The Arcane Archer arcane shot abilities 'Piercing Arrow' (and incidentally 'Seeking Arrow', which causes far fewer issues) say:
When you use this option, you don’t make an attack roll for the attack. Instead, [...]
However, if I had previous cast Lightning Arrow, which says:
The next time you make a ranged weapon attack during the spell’s duration, the weapon’s ammunition, or the weapon itself if it’s a thrown weapon, transforms into a bolt of lightning. Make the attack roll as normal. The target takes 4d8 lightning damage on a hit, or half as much damage on a miss, instead of the weapon’s normal damage.
Following the rules for Arcane Shot, as I
fire an arrow from a shortbow or longbow as part of the Attack action,
apply one of your Arcane Shot options to that arrow.
I choose to apply the Piercing Arrow option. At this point I haven't rolled resolved the attack, which is a good thing as by then it would be too late. But I've still taken the Attack action to make a ranged weapon attack.
I know that:
if you're making an attack roll, you're making an attack.
But I don't think that necessarily implies the opposite, i.e. if you aren't making an attack roll, you aren't making an attack.
When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. [...] you try to seize the target by making a grapple check instead of an attack roll [...]
Using the Attack action, you can make a special melee attack to shove a creature, [...] [...] Instead of making an attack roll, you make a Strength check [...]
So these are instances of the Attack action being used to make 'special melee attacks', that explicitly don't make an attack roll. Might surmise that my Lightning Piercing Arrow is still some kind ranged weapon attack (special or otherwise)? Or is it implied to not still be a 'ranged weapon attack' by a rule I have missed?
So how do I determine what happens? Is the Attack action I took still a ranged weapon attack if I'm not making an attack roll because of the arcane shot? Is that shot option not possible given my arrow is now a bolt of lightning, for ... reasons? If the two do combine, how is the damage from Lightning Arrow applied, and what counts as its 'target'? Or is that attack wasted and the spell fizzles as you can't make the attack roll as normal? If it doesn't fizzle from not having a target, which creatures are considered in range of the splash damage?