M&M Mix and Mega

[dhelmise]

Not really.
We actually see that Solar Beam ignores the damage reduction in Rain in the Champions trailer with Solar Beam KOing Politoed.
252 SpA Meganium-Mega Solar Beam (60 BP) vs. 0 HP / 0 SpD Politoed in Rain: 212-252 (66 - 78.5%) -- guaranteed 2HKO
So Mega Sol should be able to use damage dealing Fire moves in Heavy Rain, like how Solar Beam keeps it power in opposing Rain.

And you know, the ability is "lets the Pokémon use its moves as if the weather were harsh sunlight.", so if you use a Fire type move it should be treated like its in Harsh Sunlight and get a 50% bonus. So it'd be pretty surprising if GameFreak just made an exception to screw over Fire moves.

i moreso meant it was wishful thinking because youre coming to conclusions based on how PS speculatively implemented an ability. We have no idea how this is supposed to work; the implementation was figured out by talking with researchers and trying to come to conclusions from the description in the move. The base power reduction for Solar Beam is not handled in the same way as the damage increase for fire moves/damage decrease for water moves, so what you're doing right now is conflating the way the game treats both interactions (See how I mentioned base power for solar beam and damage for fire/water moves).

 

[Hydrametr0nice]

We actually see that Solar Beam ignores the damage reduction in Rain in the Champions trailer with Solar Beam KOing Politoed.
252 SpA Meganium-Mega Solar Beam (60 BP) vs. 0 HP / 0 SpD Politoed in Rain: 212-252 (66 - 78.5%) -- guaranteed 2HKO

Or Solar Beam is still 120 BP because of Mega Sol.