А у Node2.Next будет null?

public static IEnumerable<Node> EnumerateForward(Node? list) {
  while (list) {
    yield return list;
    list = list.Next;
  }
}

this.next = next ?? throw new ArgumentNullException(nameof(next))

S>public static IEnumerable<Node> EnumerateForward(Node? list) {
S>  while (list) {
S>    yield return list;
S>    list = list.Next;
S>  }
S>}
S>

while (!list.IsEmpty)

S>this.next = next ?? throw new ArgumentNullException(nameof(next))
S>

S>

SP>почему только локальные функции? Как бы чего не вышло?

avoiding unexpected nulls is a huge benefit, particularly in larger codebases where it's harder to keep track of the edge cases where things can be null by yourself

S> А вам не приходило в голову задуматься: а почему этому кой-кому не нужен Nullable Reference? Если не приходило — перечитайте стартовый пост, там всё написано.

Так как Next для последнего и Previous для первого будет null

#продвинутый nullable enable
        var obj1 = new SomeType1();
       var obj2 = new SomeType2();
obj1.Prop = obj2;
obj2.Prop = obj1;
// Local functions are allowed
void Connect() => obj1.Child = obj2;
Connect();
#продвинутый nullable disable

S>Попробуйте переписать пример с использованием Optional. Станет хуже

using System;
using System.Collections.Generic;
using Optional;

public class Node<T>
{
    public required T Value { get; set; }
    public Option<Node<T>> Next { get; set; }
    public Option<Node<T>> Previous { get; set; }

    public Node(T value)
    {
        Value = value;
        Next = Option.None<Node<T>>();
        Previous = Option.None<Node<T>>();
    }

    public static Option<Node<T>> Init<T>(IEnumerable<T> values)
    {
        using var enumerator = values.GetEnumerator();
        if (!enumerator.MoveNext())
            return Option.None<Node<T>>();

        var head = new Node<T>(enumerator.Current);
        var current = head;

        while (enumerator.MoveNext())
        {
            var newNode = new Node<T>(enumerator.Current);

            current.Next = Option.Some(newNode);
            newNode.Previous = Option.Some(current);

            current = newNode;
        }

        return Option.Some(head);
    }

    public static IEnumerable<T> TraverseForward<T>(Option<Node<T>> start)
    {
        var current = start;

        while (current.Match(
            some: node =>
            {
                yield return node.Value;
                current = node.Next;
                return true;
            },
            none: () => false
        )) { }
    }
}

class Node
{
    public required Node Next { get; set; }
    public required Node Previous { get; set; }
    public string Name {get; init;}
    public Node(string name) => Name = name;
}

public static IEnumerable<string> Iterate(Node start) {
  var current = start;
  do { 
    yield return current.Name; 
    current = current.Next;
  } while(current != start);
}

var node1 = new Node("Hello"); 
var node2 = new Node("World") { Next = node1, Previous = node1 }; 
foreach(var n in Iterate(node1))
  Console.WriteLine(n);

Compile time error: Required member 'Program.Node.Next' must be set in the object initializer or attribute constructor.
Compile time error: Required member 'Program.Node.Previous' must be set in the object initializer or attribute constructor.

var node1 = new Node("Hello") { Next = null!, Previous = null! }; 
var node2 = new Node("World") { Next = node1, Previous = node1) }; 
node1.Previous = node2;
foreach(var n in Iterate(node1)
  Console.WriteLine(n);

S>#продвинутый nullable enable
S>        var obj1 = new SomeType1();
S>       var obj2 = new SomeType2();
S>obj1.Prop = obj2;
S>obj2.Prop = obj1;
S>// Local functions are allowed
S>void Connect() => obj1.Child = obj2;
S>Connect();
S>#продвинутый nullable disable
S>

S>>#продвинутый nullable enable
S>>        var obj1 = new SomeType1();
S>>       var obj2 = new SomeType2();
S>>obj1.Prop = obj2;
S>>obj2.Prop = obj1;
S>>// Local functions are allowed
S>>void Connect() => obj1.Child = obj2;
S>>Connect();
S>>#продвинутый nullable disable
S>>

 #nullable disable
 #продвинутй nullable enable
 код c проверкой на присваивание всем полям не null значений
 #продвинутй nullable disable
 #nullable enable

S>var node1 = new Node("Hello") { Next = null!, Previous = null! }; 
S>var node2 = new Node("World") { Next = node1, Previous = node1) }; 
S>node1.Previous = node2;
S>foreach(var n in Iterate(node1)
S>  Console.WriteLine(n);
S>

var node1 = new Node("Hello") { Next = Node.EmptyNode, Previous = Node.EmptyNode }; 
var node2 = new Node("World") { Next = node1, Previous = Node.EmptyNode) }; 
node1.Previous = node2;
foreach(var n in Iterate(node1)
Console.WriteLine(n);