Tempo verbal da seguinte frase: Como usar?

Não precisa complicar tanto:

The task would have been done if I had finished it.
O trabalho teria sido feito se eu o tivesse completado.

Invertendo:

If I had finished the task, it would have been done.

Não faz sentido "would have been being done".

Does that make sense to you?

Concordo contigo, Henry, mas é porque minha professora falou que a resposta desta frase é "The task would have been being done if I had been doing it." e eu achei um tanto quanto duvidoso, ela está gramaticalmente correta?

Actually, she's right. Here's another version, but inverted.

If I had been writing the letter, the letter would have been being written.

In other words:

If I had been writing the letter, the letter would have been in the process of being written.

So,
If I had been doing the task, the task would have been (in the process of) being done.

The question is, who writes English like that? Does your teacher believe in torture?