In GHC 7.8.4, the following code is legal:

{-# LANGUAGE NullaryTypeClasses, TypeFamilies #-}
module NullaryData where

class NullaryClass where
    data NullaryData

In GHC 7.10, NullaryTypeClasses was deprecated in favor of MultiParamTypeClasses. However, running this code on GHC 7.10-rc1:

{-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-}
module NullaryData where

class NullaryClass where
    data NullaryData

results in this error:

NullaryData.hs:4:1:
    The associated type ‘NullaryData’
    mentions none of the type or kind variables of the class ‘NullaryClass’
    In the class declaration for ‘NullaryClass’

It's probably related to this earlier bugfix.