GHC rejects typeclass with associated type

[mhwombat]

In GHC 7.8.4, the following code is legal:

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}
    
class FiniteGrid g where
  type Size s
  size :: g -> Size g
  maxPossibleDistance :: g -> Int

But with GHC 7.10.1, it no longer compiles. The error message is:

The associated type ‘Size’
mentions none of the type or kind variables of the class ‘FiniteGrid g’
In the class declaration for ‘FiniteGrid’

I didn't see any mention of this in the migration guide, but perhaps there's another pragma I need to enable. I thought this might be related to ​https://ghc.haskell.org/trac/ghc/ticket/10020, but that bug is apparently fixed in GHC 7.10.1.

[simonpj]

There's nothing to connect Size with FiniteGrid; it's not an associated type at all. You can just define it separately:

type family Size s

class FiniteGrid g where
  size :: g -> Size g
  maxPossibleDistance :: g -> Int

Or, if you want to guarantee you always have an instance of Size g whenever you have an instance of FiniteGrid g, then use g in Size's definition:

class FiniteGrid g where
  type Size g
  size :: g -> Size g
  maxPossibleDistance :: g -> Int

This isn't a bug in 7.10; rather it was a bug in 7.8.