Arceus (Flying) uses roost?

Question

If an Arceus with a sky plate uses roost, what type does it become?

Answer 1

Arceus can't learn Roost legally, but if you somehow get it to learn it, it will become ??? or Normal type.

Gen IV

>If a Pokémon that has become a pure Flying-type uses Roost, it will become a ???-type Pokémon until the end of the turn.

Gen V

>If a pure Flying-type Pokémon uses Roost, it will become Normal-type until the end of the turn..

Source

Comments

Actually, Roost will become Normal-type in Gen V, as said in the source.

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woops, my bad, sorry xP

Answer 2

It cant learn Roost legibly.

Source