Typechecker loop with type families

The following code (which I think is ill-typed) causes GHC to hang, both on 6.12.1 and 6.13.20100616:

{-# LANGUAGE TypeFamilies, ScopedTypeVariables, FlexibleContexts #-}
module Crash where

class Family f where
  terms :: f a -> a

class Family (TermFamily a) => TermLike a where
  type TermFamily a :: * -> *

laws :: forall a b. TermLike a => TermFamily a a -> b
laws t = prune t (terms (undefined :: TermFamily a a))

prune :: TermLike a => TermFamily a a -> TermFamily a a -> b
prune = undefined

The compiler still hangs if I remove the typeclass constraints (everything to the left of a =>, I mean), but everything else seems to be necessary for the compiler to loop. Using -dshow-passes shows that it's the typechecker that loops.

Trac metadata

Trac field	Value
Version	6.13
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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