Typechecker allows a skolem to escapt

This program make GHC 7.4 and GHC 7.6 rc1 give a Lint error, because a skolem is allowed to escape

{-# LANGUAGE MultiParamTypeClasses, TypeFamilies, FlexibleContexts #-} 

module Foo where

type family F a

class C b where {}

foo :: a -> F a
foo x = error "urk"

h :: (b -> ()) -> Int
h = error "urk"

f = h (\x -> let g :: C (F a) => a -> Int
                 g y = length [x, foo y]
             in ())

This is the result with 7.6 rc1

*** Core Lint errors : in result of Desugar (after optimization) ***
<no location info>: Warning:
    In the expression: Foo.h @ (Foo.F a_o) (\ _ -> GHC.Tuple.())
    @ a_o is out of scope
*** Offending Program ***
Foo.h :: forall b_aeJ. (b_aeJ -> ()) -> GHC.Types.Int
[LclIdX]
Foo.h =
  \ (@ b_d) ->
    GHC.Err.error
      @ ((b_d -> ()) -> GHC.Types.Int) (GHC.CString.unpackCString# "urk")

Foo.f :: GHC.Types.Int
[LclIdX]
Foo.f = Foo.h @ (Foo.F a_o) (\ _ -> GHC.Tuple.())

Foo.foo :: forall a_aeL. a_aeL -> Foo.F a_aeL
[LclIdX]
Foo.foo =
  \ (@ a_g) _ ->
    GHC.Err.error @ (Foo.F a_g) (GHC.CString.unpackCString# "urk")

*** End of Offense ***

Trac metadata

Trac field	Value
Version	7.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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