key.identifierArray.rawValue)

            } catch {
                print("Unable to Encode Note (\(error))")
            }
        }
        if let newDevice = BASS50(device.peripheral, device.advData, device.rssi) {
            print("it's a new bass being added")
            self.bass50 = newDevice
            self.bass50?.state = .connected
        }
    }

@ygeras

Thank you, this was the solution. I feel silly for missing it, I thought i had certainly tested that one out.