Yesterday my druid used Share Spell to cast Primal Form (Spell Compendium, p. 161) on her brown bear companion for the nice earth form damage reduction.
But after re-reading the spell, we started to have some doubts whether that is actually possible. Primal Form states:
You cannot cast spells, and you lose any spell-like, supernatural, and extraordinary abilities of your own form.
As Share Spells is an extraordinary ability, wouldn't using it for Primal Form immediately result in Primal Form getting deactivated because Share Spells is suppressed?
This is for using the second paragraph of Share Spell:
Additionally, the druid may cast a spell with a target of “You” on her animal companion (as a touch range spell) instead of on herself. A druid and her animal companion can share spells even if the spells normally do not affect creatures of the companion’s type (animal).
...but I think it applies for the first paragraph (having a spell affect both the druid and her companion) as well.
My DM allowed it yesterday and will allow it in the future, but RAW it should be forbidden, RAI we are not sure. Is this interpretation correct?