Trick-or-Treat adds the Ghost-type, but fails if the target is already Ghost.
Forest's Curse adds the Grass-type, but fails if the target is already Grass.
[Note that when I ask if these moves work on part-Grass-/part-Ghost-types respectively, I mean that if the target is NOT a Grass/Ghost-type already. So, Trick-or-Treat working on part-Grass-types and Forest's Curse working on part-Ghost-types.]
Will Trick-or-Treat work if the target is Grass-type?
Will Forest's Curse work if the target is Ghost-type?