:print in ghci: impossible happened

GHCi, version 6.8.1: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
Prelude> let fac n = foldl (*) 1 [1..n]
Prelude> :print fac
fac = <function>
ghc-6.8.1: panic! (the 'impossible' happened)
  (GHC version 6.8.1 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

This also happens with:

Prelude> let sum' a b = a+b
Prelude> let product' a b = a*b
Prelude> let sum' a b = (+) a b

but not with:

Prelude> let sum a b = a
Prelude> :print sum
sum = <function>
Prelude> let summ a b = id b
Prelude> :print summ
summ = <function>
Prelude> let id' x = x
Prelude> :print id'
id' = <function>
Prelude> let product' = (*)
Prelude> :print product'
product' = <function>
Prelude> let sum' = (+)
Prelude> :print sum'
sum' = <function>

All these functions do work properly when used in expressions

Trac metadata

Trac field	Value
Version	6.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown