inferred type of an implicit parameter rejected (associated type)

The following module:

{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE NoMonomorphismRestriction #-}

module Test where

class C t where
    type TF t
    ttt :: TF t -> t

b :: (C t, ?x :: TF t) => t
b = ttt ?x 

compiles fine with ghc 6.12.3 but ghc 7.0.3 says:

    Could not deduce (?x::TF t)
      arising from a use of implicit parameter `?x'
    from the context (C t, ?x::TF t)
      bound by the type signature for b :: (C t, ?x::TF t) => t
      at Test.hs:13:1-10
    In the first argument of `ttt', namely `?x'
    In the expression: ttt ?x
    In an equation for `b': b = ttt ?x

Moreover, when I comment out the type declaration for b, ghc 7.0.3 compiles it and the inferred type for b is identical to the one that was commented out:

*Test> :t b
b :: (C t, ?x::TF t) => t

Trac metadata

Trac field	Value
Version	7.0.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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