Say I have Hypersonic Dragon in play and go to combat phase. Dring the declare attackers step I tap and declare Hypersonic Dragon as attacking. After I have declared Hypersonic Dragon as attacking I get priority (as there are nothing else on the battlefield generating triggers).
Now I play Relentless Assault and it resolves. Since this is during combat, there is no additional Combat or Main Phase added, but is Hypersonic Dragon untapped now?
If not, at what point is Hypersonic Dragon "a creature that attacked"? What is the earliest Relentless Assault can be played to get the untap effect?