One of the innovations of 13th Age are flexible attacks. Here're the relevant rules:
When you decide to make a flexible attack, you choose your target first, make your attack roll, and then use the natural unmodified die result to determine which of your eligible flexible attacks to use.
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If you have some attacks that are flexible and some that are not, declare whether you are making a flexible attack or a specific non-flexible attack before you roll.
Bards use flexible attacks in the form of battle cries:
Battle cries are triggered by flexible melee attacks. The bard makes a melee attack and is able to use a battle cry that corresponds to the attack's natural result, sometimes whether or not the attack hits.
Here's an example of such a battle cry for completeness:
Stay Strong! Triggering Roll: Natural 16+; Effect: …
Then, there's a spell bards can take in case they don't feel like hitting people with sticks:
Battle Chant
Ranged spell, At-Will
Special: When you use battle chant, you can choose any battle cry effect you know as if you were making a basic melee attack, with the battle chant attack roll taking the place of the basic melee attack roll.
Target: One nearby creature; Attack: …; Damage: …
This is the one time flexible attacks are explicitly linked to basic attacks. So a flexible attack is a rider effect that happens in addition to the attack that triggers it. Does this mean that any basic melee attack can be flexible?
This question is relevant because bards can learn spells belonging to other classes, which creates edge cases. Would I still be able to use my battle cries with the following spells?
Hammer of Faith
Close-quarters spell, Daily
Effect: Until the end of the battle, your basic melee attacks use d12s as their base weapon damage dice.
Seems likely, it's just a modifier of the basic attack. A trickier case:
Combat Boon
Close-quarters spell, At-Will.
Effect: Make a basic melee attack. If the attack hits, you or one conscious nearby ally can roll a save against a save ends effect.
Trickier, considering you technically cast a spell that grants you an attack.
Finally, a further stretch, elementalist's powers such as:
Resounding Thunder
Daily. Quick action.
Effect: Until the end of the battle, you may make the following attack:
Melee or Ranged Attack
Target: One nearby enemy
Attack: Strength OR Dexterity (melee) or Dexterity (ranged) + Level vs. AC
Hit: WEAPON + Strength (melee) or Dexterity (ranged) damage. At the end of your turn, one random nearby enemy takes your Wisdom modifier in damage. This increases to 2X your Wisdom modifier at 5th level, and 3X at 8th level.
This would most likely not count as a flexible attack, but at the same time it is a melee attack, with just an extra bit on top. The benefit is not significantly different from those granted by previous examples, so if they are fair game, shouldn't this one be too?
I understand that 13th Age has a much more relaxed attitude towards strict wording than some of its predecessors, so I'm more interested in the intent of the rules rather than their lexical analysis.
Best Answer
The question will be a real issue in case of multi-classing, since Combat Boon is a Cleric's power, while only Fighters and Bards have flexible attacks.
My conclusion is:
Flexible attacks are not basic melee attack
Hence, you can't use Combat Boon with Flexible attacks, but I would rule Hammer of Faith as "yes" because it is a passive buff, and because it's a Daily. The game goes out of the way to make Daily powerful (transparent targeting being the first thing I can think of).
My reasoning is base on the text for hampered, page 172 of the rule-book.
Also, additional attacks per turn granted by powers or talents (the Fighter's Counter Attack, for instance) cannot be used as triggers for flexible attacks. This reinforces that the two (basic attack, and flexible attack) are two different beasts.
Hence, I guess the intent of the rules is for flexible attacks to be considered as powers with a chance of triggering depending on the dice roll. Essentially, I guess my stance is "only one effect from one attack" - which I would argue it's the rules' intent because you can only trigger one flexible attack per trigger, despite being eligible for more than one.