Type pattern synonyms

Hi, it would be nice if the following example were acceptable:

{-# LANGUAGE FlexibleInstances, TypeFamilies, TypeSynonymInstances #-}
data X a = X a
type family TX a
type instance TX a = X a

instance Show (TX Int)

But ghc complains that it cannot substitute the instance of TX:

ts.hs:6:10:
    Illegal type synonym family application in instance: TX Int
    In the instance declaration for ‛Show (TX Int)’

A more practical (but not self-contained) example involving extensible records looks http://lpaste.net/100436

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Trac field	Value
Version	7.6.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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