Debugging info(?) leaks out: "Urk! Inventing strangely-kinded void TyCon"

With GHC 6.6, this module:

module G where

testL = foo undefined

class Foo t where
    foo :: m a -> t m a

leaks some debugging info(?):

Urk! Inventing strangely-kinded void TyCon:
    :t{tc ae2}
    (* -> *) -> * -> *

tmp.hs:4:8:
    Ambiguous type variable `t' in the constraint:
      `Foo t' arising from use of `foo' at tmp.hs:4:8-20
    Possible cause: the monomorphism restriction applied to the following:
      testL :: forall (m :: * -> *) a. t m a (bound at tmp.hs:4:0)
    Probable fix: give these definition(s) an explicit type signature
                  or use -fno-monomorphism-restriction

I haven't managed to get this message in an acceptable program.

(found by fasta on IRC)

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown