One more code sample which compiled with GHC 7.2.1 and does not with the new RC:
{-# LANGUAGE FlexibleContexts, FlexibleInstances, FunctionalDependencies, MultiParamTypeClasses, RankNTypes, UndecidableInstances, TypeFamilies #-}
newtype MyMonadT m a = MyMonadT (m a) class MyClass b m | m -> b where data StM m :: * -> * myFunc :: b a -> m a instance MyClass b m => MyClass b (MyMonadT m) where newtype StM (MyMonadT m) b = StMMine [b] myFunc = undefined <<<<< In the instance, in GHC 7.2 the 'b' on the LHS of the newtype introduces a fresh type binding. In the RC I get a kind error, because the compiler seems to be trying to make all of the 'b' labeled types the same type. Since the 'b' isn't an indexing parameter, it doesn't make since for it to not introduce a new binding. This seems like an odd UI corner case, so I'm not sure what the correct answer is. But it is a regression, so I thought I would ask and make sure it was on purpose. Antoine