Re-implement an `IEnumerable` method without `yield`

GenerateWithoutYield.cs

// https://twitter.com/noseratio/status/1491189525061980160

using System.Collections;
using System.Diagnostics.CodeAnalysis;

foreach (var item in Generate())
{
    Console.WriteLine(item);
}

foreach (var item in GenerateWithoutYield())
{
    Console.WriteLine(item);
}

static IEnumerable<int> Generate()
{
    var sum = 0;
    for (int i = 0; i < 10; i++)
    {
        yield return sum += i; 
    }
}

static IEnumerable<int> GenerateWithoutYield()
{
    return new Generator<int>(() =>
    {
        var sum = 0;
        var i = 0;

        return () =>
        {
            if (i < 10)
            {
                return (sum += i++, true);
            }
            return (default, false);
        };
    });
}

public partial class Generator<T> : IEnumerable<T>
{
    public delegate (T item, bool itemProvided) OnNext();

    readonly Func<OnNext> _onNextFactory;

    public Generator(Func<OnNext> onNextFactory) => _onNextFactory = onNextFactory;

    public IEnumerator<T> GetEnumerator() => new Enumerator(_onNextFactory());

    IEnumerator IEnumerable.GetEnumerator() =>
        throw new NotImplementedException();
}

public partial class Generator<T>
{
    class Enumerator : IEnumerator<T>
    {
        readonly OnNext _onNext;

        [AllowNull]
        T _item = default;
        bool _itemProvided;

        public T Current => _itemProvided ? _item : throw new InvalidOperationException();

        object IEnumerator.Current => throw new NotImplementedException();

        public Enumerator(OnNext onNext) => _onNext = onNext;

        public void Dispose()
        {
        }

        public bool MoveNext()
        {
            (_item, _itemProvided) = _onNext();
            return _itemProvided;
        }

        public void Reset() => throw new NotImplementedException();
    }
}