Premise
Sneak Attack, like Divine Smite, is not weapon damage according to the ruling for the Great Weapon Fighting fighting style:
Does the benefit of the Savage Attacker feat apply to ad-ditional effects like the rogue’s Sneak Attack or the pal-adin’s Divine Smite? No. The benefit of Savage Attacker applies only to rolls of the weapon’s damage dice, not to any extra damage that a feature or other ability might grant
Consider a weapon such as the Sun Blade that is finesse and can be used two-handed. The sneak attack damage is not applicable for the GWF re-rolls, because the damage isn't from the weapon according to the cited ruling.
Question
If it's not weapon damage, then what kind of damage does sneak attack do? and
where in the rules, errata, or tweets is this clarified?
Implications to consider:
- Can sneak attack deal magical damage?
- If no contradiction exists and sneak attack is weapon damage, are GWF re-rolls applicable to sneak attack damage?
Best Answer
Sneak attack does damage per weapon type1
If using a scimitar*; slashing damage.
If using a short sword, rapier, dagger, bolt, dart, or arrow; piercing damage.
If using a sun blade; radiant damage.
The text is clear on what sneak attack represents: more lethal use of the weapon used in the attack.
If the attack is magical (due to the weapon being magical) the damage bypasses immunity / resistance to the damage type.
A magical weapon is a magic item. That's how you get a magical attack with a weapon. The sun blade is a +2 magical weapon, so the attack is magical.
There isn't "magical damage" per se, beyond Force damage. There are magical attacks.
Magical isn't one of the listed damage types. Shortswords do piercing damage. (Weapons table, PHB, p. 48). Sun blades do radiant damage, and they provide magical attacks since they are magical swords. (Magic Item, see above).
Note from the above: Sneak Attack's extra damage dice are bound to the weapon being used to inflict them. Many weapons cannot be used for a sneak attack, such as a quarter staff, a great club, a greataxe, a pike, a glaive, et cetera.
From a practical standpoint, if the sneak attack did not do the weapon's damage type, there would be a serious nerf on the Rogue class since that is a major feature of their combat capability.
Sneak Attack and the Great Weapon Fighting fighting style have no relationship to each other. There are no weapons on the weapon table that are finesse that you can use for Great Weapon Fighting. (There is one magical weapon (rare) that is an exception to the rule: Sun Blade, which is has the finesse property. The rule on Specific Beats General (Basic Rules, p. 5) handles that lone exception well enough.
There is no contradiction; there is an exception. And here's the thing: GWF is a Fighter class feature. The tweet response was about a Fighter class feature, not a Rogue class feature. Sneak attack is a Rogue class feature. You are in the land of rulings, not rules, at this point if you are uncomfortable with this exception.
RAW supports the nova strike.
Great Weapon Fighting (Fighter and Paladin class feature)
Sun blade fits all of this, so far.
Sneak Attack / Critical hit
Sneak Attack: you can deal an extra 1d6 damage to one creature you hit with an attack ...
GWF fighting style? Reroll damage dice. Crit? Added damage dice. Sneak Attack? Added damage dice. Reroll those Sneak Attack / Crit damage dice that come up 1 or 2.
So what do I do as a DM?
If this exception seems to you to be a problem, then the simple answer is "no sun blades in this world" or "no Rogue Fighter MC with that combo." Problem solved. (Tell the players up front, please). If, on the other hand, you are willing to now and again wallow in a Sneak-Attack-Sun-Blade-Radiant-Damage nova from the Thief/Fighter Multi class, then make that ruling and play on. (The Angels and other Celestials who resist radiant damage won't worry in any case ...)
As a side note, your question mixes apples and oranges. Divine smite has no relationship to Sneak Attack. Also, Divine Smite does radiant damage, not "magical" damage.
Dev RAI commentary
I figured out all of the above by myself. Mr Crawford has offered his commentary.
1And again, the weapon dictates the damage type; Jeremy Crawford agrees.
A further point about how weapon type is tied to sneak attack damage: