Additional cost: discard v. Discard: Add mana

[Korath]

Describe the Bug:
Cards with "As an additional cost to cast this spell, discard [a specific number of cards, maybe of a specific type] [maybe at random]" let you cast them without discarding if you use all the cards in your hand to produce mana.

Fixture: A Mountain and a Skirge Familiar on the bf, a Sonic Burst and a Swamp in your hand.
Observed: Sonic Burst highlighted yellow. Clicking on it charges mana; you click on the Mountain for , you click on the Skirge Familiar and discard the Swamp for . No card is discarded for Sonic Burst's additional cost, and it still goes on the stack, resolves, and deals damage.

Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
dev 58bc91e3e.

What exactly should be the correct behavior/interaction?
Expected: Sonic Burst preferably unhighlighted. Clicking on it preferably does nothing. Being yellow and clickable, but cancelling when Sonic Burst finds it no long has a card to discard, is minimally acceptable, though it'll make the AI try to activate it, fail, and waste mana.

Are any other cards possibly affected by this bug?
Simian Spirit Guide notably is not affected, since you incorrectly can't exile it to add mana during mana payment.

"As an additional cost to cast this spell, discard X cards" likely isn't affected either, though it may be if is part of the casting cost. I haven't checked.