vala iterator next_value method return type - iterator

Question

From vala tutorial:
alternative iterator protocol: "T? next_value()"
If the iterator object has a .next_value() function that returns a nullable type then we iterate by calling this function until it returns null.
I write some code, but got an error.
the error message:
error: return type of `Something.next_value' must be nullable
foreach (string s in A) {
^
I don't understand, it is nullable already in my next_value return type.
How to correct this code?
public static void main() {
stdout.printf("hello\n");
var A = new Something<string> ({"aa", "bb"});
foreach (string s in A) {
stdout.puts(s);
stdout.puts ("\n");
}
}
public class Something<T> : Object {
public T[] data;
private int _size;
private int _i = 0;
public Something (owned T[] a){
this.data = a;
this._size = data.length;
}
public Something<T> iterator(){
return this;
}
// error: return type of `Something.next_value' must be nullable
// foreach (string s in A) {
// ^
public T? next_value () {
return _i < _size ? data[_i++] : null;
}
}

Answer 1

You have declared that the generic data type must be nullable, which is correct. So when you pass the specific data type as a type argument make sure it is also nullable. The line:
var A = new Something<string> ({"aa", "bb"});
should be:
var A = new Something<string?> ({"aa", "bb"});