A property wrapper for values that can be initially nil, but once it has a value, it cannot be reset to nil afterwards.

GetOnlyOptional.swift

@propertyWrapper
struct GetOnlyOptional<T> {
    init(wrappedValue: T?) {
        self.wrappedValue = wrappedValue
    }
    var wrappedValue: T? {
        get {
            _value
        }
        
        set {
            guard let newValue = newValue else { return }
            _value = newValue
        }
    }
    private var _value: T?
}

I don't like the name. Is NilOnce any better? Comments are welcome.