If Ditto is holding the Eviolite and transformed into an un-evolved Pokemon would he get the boosts from the evolite? Also if Ditto was not holding the Eviolite but he transformed into a Pokemon that was holding the Eviolite would he get the boosts, like how he copies the stat boosts from things like S-dance? Also if he was holding the Eviolite and he transformed into a Pokemon with the Eviolite would he get the boost from both of them?
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No, even if ditto has the eviolite and transforms into an un-evolved pokemon the game still registers it as a ditto which has no evolution so sadly this would not work.