suppress warning "Defined but not used: type variable ‘x’" in GHC-8.0

[Henning Thielemann]

GHC-8.0 emits several new warnings of this kind:

    Defined but not used: type variable ‘x’

for declarations like

    type instance Snd x y = y

Enthusiastically, I started to replace unused type function arguments by 
underscores, only to find out that older GHC versions do not accept that. 
With what option can I disable this warning? Or can it be removed from 
-Wall for now?