Require XOverlappingInstances for the most specific instance only

I tried compiling the attached snippet with both ghc 6.12.1 and 6.10.4. The result is that even with XOverlappingInstances flag it still does not allow the program although it is provided with a concrete instance.

The code snippet is:

{- File overlap.hs -}
{-# OPTIONS_GHC -XOverlappingInstances -XTypeSynonymInstances #-}

type PairIS = (Int,String)
instance Show PairIS where
 show (a,b) = "(("++(show a)++","++(show b)++"))"

test :: Int -> String -> String
test a b = show (a,b)

The OS is :Linux 2.6.31-19-server #56 (closed)-Ubuntu SMP x86_64 The gcc version is (irrelevant): gcc version 4.4.1

The error message is:

Glasgow Haskell Compiler, Version 6.12.1, for Haskell 98, stage 2 booted by GHC version 6.8.2
Using binary package database: /usr/local/lib/ghc-6.12.1/package.conf.d/package.cache
wired-in package ghc-prim mapped to ghc-prim-0.2.0.0-9d35c97e886f807a1e6d024aaa91dcec
wired-in package integer-gmp mapped to integer-gmp-0.2.0.0-9a51ffb34a83618a1a3d4e472b9977a0
wired-in package base mapped to base-4.2.0.0-b340bbd470b5859bcbb920aea62a12cf
wired-in package rts mapped to builtin_rts
wired-in package haskell98 mapped to haskell98-1.0.1.1-0fdaf3b26bc38c43ce8371edf538dbf6
wired-in package template-haskell mapped to template-haskell-2.4.0.0-fc13d9708caa0cfdc4173ee31dc2bf26
wired-in package dph-seq mapped to dph-seq-0.4.0-b8a71915f9569cbd3a6895da8311707a
wired-in package dph-par mapped to dph-par-0.4.0-68df56bb8ae18f20e3518a7b338d7ebc
Hsc static flags: -static


D:\code\overlap.hs:8:11:
    Overlapping instances for Show (Int, String)
      arising from a use of `show' at D:\code\overlap.hs:8:11-20
    Matching instances:
      instance (Show a, Show b) => Show (a, b) -- Defined in GHC.Show
      instance [overlap ok] Show PairIS
        -- Defined at D:\code\overlap.hs:4:9-19
    In the expression: show (a, b)
    In the definition of `test': test a b = show (a, b)