MinimalEnumerables.cs

public class Enumerable<T>
{
    public Enumerable<T> GetEnumerator() 
        => this;

    public T Current 
        => default;

    public bool MoveNext() 
        => default;
}

public class AsyncEnumerable<T>
{
    public AsyncEnumerable<T> GetAsyncEnumerator() 
        => this;

    public T Current 
        => default;

    public ValueTask<bool> MoveNextAsync() 
        => default;
}