Deduct and return class type

[Виталий Фадеев via Digitalmars-d-learn]

Context:
    data + GUI List

Goal:
    auto list = new List( data );

Concept:
    class is created in the usual way : new List( data )
    store inside the class            : T data;
    type T deducted                   : ( T )( T data )


Tried way:
    template List( T )
    {
        class List
        {
            T data;

            this( T data )
            {
                this.data = data;
            }


            // data usage...
        }
    }


    void main()
    {
        string[] extensions = [ ".d", ".di" ];

        auto list = new List( extensions );
    }


    Source: https://run.dlang.io/is/Bw2zHB

Question:
    How to implement on D beauty clean flexible code ?
    like a:
        auto list = new List( data );

    How to return from 'List( data )' class type ?