automatic show instance generating loop

I have the following code :

{-# LANGUAGE TypeFamilies, DataKinds #-}

data Status = Valid | Invalid deriving(Show)

data A s = A (TF s)

type family TF (s:: Status) where
    TF Valid = Int
    TF Invalid = Either String Int

instance Show (A Valid)
instance Show (A Invalid)

a :: A Valid
a = A 3

Printing a results in a stackoverflow.

I'm using GHC-7.10.3 (and 7.8.4) on Ubuntu under a VM

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture