I just don't see that intent in how its worded. I think they willfully rolled the cantrip & attack into one to free up action economy.If that was the intent, I don't think the melee attack would be part of the spell, but rather, the spell would be something entirely different. Like, a bonus action spell that states something like "the target of this spell must be a creature you have hit with a melee weapon attack using the Attack action." Or a class feature that bladedancers could apply on a hit with a melee weapon attack. Or something similar.
Making the attack part of the effect of the spell implies to me that the attack, like other spell effects, is magical.
For all practical purposes, the attack made as a part of the casting of Greenflame Blade and other such cantrips is a somatic component and not an attack granted by the spell. The intent is clear to me that the damage is based on the qualities of the weapon used.
Trying to apply hard logic to 5th edition is only going to result in a lot of wasted time and contradictory results.
I think it is very likely one hand didn't know what the other was doing.
but it doesn't say that does it? This is you changing the words in the spell to justify your position. And even if it did say what you modified, it doesn't matter:
What is the source of the attack?
A spell.
What counts as a magical attack for the purposes of bypassing resistance and or immunity to nonmagical attacks?
An attack with the following sources:
a spell, magical item and/or magical source.
You're adding words into a spell, ignoring that the errata specifically talks about sources (apparently also ignoring that SPELLS count as a source!) and NOT damage type, etc.. to justify your position that you can dismiss dev intent, errata and the general rules of the game. Tell me again how my interpretation is inherently flawed?
It was a simple yes or no question independent of anything else to help me understand your position. And I'm still unsure. Do you think the current errata would allow a spell with a description of doing nonmagical slashing damage to do nonmagical slashing damage? If that answer is no I would say either your interpretation of or the errata itself is flawed.
You're asking me if a theoretical spell that doesn't exist, which is designed to specifically to conflict with the current errata, would function or not? What does me answering yes or no to this question represent or even mean? First off I don't think that they would create a spell to specifically conflict with errata without also including rules to create an exception. Second they would have to create the spell so that it specifically explains that it circumvents how the general rules interact with attacks which have the sources being discussed: Spells, Magical Items and Magical Sources. 3rd This is an exception based system, the entire point is to create general rules and then specific rules which can modify, bypass or break them. But these exceptions have to be specifically called out.
So if they do all of this, you could end up with a spell that looks like this:
As part of casting this spell make a melee attack, this attack cannot bypass nonmagical resistance or immunity. The attack functions as a normal weapon attack for the purposes of effects. If the attack hits, a burst of flame leaps out from the blade and deals x amount of damage to another target.
Just a rough draft but this is how a specific beats general system works. Green Flame Blade is a spell, it let's you make an attack, that attacks source is a spell so it bypasses by default resistance and immunity.
Both my my first and second points were in regards to your custom (theoretical if you prefer) version. I was explaining what your changes would mean if it was really worded that way to cover all the bases.
And again, you still don't understand that the properties of the weapon you're using or any factor beyond the fact that the SPELL is letting make the ATTACK doesn't matter for determining if the attack bypasses or not. It's like you're glossing over the fact that the errata is talking about SOURCES of attacks. Spells, Magical Items, Magical Sources. Not weapon properties, attack effects, damage dice, damage type.
You're completely stuck on item properties and attack effects, damage type and so on. If you can't see beyond this point nothing I say will get through to you. You're even now changing the words in the spell text to justify your point of view. The 1d6 nonmagical slashing damage that you just posted IS IRRELEVANT. IT doesn't matter. All that matters is the ATTACK is granted by the SPELL which is the SOURCE. You can substitute whatever the hell you want into the spell text, it doesn't matter. It's a spell letting you make an attack ERGO it bypasses. lol
This is a specific beats general rules system. Something has to specifically call out that it allows a game element to bypass the general rules. Simply saying that an attack does 1d6 nonmagical slashing damage doesn't change any of the facts I've just explained to you.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.