Fling will fail if used by a Pokémon holding a Mega Stone that can Mega Evolve the user.
It is not clear wether it is possible to fling a Mega Stone that cannot Mega Evolve the user, but I tested it and found that you can. I have yet to figure out the base power, because the Damage Calculator doesn't allow Mega Stones on Pokémon that are not compatible.
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