As David Melding said quite clearly, the Government of Wales Act states that:

'A person holding office as the First Minister, a Welsh Minister appointed under section 48 or a Deputy Welsh Minister may not be appointed as the Counsel General...and the Counsel General or a person so designated may not be appointed to any of those offices.'

Now, I assume we will now be told that he wasn't appointed as a Minister under section 48, but, frankly, he himself, as the nominee for Counsel General, says on his Twitter page that he is a Brexit Minister, not that he has responsibilities for Brexit. Now, if that's a legal slip, we'll leave it there, but he himself calls himself a Brexit Minister, the Government already calls him a Brexit Minister, and the legislation states clearly that a Minister cannot also be appointed as Counsel General.