Some stores sell products that can only be purchased once (for example, on iTunes, you can only buy a song for yourself once, or on Steam, you can only buy a game for yourself once).
So for products like this, how can I limit the quantity to 1 in Commerce? (Users should never be able to order 2+ of any individual product of type myproducttype.)
There are some answers for doing this in Drupal 7:
Unfortunately, these solutions use Rules, which is still in alpha for D8.
So how can I programatically restrict the product quantity to 1 for a given product type?