Cabal build problem with GHC <= 6.4

ghc-6.2.2 -H16m -O  -I. -Iinclude -Rghc-timing -H32m -O  -I../libraries -fglasgow-exts -no-recomp    -c Distribution/Compat/Map.hs -o Distribution/Compat/Map.o  -ohi Distribution/Compat/Map.hi

../libraries/Cabal/Distribution/Compat/Map.hs:68:
    Inferred type is less polymorphic than expected
        Quantified type variable `b' is unified with another quantified type variable `k'
    When trying to generalise the type inferred for `difference'
        Signature type:     forall k a b.
                            (Ord k) =>
                            Map k a -> Map k b -> Map k a
        Type to generalise: Map k a -> Map k k -> Map k a
    When checking the type signature for `difference'
    When generalising the type(s) for `difference'

Trac metadata

Trac field	Value
Version	6.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	libraries (other)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown