Panic in tidyNPat when using N+k patterns with overloaded strings

Spotted this while reading the source code to tidyNPat. It looked dubious to me, and indeed it is:

{-# OPTIONS_GHC -XOverloadedStrings #-}
module Main where

import Data.String

instance IsString Int where
        fromString x = 1337

f :: Int -> String
f "hello" = "correct"
f _       = "false"

main = do
        print $ f 1337
        print $ f 1338

Result:

mbolingbroke@mb566 ~/Programming/Checkouts
$ ghc --make Boog.hs
[1 of 1] Compiling Main             ( Boog.hs, Boog.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.3 for i386-apple-darwin):
	tidyNPat

Trac metadata

Trac field	Value
Version	6.10.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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