Buy a variant get another variant free in the same product

I'm searching for a way to let the user get a free downloadable product when he purchases another variant product in the same group product,
So I have a group product (X) which has two variants, the first variant is a downloadable product (A) and the other variant is normal product (B). What I need is that, when the user purchases the normal product (B), he gets the product (A) as well for free, but if he purchased the downloadable product (A), then he will get product (A) only.

Try setting up Discount with type Assigned to Products for 100% for product A, having requirement Has Product for B