Dementia Bat

• WWizards of the Coast2011-06-01
  Official Ruling
  You can’t force an opponent to discard a card they cast by activating Dementia Bat’s ability in response. The spell is on the stack by then.
• WWizards of the Coast2011-06-01
  Official Ruling
  Unlike most abilities that force a player to discard cards, this ability may be activated whenever you could cast an instant, including during your opponent’s draw step after they have drawn a card.

Rulings via Scryfall