'impossible' happened in reportNoInstance?

This program, though it shouldn't typecheck, shouldn't make the compiler panic like this either:

ghc-6.5: panic! (the 'impossible' happened)
  (GHC version 6.5 for i386-unknown-linux):
        reportNoInstance
    $dPrintfType{v alY} :: {<pred>base:Text.Printf.PrintfType{tc r5j} (a{tv alO} [sk]
                                                                       -> b{tv alP} [sk])}

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Presumably the problem is in the phex part...

{-# OPTIONS_GHC -fglasgow-exts #-}
#!haskell

-- Actually, this exercises prelude/PrelRules, but this is the closest
-- place I could find to put it...

import GHC.Exts
import Numeric
import Text.Printf

main = do phex (I# (uncheckedIShiftL#  (negateInt# 5#) 2#))
          phex (I# (uncheckedIShiftRA# (negateInt# 5#) 1#))
          phex (I# (uncheckedIShiftRL# (negateInt# 5#) 1#))
          phex (W# (uncheckedShiftL#   (int2Word# (negateInt# 5#)) 2#))
          phex (W# (uncheckedShiftRL#  (int2Word# (negateInt# 5#)) 1#))

-- this doesn't work on negative numbers :-(
-- phex x = putStrLn ("0x" ++ showHex x "") 

phex :: a -> b
phex x = printf "0x%x" x

Trac metadata

Trac field	Value
Version	6.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown