Can't write type signature for top-level polymorphic pattern binding

Hi,

In GHC 7.4.1 and 7.6.3 I can't write a type signature for a top-level polymorphic pattern binding. Consider this program:

{-# LANGUAGE RankNTypes #-}

newtype T = T (forall t. t -> t)

tf :: T
tf = T id

-- Can't write this type signature:
-- f :: t -> t
T f = tf

-- But with an indirection we can:
g :: t -> t
g = f

-- We can still use f as it were fully polymorphic (which is good):
a :: ()
a = f ()
b :: Char
b = f 'b'

I expect to be able to specify a type for f.

The same applies for a data family constructor, which is my original use case. I don't think it matters much, but here is an similar test case that uses a data family:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}

class C t where
  data F t :: *
  mkF :: t -> F t

instance C () where
  data F () = FUnit (forall t. t -> t)
  mkF () = FUnit id

-- Can't write a type for f here either:
FUnit f = mkF ()

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture