Some creatures have entries in their stat block that are classified as 'melee spell attack', or 'ranged spell attack', without actually being tied to the casting of a spell. Some PC subclasses also get some of those, notably the Way of the Sun Soul and the Circle of Stars, but they are framed in such a way that still leaves no doubt as to when you can use it (with the Attack action, for the Monk, and as a bonus action on your turn, for the Druid). So let's say I get access to the former, monster-like spell attacks, a possible way this can happen is the shapechange spell.
Can I use such an attack option when I take the Attack action? I understand that these actions on monster stat blocks are, indeed, actions, so they are not just attacks when they use it, still the ruling on opportunity attacks leads me to believe this would be an option (thanks, Medix2, for your comment pointing me to SAC).
Q. Can you use a melee spell attack to make an opportunity attack? [...]
A. [...] A few monsters can make opportunity attacks with melee spell attacks. Here’s how: certain monsters—including the banshee, the lich, and the specter—have a melee spell attack that isn’t delivered by a spell. For example, the banshee’s Corrupting Touch action is a melee spell attack but no spell is cast to make it. The banshee can, therefore, make opportunity attacks with Corrupting Touch.
The wording of opportunity attacks and the attack action are, in fact, the same (emphasis mine):
To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature.
With this action, you make one melee or ranged attack.
For a possible scenario: an 18th level Bladesinger with Extra Attack uses shapechange to turn themselves into an Amnizu. Can they use Disruptive Touch as part of their Attack action?
I'd like to note, here, that Disruptive Touch is a spell attack. While I've seen similar questions arise involving natural weapons, it is often understood that natural weapons are a particular kind of weapon by the rules, so that using claws or a bite as a part of the Attack action (e.g. a Circle of the Moon Druid) would be completely analogue to what a PC normally does. Does it being a spell attack make any difference here? Or is it the same thing, being that no spellcasting is involved?