Cannot assign protocol array to generic array

Question

There are some codes below, some gives compile time error some doesn't. Is there a bug or do I miss something about generics here?

1) Doesn't work:

class DataSource: NSObject {
    var dataObjects: [DataType]

    init<T where T: DataType>(dataObjects: [T]) {
        self.dataObjects = dataObjects //Cannot assign value of type [T] to type [DataType]
    }
}

But this works:

class DataSource: NSObject {
    var dataObjects: [DataType]

    init<T where T: DataType>(dataObjects: [T]) {
        self.dataObjects = []
        for dataObject in dataObjects {
            self.dataObjects.append(dataObject)
        }
    }

}

2) Doesn't work:

class DataSource: NSObject {
    var dataObjects: [DataType]

    init<T:DataType>(dataObjects: [T]) {
        self.dataObjects = dataObjects //Cannot assign value of type [T] to type [DataType]
    }
}

But this works:

class DataSource: NSObject {
    var dataObjects: [DataType]

    init<T:DataType>(dataObjects: [T]) {
        self.dataObjects = []
        for dataObject in dataObjects {
            self.dataObjects.append(dataObject)
        }
    }
}

3) This also works:

class DataSource<T: DataType>: NSObject {
    var dataObjects: [T]

    init(dataObjects: [T]) {
        self.dataObjects = dataObjects
    }
}

Also what is the difference between T where T: DataType and T:DataType

P.S.:DataType is an empty protocol

Answer 1

Most likely, the problem is that your protocol is not inheriting from a reference DataType, while the array expects objects.

For example, Any is not always by reference

protocol DataType: Any {
}

class DataSource: NSObject {
    var dataObjects: [DataType]

    init<T:DataType>(dataObjects: [T]) {
        self.dataObjects = dataObjects //Cannot assign value of type [T] to type [DataType]
    }
}

on the other hand, AnyObject always is:

protocol DataType: AnyObject {
}

class DataSource: NSObject {
    var dataObjects: [DataType]

    init<T:DataType>(dataObjects: [T]) {
        self.dataObjects = dataObjects //Works fine
    }
}