GHCi debugger crashes with :print

To reproduce:

1. Enter the following code into a source file:

{-# LANGUAGE NoMonomorphismRestriction #-}

d (i, j) = i + j

dmerge xs     [] = xs
dmerge []     ys = ys
dmerge (x:xs) (y:ys) | d x <  d y = x : dmerge xs     (y:ys)
                     | d x == d y = x : dmerge xs     ys
                     | otherwise  = y : dmerge (x:xs) ys

pairs xs = head xs : pairs (tail xs `dmerge` map (\(i,j) -> (i+1,j+1)) xs
                                    `dmerge` map (\(i,j) -> (i,  j+1)) xs)

allpairs = pairs [(1,2)]

1. Start ghci, and pass this file as a parameter.
2. Type :print allpairs

GHCi will crash.

cdsmith@devtools:~/haskell/projects/euler$ ghci -ignore-dot-ghci BugTest.hs
GHCi, version 6.9.20071101: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( BugTest.hs, interpreted )
Ok, modules loaded: Main.
*Main> :print allpairs
*** Exception: ghci/Debugger.hs:84:11-72: Irrefutable pattern failed for pattern Data.Maybe.Just subst

ghc-6.9.20071101: panic! (the 'impossible' happened)
  (GHC version 6.9.20071101 for i386-unknown-linux):
        ghci/Debugger.hs:84:11-72: Irrefutable pattern failed for pattern Data.Maybe.Just subst


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

Note that it does not crash without the LANGUAGE directive, so this appears to have something to do with the polymorphic type.

cdsmith@devtools:~/haskell/projects/euler$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.9.20071101

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	cdsmith@twu.net
Operating system	Unknown
Architecture	Unknown