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A level 20 barbarian has the Primal Champion class benefit, which allows him to have a Constitution score of 24 (and a modifier of +7). The Durable feat gives your Hit Die a minimum equal to your CON modifier ×2. But the barbarian hit die is a d12.
Do you regain 12 or 14 hit points, since your maximum is 12, and your minimum is 14?
You Regain 21 Hit Points
That at least seems to be the final word from Mike Mearls: http://www.sageadvice.eu/2014/09/01/durable-feat/
He asserts that a character can use durable to get back more than max possible hit points. For your barbarian, when you roll the hit die, the result is 14 no matter what you roll, and then you add your Con modifier to it.
You heal 14 HP at minimum. You can heal even more than that.
The rules don't say anything about maximum HP healed, it just so happens the nature of your hit dice prevents you from healing more than 12 HP from the dice.
Short Rest
[..] For each Hit Die spent in this way, the player rolls the die and adds the character’s Constitution modifier to it. The character regains hit points equal to the total.
If you rolled a 12 on your d12 hit die, you heal up 12 + CON modifier, or 19 HP for a Lvl 20 Barbarian.
