Is hitting creatures with Steel Wind Strike considered "using" a weapon?

Question

Steel wind strike's spell description (XGtE, p. 166) states:

You flourish the weapon used in the casting and then vanish to strike like the wind. Choose up to five creatures you can see within range. Make a melee spell attack against each target. On a hit, a target takes 6d10 force damage.

Now, we know that steel wind strike isn't a melee weapon attack. Attacks can either be a melee or a ranged attack and they can either be a spell or a weapon attack (so 4 types total). Sometimes it matters, but for the purposes of this question, I wanted to get that out of the way. Jeremy Crawford confirms this.

My question is in regards to the Great Weapon Fighting fighting style and the Great Weapon Master feat, both of which have similar wording.

The Great Weapon Fighting fighting style is described as follows (emphasis mine):

When you roll a 1 or 2 on a damage die for an attack you make with a melee weapon that you are wielding with two hands, you can reroll the die and must use the new roll, even if the new roll is a 1 or a 2. The weapon must have the Two-Handed or Versatile property for you to gain this benefit.

The Great Weapon Master feat (PHB, p. 167) is described as follows (emphasis mine)

• On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

• Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage.

In my mind, this all hinges on whether or not steel wind strike is considered to be using your weapon or not. All other requirements are satisfied. You are making a melee attack (weapon attack is not specified) with steel wind strike's melee spell attack, which satisfies Great Weapon Master. You are obviously making an attack, so Great Weapon Fighting is satisfied.

This leads me to conclude that if I am using that weapon, I can take a penalty for +10 damage on the spell effect, I can reroll 1s and 2s for the damage dice rolled, and if I kill something I can make a single melee weapon attack as a bonus action.

The question is: Are you making these attacks with a weapon?

Answer 1

No, I would say it is not. The word "with" in this case is not interchangeable with "involving in any fashion", it means using that weapon to make a melee weapon attack, but one that doesn't necessarily have to be the Attack action.

The question is really asking, to my mind, is SWS a gish spell? Well, let's take a look at some of the classic 5e gish spells, ones we know have interactions with things like GWF and GWM:

Booming Blade

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails. (SCAG p.143)

Green-Flame Blade uses the same language.

We could even take a look at a Xanathar's spell, though it's drastically different:

Shadow Blade

You weave together threads of shadow to create a sword of solidified gloom in your hand. This magic sword lasts until the spell ends. It counts as a simple melee weapon with which you are proficient. It deals 2d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60). (XGtE p.164)

Now, none of these things really sound very much like "You flourish the weapon used in the casting and then..." To my mind, this would suggest that 'flourish' and 'weapon used in the casting' language is much more of a flavor cue and not meant to suggest that the spell will have interactions with GWF and GWM any more than any other melee spell attack would, whether or not you're casting a melee attack spell while holding a greatsword.

Answer 2

You aren't using the weapon in the attack

The melee spell attacks are part of the spell's effect while the weapon used is a material component (part of the casting):

Components: S, M (a melee weapon worth at least 1 sp)

You don't use material components to make the attacks, the components of the spell are part of the casting, which results in the spell effect (emphasis mine):

Each spell description begins with a block of information, including the spell's name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell's effect.

[...]

Material (M)

Casting some spells requires particular objects...

Material components and other elements of casting are completely separate from the spell effect (which includes the attacks). As such, you are not using the weapon to make any attack and features like Great Weapon Fighting will not apply.