Does "an attacking creature" count as "a creature that attacked"?

Question

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?

Answer 1

The Hypersonic Dragon will be untapped.

It is well established that to attack is to declare as an attacker, so a creature that attacked is one that has been declared as an attacker earlier in the current turn.

508.2. Second, any abilities that triggered on attackers being declared go on the stack. (See rule 603, “Handling Triggered Abilities.”)

508.2a Abilities that trigger on a creature attacking trigger only at the point the creature is declared as an attacker. They will not trigger if a creature attacks and then that creature’s characteristics change to match the ability’s trigger condition

The rule and ruling for "put on the battlefield attacking" further supports this:

508.4. If a creature is put onto the battlefield attacking, [...]. Such creatures are “attacking” but, for the purposes of trigger events and effects, they never “attacked.”

2011-9-22 Although the token is attacking, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example).

Answer 2

508.4. If a creature is put onto the battlefield attacking, its controller chooses which defending player or which planeswalker a defending player controls it’s attacking as it enters the battlefield (unless the effect that put it onto the battlefield specifies what it’s attacking). Such creatures are “attacking” but, for the purposes of trigger events and effects, they never “attacked.”

From this rule we can infer that "Attacking" means currently in combat attacking the defending player or planeswalker, while "Attacked" means was declared as an attacker in the Declare Attackers Step.

So if you want to untap your Hypersonic Dragon you can do it any time after the Declare Attackers Step. Note that you will not get the additional Combat and Main Phases unless the spell resolves during a Main Phase.