Trick-or-Treat adds the Ghost type to the target, in addition to the Pokémon's original type(s). If it is used against a dual-type Pokémon, that Pokémon will have three types at once. If the target already has an additional type added to it by Forest's Curse, that type is replaced with the Ghost type. Trick-or-Treat will fail if the target Pokémon is already Ghost type. This additional type can be copied by Reflect Type.
The move will fail if used on a Ghost Type. It won’t have any additional effectiveness. Ghost on ghost is super-effective.
Be sure to check out the site’s type coverage checker to see a comprehensive list of type effectivenesses.
Hope this helps.
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