Mysterious Default Value for un-assigned variable

[MojtabaHs (Seyed Mojtaba Hosseini Zeidabadi)]

@Usage before declaration

what you get if you run swiftc with a single file that has top-level code in it, or if your module has a main.swift

I can also reproduce this if none of these conditions are true. You can clone this project and change this function to:

Button("Update Name") {
    let id = toastManager.showToast() {
        OverlayTextInputToastView(toastID: id, userName: $userName)
    }
}

The module has not a main.swift file or the other condition.