public class Noded_VM
{
    // Fields
    
    private Noded_VM _parent;
    private Model _current;
    private readonly ObservableCollection<Noded_VM> _children;

    private bool _isExpanded;
    private bool _isSelected;


    // Presentation properties.

    public Noded_VM Parent
    {
        get { return _parent; }
        set { _parent = value; }
    }

    public ObservableCollection<Noded_VM> Children
    {
        get { return _children; }
    }

    /// <summary>
    /// Gets or sets whether the TreeViewItem associated with this object is expanded.
    /// </summary>
    public bool IsExpanded
    {
        get { return _isExpanded; }
        set
        {
            SetProperty(() => _isExpanded, v => _isExpanded = v, value);

            // Expand all the way up to the root.
            if (_isExpanded && _parent != null)
                _parent.IsExpanded = true;
        }
    }

    /// <summary>
    /// Gets or sets whether the TreeViewItem associated with this object is selected.
    /// </summary>
    public bool IsSelected
    {
        get { return _isSelected; }
        set { SetProperty(() => _isSelected, v => _isSelected = v, value); }
    }


    // Constructors.
    
    /// <summary>
    /// To create root of the tree.
    /// </summary>
    /// <param name="current"></param>
    public Noded_VM(Model current)
        : this(current, null)
    { }

    /// <summary>
    /// To create nodes and leafs of the tree.
    /// </summary>
    /// <param name="current"></param>
    /// <param name="parent"></param>
    private Noded_VM(Model current, Noded_VM parent)
    {
        _parent = parent;
        _current = current;
        _children = new ObservableCollection<Noded_VM>(
            _current.Children
            .Select(child => new Noded_VM(child, this))
            .ToList());
    }
}

public class Noded_VM<T> : ModeledBase_VM
    where T : Noded_VM<T>
{
    // Fields
    
    private T _parent;
    private ITreeNodeModel _current;
    private readonly ObservableCollection<T> _children;

    
    // Properties

    public T Parent
    {
        get { return _parent; }
        set { _parent = value; }
    }

    public ObservableCollection<T> Children
    {
        get { return _children; }
    }


    // Constructors

    /// <summary>
    /// To create root of the tree.
    /// </summary>
    public Noded_VM(ITreeNodeModel current)
        : this(current, null)
    { }

    /// <summary>
    /// To create nodes and leafs of the tree.
    /// </summary>
    private Noded_VM(ITreeNodeModel current, T parent)
        : base(current)
    {
        _parent = parent;
        _current = current;
        _children = new ObservableCollection<T>(
            _current.Children
            .Select(child => new T(child, this)) // ОШИБКА!   
                                                 // Cannot create an instance of the variable type 'T' 
                                                 // because it does not have the new() constraint
            .ToList());
    }
}